An Introduction to Digital Scanning

An Introduction to Digital ScanningDigital Color Prepress volume four

This Agfa scanning guide is intended to be both an introduction for beginners, anduseful resource material for more experienced operators. The techniques andterminology relating to scanning for prepress and other applications are clearlyexplained and extensively illustrated.

Dramatic increases in computer power and software capabilities, combined with theproliferation of affordable image-capturing devices now offer image-manipulationfacilities to us all. The colour knowledge and skills of an experienced scanner operatorare, however, far less easily acquired.

Printed image quality is highly dependent upon the accuracy and correct colour andtonal balance of the initial scanning process. If significant image details and tonalranges are absent from the start, even the most skilful image retoucher will find itdifficult to produce an acceptable result.

Simple rules are provided in this guide to allow the best possible results to be obtainedwhen scanning any original, taking into account the intended output device. Potentialscanning or image-processing errors are illustrated for easy identification.

A section is also included for those about to purchase image-capturing equipment,giving advice on the suitability of currently available devices for specific applications.

Although a guide of this size could never be an exhaustive description of image-capturing technology and techniques, we have aimed to provide the reader withsufficient general information to obtain the best possible image quality from a widerange of equipment. Used in combination with our other Digital Colour Prepressguides, which deal mainly with the requirements for successful prepress and printing, a comprehensive knowledge of the complete process from original to print will begained.

The terms printed in bold throughout this guide can also be found in the glossary.

1CONTENTS 1

SETTING THE SCENE 2An overview of current imaging technologies

CHOOSING YOUR INPUT DEVICE 4Advice on the suitability of image capturing devices for specific tasks

SENSING TECHNOLOGIES 6A comparison of the two methods presently used to read image data

COLOUR BASICS 8Perception of colours in nature, on computer monitors and on the printed page

THEORY OF OPACITY AND DENSITY 10Understanding the principles of measuring film and paper densities

JUDGING YOUR ORIGINALS 11Deciding whether images will require special treatment during or after scanning

PICTURE ELEMENTS 12The composition of digital images, with an introduction to binary numbers

RESIZING BITMAPS 14The effects of increasing or decreasing image size and resolution

OUTPUT BASICS 16A summary of output possibilities and an explanation of resolution terminology

LINE-ART RESOLUTION 18Advice on scanning black-and-white originals, plus simple resolution rules

GREYSCALE RESOLUTION 20Advice on scanning greyscale originals, plus simple resolution rules

COLOUR RESOLUTION 22Advice on scanning colour originals, plus simple resolution rules

HISTOGRAMS AND TONE CURVES 24How to check and adjust the tonal ranges of scanned images

LINEAR AND NON-LINEAR TONE CORRECTIONS 26Various methods of changing image brightness and contrast

SCANNER DENSITY CONTROLS 28Use of automatic and manual controls to obtain optimum image density

GLOBAL AND SELECTIVE COLOUR CORRECTIONS 29Removal of unwanted colour casts and the selective modification of colours

SHARPEN YOUR IMAGE 30The explanation of a sharpening process, highlighting its potential pitfalls

COLOUR DEFINITIONS 32Precise descriptions of colour for accurate measurement and communication

COLOUR MANAGEMENT 34Automatic systems that ensure consistent colour matching from original to print

FILE FORMATS AND FILE STORAGE 36Popular formats, compression techniques, size calculations and storage methods

GLOSSARY 38

32

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SETTING THE SCENE

Technological advances in photography have enabled us allto cheaply record picturesque scenes such as the illustratedstone arch, for distribution to a wider audience. However,image reproduction using light-sensitive film or paper requirestime-consuming and exacting processing techniques. Multiplephotographic copies are expensive and final results oftendiffer widely in colour from the original scene.

Ink-based printing processes, such as offset lithography, allowhigh-volume image reproduction of photographic or otheroriginals at a reduced cost per copy. These processes requireimages to be separated into cyan, magenta, yellow and blackcomponents (CMYK), the four process ink colours used inprinting presses. In the past, CMYK separation methodsemployed either a large-format reprographic camera (1)equipped with coloured filters or a combined drum scannerand recorder (2).

Repro camera operators used RGB filters to record the red,green and blue components of colour images on black-and-white (monochrome) films. Many intermediate positive andnegative films had to be created before CMYK separationswere obtained. The more productive drum scanner recorderemployed three RGB signal amplifiers known asphotomultiplier tubes (PMT’s) to read RGB colour valuesfrom an original that was mounted on a rotating drum. Thesevalues were translated into CMYK colour separations anddirectly exposed onto monochrome film attached to a secondrotating drum. Both methods produced film separations fromwhich printing plates were made. New digital scanning andrecording methods have now eclipsed these highly specialisedand costly systems, opening the world of image processing tous all.

Digital inputModern digital input techniques allow images to bemanipulated and retouched on a computer, with precisecontrol and great flexibility. Final results are easily copied,any number of times, without loss of quality. In contrast tofragile photographic and hand-drawn original images,multiple digital copies stored on magnetic tapes or othermedia ensure reliable data integrity. The main disadvantageof digital images is that their quality is generally matched tothe intended output size and printing process. Unforeseenchanges in application may require new digital input from theoriginal source.

Digital images consist of a grid of small squares, known aspicture elements or pixels. RGB input devices reduce thevisible colour range to a limited palette. Each pixel isallocated the palette colour that most closely matches theoriginal image. The bigger the palette, the more accurately anoriginal image is described. Palette size is specified in bits,which are explained in the following “Picture elements”section.

Scanners are used to convert photographic or hand-drawnoriginals into digital data. Recent drum scanners (3) incorporatetraditional PMT sensors, but are designed to provide only digitaldata. Adaptations have also been made to earlier drum scanners(4), which enable them to supply digital data instead of directlyexposing films. The PMT sensor technology is not easilyimplemented in compact flatbed scanners (5) or digital camerasso a new technology has evolved. Charge-coupled devices(CCD’s), consisting of thousands of minute, light-sensitivereceptors (elements) convert varying light levels into digitalsignals.

Modern still digital cameras (6) use a two-dimensional CCDarray or matrix to instantly record ‘snapshots’. Data is eitherdown-loaded directly to a computer or it is stored on aremovable disk. Detachable CCD matrix camera backs are alsoavailable to adapt professional still cameras (7). Digital videocameras or camcorders (8) use a CCD matrix to recordconsecutive frames, which are either transferred directly to acomputer, or recorded onto high-quality magnetic tape (video).Flatbed scanners normally employ a linear CCD array instead ofa matrix to record successive lines of data as an image is scannedinto the computer.

An alternative to in-house image-capturing facilities is theuse of professional scanning services to transfer film-basedimages onto compact disks (CD’s). Computer-linked CDplayers (9) give rapid access to these potentially massivedigital-image databases.

Digital outputThe conversion of pixels into printing pigments is dealt within the “Output basics” section but a summary of availabledigital output devices is included here. Interactivemultimedia presentations require either a computer-drivenprojection system (10) or a colour monitor (11) with audiofacilities to reach their audience. Digital printing deviceshave proliferated to cater for the increasing use of computer-based publishing and image manipulation programs. Filmrecorders (12) expose digital data onto colour transparencyfilm for use in slide presentations, or to obtain any number ofsecond originals (high-quality copies of an originalphotographic image). This digital photo-imaging allowsdigitally-created, modified or restored originals to be outputon positive or negative film for convenient photographicdistribution or storage in image banks. Multiple black-and-white paper copies are produced by laser printers (13), whichrely on the xerographic copier process (dry toner). Paperoutput from tabletop colour printers (14), using technologiessuch as thermal wax transfer or dye sublimation, is restrictedto proofing and low-volume printing, due to high costs andslow speeds. Digitally-driven xerographic colour copiers (15)offer slightly faster printing speeds but costs remain high.

Monochrome film separations for ink-based colour printingprocesses are output by high-resolution imagesetters (16).Some of these devices can now expose directly onto printingplates (direct-to-plate), avoiding the need for intermediatefilms (17). Efforts are being made to transfer digital datadirectly to special offset litho press rollers (18), eliminatingthe film and plate-making processes (direct-to-press). Themost exciting development in low to medium volume digitalcolour reproduction is the introduction of high-speed duplex(double-sided) web presses, based on improved xerographicimaging technologies (19). These “computer-to-paper”systems produce low-cost colour copies in any quantitywithout requiring costly and time-consuming presspreparation or clean-up time between jobs.

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54

Office, OCR, FPO,B/W printing

Black-and-whiteflatbed scanner

Rxflexible, rigidpos. 3-D objects

Can be high

Simple, oftenautomatic

Operation*

Medium to high

Productivity*

Low to medium

Cost*

Main use

Originals

Office, FPOB/W and colourprinting

Colourflatbed scanner

Rx, Txflexible, rigidpos., neg.3-D objects

Can be high


Medium to high

Medium

Office, OCR, FPO,B/W printing

Transparencyscanner

Txflexiblepos., neg.

High


High

Medium to high

B/W and colourprintingDigital photo-imaging

Professionalflatbed scanner

Rx, Txflexible, rigidpos., neg.3-D objects

High to very high


High

Medium to high

B/W and colourprintingDigital photo-imaging

Drum scanner

Rx, Txflexible, pos., neg.

High to very high

Complex, oftenrequiring specialistknowledge

Medium to high

Medium to high

Audio/VisualcommunicationB/W and colourprinting

Digital camera

Rx, Txflexible, rigidpos. 3-D stationary scenes

Medium


Medium to high

Medium to high

Audio/Visualcommunication

Video camera

Rxflexible, rigidpos. 3-D scenes

Low to medium

Simple

Medium

Low to medium

Office, OCRFPO

Hand-heldscanner

Rxflexible, rigidpos.

Low

Simple but inconsistent

Medium

Low

CHOOSING YOUR INPUT DEVICE

Drum scannersThe photomultiplier tubes (PMT’s) used in drum scanners tosense RGB colour values are capable of producing very high-quality results. Early drum scanners were complex devices,requiring skilful operators in order to reach their fullpotential. They remain the most expensive image-capturingdevices on the market, although prices have begun to fallwith the introduction of desktop models. Only flexibleoriginals may be mounted on the transparent acryliccylinders used in drum scanners, which is a time-consumingtask. Negative or positive, transparent or reflective originalsare normally accommodated.

Digital camerasSome digital CCD cameras are designed solely to recorddigital data, having no provision for loading traditional films.Others are adapted from standard film cameras by theaddition of a digital back unit. With portability in mind, thededicated digital cameras record data onto removable disks.The capacity of these disks and number of elements in theCCD matrix limits the resolution of captured images. A suitable application is journalism, where digital images areimmediately relayed via modem or satellite for publication innewspapers. Standard film cameras with digital backs tend tocapture higher resolution images, feeding data via a cable tofast hard disks. RGB primary colours are scannedsimultaneously or in three passes, requiring the camera to bemounted on a stable tripod and allowing no movement inthe subject matter. Although more suited to capturing 3-Dobjects, digital back cameras could be used as an alternativeto flatbeds.

Video camerasMoving image sequences or individual stills can be directlycaptured by special computer-based frame-grabbing systemsfrom CCD video cameras or recorded tapes. Multimediapresentation software is able to utilise and edit clips of thesemoving images, although the high volumes of digital datarestrict image quality, size and duration. When a video clip is the only record of an important event, it is possible to increase the resolution of individual frames by theresampling process described in the “Resizing bitmaps”section, so that they can be printed with reasonable quality.

Hand-held scannersThese low-cost CCD devices are manually passed over flatblack-and-white or colour originals. They are not designedfor transparency scanning and their maximum width isnormally less than A4 format. Although some hand-heldscanners have a quoted resolution of 800 ppi, their ability toproduce acceptable results is limited. Applications for thesescanners include OCR, and rapid capture of contone imagesused for position only (FPO) when developing page-layoutconcepts.

The varying designs of image-capturing devices make themmore suited to some tasks than others. A good way of decidingwhich device best fits your requirements is to establish whattype of originals must be handled and how the captured datawill be used. Are the originals flat or three-dimensional? Willflat originals flex or are they rigid? How large are the originals?By what proportion will captured images need to be increasedin size? Are the images on transparent materials (film) orreflective materials (paper)? Do they consist solely of hard-edged black-and-white areas or lines (line art)? Are theycontinuous tone images (contones) such as photographs,containing smoothly blended grey tones or colours? Haveoriginals been printed as halftones? The descreening optionfor smoothing out the dots in halftone originals is notavailable with all image-capturing devices.

Many other factors may influence the final decision, includingease of use, versatility, software features, robustness, reliability,efficiency of service and credibility of the manufacturer. Theability to read a wide range of tones, especially in shadowareas, is particularly important when scanning colourtransparencies. Well-designed software interfaces may offeradvanced features such as the conversion of colour negativesto positive images, and the direct separation of RGB data intoCMYK for ink-based printing processes.

To read printed text into a wordprocessor, a hand-heldscanner with Optical Character Recognition (OCR) softwaremay be adequate. Simple black-and-white flatbed scannerscapture greyscale images or line art. The ability tosignificantly enlarge detailed colour images without noticeabledeterioration calls for a large number of closely-spacedreadings to be taken from them. This high resolution isprovided by professional flatbed or drum scanners.

Input device resolutions are quoted in pixels per inch (ppi),whereas the maximum resolution of output devices is thenumber of dots they are able to print or record per inch (dpi).The true optical resolution of a CCD input device isdetermined by the quantity of CCD cell readings taken perinch and by the optical system. When comparing inputdevices, check whether their optical resolutions have beenincreased by software enhancement (interpolation). Thisprocess avoids visible pixels in enlarged images but capturesno additional detail.

Flatbed scannersCCD-based flatbed scanners are the most popular image-capturing devices for desktop publishing (DTP) andprofessional prepress. They can normally be operated fromwithin standard image-editing programs. The more advancedsoftware interfaces used to drive flatbeds require minimaloperator training because optimal colour balance and imagedensity are automatically determined.

A wide range of flatbed scanners is available, from the low-cost black-and-white variety to high-quality, professionalcolour devices. High-end flatbeds scan both reflective andtransparent originals but an optional unit may need to bepurchased to scan transparencies on mid-range devices.

Professional flatbeds are generally cheaper than traditional drum scanners, although they are capable of producing scans of similar quality. Another advantage of flatbeds over drumscanners is that images on rigid substrates of any thickness can be scanned, such as books or card-backed page layouts(mechanicals).

Transparency scannersThese CCD-based devices are dedicated to scanning films athigh resolutions. They are popular with service bureaus, tradeshops, newspaper and magazine publishers. When the filmformat is not restricted to 35mm, a range of standard filmholders is usually included. Automatic loading and batch organg scanning of framed transparencies is supported by somemodels. Focusing, colour control and calibration of imagedensity may also be automated.

Quality

Abbreviations OCR: Optical Character RecognitionFPO: For Position OnlyRx: Reflective

Tx: Transparentpos.: positiveneg.: negative

* Although these factors are determined partially by the design and construction of eachdevice, the quality and versatility of software interfaces will significantly affect overallperformance.

6

Continuously varying analogue voltagesare sampled into a series of steps orlevels, each having a specific numericalvalue (binary). The number of levelsdepends upon the design of the A/D converter. An 8-bit A/D convertersamples 256 levels, 10 bits give 1024 levels, 12 bits provide 4096 levelsand a 14-bit converter allows 16384 unique levels to be counted. If signal-to-noise ratio is poor, additionalsamples of a smaller size will notnecessarily improve image quality.Bits and binary numbers are explainedmore fully in the following “Pictureelements” section.

Both photomultiplier tubes (PMT’s) andcharge-coupled devices (CCD’s) convertdifferent brightness levels intocontinuously varying or analogue voltages.These are chopped into a specific numberof steps or levels by an analogue-to-digital(A/D) converter, in a process known assampling. The purity of small analoguesignals is easily affected by electricalinterference, producing noise or inaccuratereadings. Good signal-to-noise ratios areimportant in the design of sensors andassociated circuitry. Most light sources andanalogue electrical devices require a periodof time to reach a stable operatingtemperature or condition. It is thereforeadvisable to wait a few minutes afterswitching on any scanner before makingthe first scan.

PMT sensorsTraditional drum scanners use a xenon ortungsten-halogen light source, which isfocused onto a small area of the original byfibre optics and condenser lenses.Transparencies are lit from inside the drumand reflective materials from outside.Transmitted or reflected light from aminute point on the image enters thesensor unit travelling along the outside ofthe rotating drum. The light is directedonto semi-transparent or dichroic mirrors,which are angled at 45° to the beam. Somelight is reflected from each mirror, whilstthe remainder is transmitted to the nextmirror. Reflected light passes througheither a red, green or blue filter and theninto one of three optical amplifiers knownas photomultiplier tubes. A/D convertersthen chop the analogue voltages intodigital data. A fourth PMT may provideimage-sharpening information, althoughsoftware-applied sharpening after scanningoffers greater flexibility.

PMT technology is capable of registering awide density range, but its complexitycauses manufacturing and maintenancecosts to be higher than those for CCDdevices. Extensive manual controls in mostPMT scanners require the knowledge andpractice of an expert to procure the bestsettings. Only flexible originals aresupported, the mounting of which is atime-consuming operation. Rigid originalsmust be reproduced on flexible material.Precious flexible originals may need to beduplicated to avoid possible damage duringthe mounting and rotational scanningprocess.

PMT drum scanning

Light sourcePhoto-multipliertubes(PMT‘s) forred, greenand blue

Colour filters

To A/D converter andoutput processing

The photomultiplier A PMT is a vacuum light sensor in whichelectrons are multiplied by secondaryemission. Light (photons) falling on thephotocathode releases electrons. Theseare conducted to the dynodes. Eachdynode releases more electrons by what is known as secondary emission. Severaldynode layers are needed to convert asmall quantity of light into a usableelectrical signal. Variations in electricalcurrent are measured at the anode. After amplification, the analogue signal is converted by an A/D convertor into adigital signal.

Lens

Original

Mirror

Dichroic mirrors

Cathode

Filtered light

Dynode

Anode

time0

255

A/D converters

Analogue Digital

SENSING TECHNOLOGIES

7Greyscale scanners take a single set of lightintensity readings from originals. Colourscanners capture three sets of readings fromcolour originals by the use of red, green andblue filters. Scanners that incorporate asingle linear CCD array sometimes rotatean RGB colour filter wheel in the lens unitbefore each of the three separate passes ofthe original are made. Single-pass scannersmay use three linear CCD arrays, which areindividually coated to filter red, green andblue light. The same image data is focusedonto each array simultaneously.

Although three-pass scanning is slowerthan single-pass scanning and registrationbetween colours is more critical, there are anumber of advantages. CCD sensors areless sensitive to blue light than to greenlight and are most sensitive to red light.Red, green and blue light come into sharpfocus at slightly different points from eachother. Some three-pass scanners optimiseboth the scanning speed and lens focusingfor the specific colour being read.

Well-designed and integrated CCD sensorsare able to read an image density rangesimilar to that of PMT sensors, oncecertain characteristics are taken intoaccount. The individual CCD elements inan array have a slightly different sensitivityfrom each other, and may also give a smallreading when no light is falling on them(dark current). Some devices compensatefor these anomalies by precisely calibratingeach CCD element. When an excess oflight falls on a CCD element, its chargecan spread or bloom into neighbouringelements, causing incorrect readings. Thesolid-state design of CCD sensors makesthem far more compact and mechanicallysimple than PMT sensors. They are alsocheaper, more stable and requireconsiderably lower voltages than PMT’s.

CCD sensorsFlatbed scanners employ a linear CCDarray, which consists of severalthousand charge-coupled deviceelements arranged in a row on a singlesilicon chip. Originals to be scanned areplaced on a glass plate. Transparenciesare evenly lit from above and reflectiveoriginals from below by a fluorescent orhalogen light source. Lengthwisemovement of the light source togetherwith a mirror directs consecutive linesof image data onto the static CCDarray, via a second mirror and asynchronously focused lens unit.

The full width of an image is readsimultaneously as a line. Light of aspecific colour and intensity falling oneach CCD element creates aproportional electrical charge within it.This analogue charge is systematicallypassed along chains of cells to an A/Dconverter, where it is sampled intodigital data. The CCD is now clear toreceive the next light-induced charge.

CCD flatbed scanning

Light source

Mirror

Mirror

Lens

RGB-coated CCD chip

Light-capturingCCD elements

To A/D converterand outputprocessing

Original

98

75°45

°

105°

90°

COLOUR BASICS

Our perception of colours in nature isdetermined by three factors - the type oflight source, how substances change thereflected or transmitted light, and thesensitivity of our eyes to the resultinglight.

The sun radiates a wide variation ofelectromagnetic waves, each having adifferent wavelength. The human eye issensitive to only a small range of thesewavelengths, known as white light.

Rainbows are created when white light issplit up by droplets of water. Passing abeam of white light through a glass prismproduces a similar effect. The shorterwavelengths are bent (refracted) morethan the longer ones, splitting the whitelight into its component spectrum ofvisible colours. Each colour causes aspecific reaction in the eye’s red, greenand blue cones or receptors. Yellow isperceived by both the red and greencones, for example.

The spectrum colours are the basicbuilding blocks of a much wider range orgamut of colours. When selections ofthese pure wavelengths are mixed oradded together in differing proportions,thousands of different colour sensationscan be perceived.

Additive coloursColour monitors and TV’s mirror thefunction of the eye by emitting red, green and blue colours (RGB) - the three primary colours of light. All othercolours can be composed by adding theseprimaries in different proportions andintensities, giving rise to the termadditive mixing. Green and blue lightresult in cyan (C), red and blue lightmake magenta (M), and red and greenlight form yellow (Y). C, M and Y areknown as the secondary colours of light,or the primary colorants when referringto pigments. White light is producedwhen red, green and blue are added insimilar proportions, whereas black resultsfrom their total absence. In reality, theblack displayed on colour monitors islikely to be a dark green or brown greydue to stray light emissions. The gamut of colours that can be displayed on amonitor is smaller than that seen innature because it is limited by thecharacteristics of the phosphor screencoatings that emit the light.

Subtractive coloursAll substances absorb, transmit or reflectspecific wavelengths of white light. Whenan object absorbs some light, only theremaining mixture of reflected ortransmitted wavelengths is detected byour eyes. An opaque white materialreflects all wavelengths, whilst a blackone absorbs them. Translucent ortransparent materials absorb or subtractcertain wavelengths of white light andtransmit the others. All of the spectralcolours can be produced from a whitelight source by passing it through single orpairs of translucent CMY filters. This is asubtractive process since the transmittedlight will be less intense than the lightsource. A cyan filter, which transmits blueand green light but subtracts red light,followed by a magenta filter, whichsubtracts green light, results in only theblue light being transmitted. Weakeningthe cyan filter allows some red light to betransmitted, producing violet light.

Colour photography materialsincorporate variable density, subtractiveCMY dyes, which filter light toreproduce life-like images. In printingtechniques such as offset lithography thedensity of the CMY process inks cannotbe continuously varied across an image,so a range of colours is produced by ahalftone technique, where CMY dots ofvariable size are printed in overlappinggrids. The smaller the dot, the less lightit will absorb, decreasing apparentdensity by increasing the amount ofreflected light. Process ink pigments areless pure than photographic dyes, so pure black cannot be obtained by over-printing solid CMY inks. For this reason,black (K) ink is printed in addition to, orinstead of dense CMY combinations.Process ink impurities, combined withthe incomplete reflectance of printingpaper generally result in a smaller colourgamut than photographic materials.

The visible spectrum

Monitors display a colour gamut thatis smaller than the visible spectrum.The merging of light emitted from

coloured light sources is an additiveprocess. All spectral colours andwhite light can be created by addingred, green and blue light.

Cyan, magenta and yellow filters orpigments subtract varying quantities ofred, green and blue from white light toproduce a limited gamut of spectralcolours.

Cyan Magenta

Yellow

Red Green

Blue

Halftone colour printing normally employsfour overlapping grids of dots (CMYK),which subtract differing amounts of RGBlight in proportion with dot size.

Printing inks also produce a smallercolour gamut than the visiblespectrum, but this is not the same asthe monitor gamut.

Electromagneticspectrum

Gamma

Röntgen

Ultra violet

Visible spectrum

Infra red

Microwave/Radar

T.V.

AM radio

Sound

Violet

Dark blue

Blue

Green

Yellow

Orange

Red

Spectrum colours

Spectrumcolours

White light(visible spectrum)

Natural image

Subtractive colour Print reproduction

Additive colour Monitor reproduction

1110

O = 10

10%

O = 1

100%

O = 2

O = 10O = 1 O = 2

50%

100%100%

100%

100%100%

100%

100%50%

10%

Clear film(carrier) Emulsion

Density D = log O 0 0.3 1 1.3 2 3 3.3

Opacity O = 1 2 10 20 100 1000 2000

Transmission T 100% 50% 10% 5% 1% 0.1% 0.05%or reflectance R

JUDGING YOUR ORIGINALS

Before beginning to scan any original, it isworthwhile checking whether it containsa restricted tonal range or unusual colourbalance. Try to locate the most denseshadow area (Dmax).Photographic images without darkshadows could be intentionally high key,or may have been incorrectly exposed orprocessed. Conversely, predominantlydark images may be intentionally low key.When the lightest image area (Dmin) is abright, reflected specular highlight, it will probably contain no detail at all. An image containing bright highlightsand deep shadows has a high contrast.This is more noticeable when few midtonesare present. A small tonal range (Dmax-Dmin), lacking extreme highlights andshadows, indicates a low contrast image.

When any of these characteristics areintentional, the automatic exposurecontrols offered by some scannerinterfaces may need to be bypassed oradjusted manually to prevent unwantedchanges to images. The black-and-whitereflective original of a man in sun glassescontains a wide tonal range. A whitecircle on the sun glasses indicates theposition of Dmax. The black circle on hisnormal glasses shows Dmin.

The colour transparency of a woman hasan intentional colour cast throughout theimage. Automatic location of Dmin findsthe pixel having the brightest, combinedcolour values. In this case, Dmin in thewoman’s eye will not be a neutral tonedue to the colour cast. Likewise, Dmax inher hair is not a neutral shadow area.Certain colours, such as deep reds, aremore difficult to scan than others.

Colour negatives contain a strong orangemask, which is removed before the imageis reversed to a positive version by someadvanced scanner drivers.

Screened originals need to be descreenedduring the scanning process by use of asoftware blur filter or by defocusing thescanner optics. This avoids moirépatterning and colour shifts in printedoutput.

For an image-capturing device tofaithfully reproduce a transparent orreflective original, it must be able toregister virtually the complete densityrange present. This range is thedifference between the most densearea and the least dense area, which isDmax - Dmin.

Transparencies generally have a Dmaxof about 3.3 and a Dmin of 0.3, givinga density range of 3.0 D.

B/W original

Colour original

Reflective materials may have adensity of 2.0 D, but in most cases therange is nearer to 1.7 D. The fact thata transparency film contains a tonalrange ten times wider than reflectivematerials requires devices intended fortransparency scanning to be far moresensitive.

Transparent and reflective originals

Amount of light transmitted Transmission (T) =

Total light source

Amount of light reflectedReflectance (R) =

Total light source

1 1Opacity (O) =

T or R

log 1

log 1

Density (D) = log (Opacity) =T

=R

Exposing a film to increasing light levelsraises the opacity (O) or blackness of itsemulsion after development. The opacityof a film is defined by the total amount oflight falling on it, divided by the amounttransmitted through it. A clear film thattransmits 100% of the light falling on ithas an opacity of 1 (100% / 100%),although in practice a small proportion oflight is always absorbed. When only 50%of light is transmitted by a film, itsopacity is 2 (100% / 50%). A 10%transmission indicates an opacity of 10(100% / 10%). Exactly the samecalculations can be applied to reflectivematerials. If a printed area on paper gives50% light reflectance, its opacity is 2.Opacity is the inverse of transmission (T)and is also the inverse of reflectance (R).An opacity of 2 results in a transmissionor reflectance of 1/2 of the incident light.

When the thickness (mass) of a filteringdye or exposed film emulsion is increasedin equal steps, its opacity rises at aprogressively faster rate. For this reasonthe term density was introduced, whichdirectly corresponds with the thickness ofa filtering layer. Density is proportional tothe logarithm of opacity, as explained in“Density in depth”.

If a light source of 2000 units is shone ontoan exposed film of opacity 10, there will be200 light units transmitted (2000 / 10). Adding a second film of the same opacitycuts the transmitted light again by a factorof 10, leaving 20 units (200 / 10). Theoverall opacity of the two films is 10 x 10 =100. Another way of writing this is 102,which is “ten raised to the power of two”. The total opacity of three identical films is 10 x 10 x 10 = 1000 or 103, which willtransmit only 2 units of light.

Density indicates the thickness or mass of afiltering dye or exposed film emulsion. It canbe seen from the simple calculations abovethat doubling the thickness of a filter doesnot double its opacity but instead raises it tothe power of two.

Density is proportional to the power bywhich opacity is raised, or in other wordsthe logarithm of opacity.

A film with a density of 1.0 D has anopacity of 101 or 10, transmitting 10% ofincident light. A 2.0 D film has an opacityof 102 or 100, giving a transmission of 1%.A 3.0 D film has an opacity of 103 or 1000,which means that it transmits only 0.1% ofthe light falling on it. An increase of 0.3density doubles the opacity range. A 3.3 Dfilm contains twice as many tonalvariations as a 3.0 D film (103.3 is close to2000).

Density in depth Density formulas

1 T

Colour negative original

THEORY OF OPACITY AND DENSITY

Dmin

Dmax

Dmax

Dmin

1312

Digital computers use millions of linked electronic switches to make calculations and process alldata. Each switch is either on or off, representing a value of one or zero respectively. In order tocount in ones and zeros it is necessary to use the binary number system. With the standarddecimal system, each digit increases from zero to nine before it is reset to zero and the digit tothe left is incremented (09 becomes 10). Binary digits, known as bits, only increase from zero toone before the next digit is incremented. A 2-bit binary number (22) has only four possiblevalues: 00, 01, 10, 11 (representing 0, 1, 2 and 3 in decimal values). An 8-bit binary number(28) provides 256 different values.

PICTURE ELEMENTS

A scanned digital image is composed of amatrix or bitmap of touching pixels(picture elements), which are smallsquares of solid black, white, varying greytones or colour. Bitmaps are either squareor rectangular.

Every digital or bit-mapped image hasfour basic characteristics: resolution,dimensions, bit depth, colour model.When an image is scanned, the numberof samples or readings to be recorded in agiven distance must be specified. This isknown as the scanning resolution, whichis normally specified in pixels per inch(ppi) or samples per inch (spi). The useof metric resolutions is increasing - “Res 12” means 12 pixels per millimetre (305 ppi). The physical size of pixelschanges according to the chosenresolution. Appropriate resolutions areindicated in the following “Line-art,Greyscale and Colour resolution”sections.

Bitmaps always consist of whole numbersof pixels, so although dimensions may begiven in inches or centimetres,measurements are more simply stated in pixels. Division of the number of pixels in the height and width of a bitmapby its resolution provides the physical size.For example, if an image is scanned at 300 ppi and the width and height are 900 pixels, the physical size is three inchessquare (900 / 300). When the resolution ischanged to 150 ppi, the physical size willbe six inches square (900 / 150). Thenumber of pixels has not changed but theyare now four times as big (double thewidth and height).

Bit depth (also known as pixel depth)defines how many tones or colours everypixel in a bitmap can have. In otherwords, the depth of information recordedduring the scanning process is limited bythe chosen bit depth.

If an image is scanned digitally to a depthof one bit, each pixel can have only twostates - black or white (zero or one).Images reduced to pure black and whitepixels are called bilevel images or flatbitmaps. When more than one bit is usedto describe each pixel, a range of greytones or levels can be placed between theblack and white. A depth of two bits addstwo grey tones to the black and white,giving four levels in total. Eight-bit data

Dimensions, resolution, bit depth and colour model all affect the digital file size of an image,determining the disk space required to store it. File size also has a direct relationship to thecalculation time used by a computer's processor during image editing. If the resolution of animage is doubled, the file size will increase by a factor of four since there will be twice thenumber of pixels in both width and height. A 32-bit CMYK file is 32 times as large as a 1-bitline-art version of the same image.

RGB colour 3 x 65536 colours3 x 16 bit

RGB colour 3 x 256 colours3 x 8 bit

Greyscale 256 grey levels8 bit

offers 256 different grey levels (includingblack and white). This is normallysufficient levels to reproduce a smoothgradation from black to white withoutseeing tonal jumps or bands.

Colour modelsIn order to record coloured pixels, tonalinformation is required for individualprimary colour channels. RGB imagesnormally use 24-bit depth (3 x 8 bits) and 32-bit depth is needed for CMYKimages (4 x 8 bits). When each colourchannel is defined by an 8-bit number,256 brightness levels per channel arepossible. The combination of 256 levelsof red, green and blue allows more than16 million colours to be described.

SupersamplingMost colour scanners are able to

differentiate 256 tonal levels for eachof the RGB primary colours.

Some are designed to record manymore levels, extending the bit depth

to 10, 12, 14 or even 16 bits percolour. This additional or super-

sampled data is rarely used by outputdevices, but it allows a wider range ofshadow details to be captured andsubsequently heightened. When scanninghigh-density transparencies this isparticularly important and it providesflexibility when RGB images areconverted to CMYK.

Some image-editing programs are nowable to work internally with 16-bit data,providing greater flexibility in colourcorrection prior to down-sampling to 8-bit data for output.

The fact that a scanner records a highernumber of bits per colour does notnecessarily mean that it can differentiateadditional tonal levels. If sensors andelectronic circuitry are poorly designed,the scanner may incorrectly register thesame numerical value for various tones.

Greyscale 4 grey levels2 bit

BilevelBlack and white1 bit

Binary number system

Bitmaps and file size

1514

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Resolution adaptations for resizing

RESIZING BITMAPS

ResamplingIf an image needs to be changed indimension and it is not possible to rescanit, pixels must be added or removed tomaintain the same resolution. Thisprocess is called resampling. The removalof pixels, known as down-sampling, is arelatively simple calculation, oftenachieved by pixel skipping. When toogreat a reduction is made, staircasing willbe visible in diagonal lines and finedetails will break up.

Resampling an image upwards by theaddition of pixels is known asinterpolation. Image-capturing devicesmay incorporate interpolation software toenhance their maximum opticalresolution. Whilst increasing resolutionby interpolation helps to reduce visiblestaircasing, it adds no extra detail toimages. In fact excessive interpolationwill result in a blurred, out-of-focusappearance. Subsequent application ofunsharp masking (USM) will resharpenthe image to a certain extent.

When an image is disproportionallyresized, more in one direction than theother (anamorphic distortion), new pixelsmust be interpolated or redundant onesremoved. This also applies when imagesare warped, sheared (skewed), or placed inperspective.

Whenever possible it is best to avoidresampling by scanning images at thecorrect resolution. If the scanner is notable to reach the resolution required toallow for enlargement, interpolation isthe only solution.

Any bitmapped image has a specificresolution or number of pixels per inch. If an image is enlarged without addingextra pixels, the size of each pixel mustincrease. This means there will be lesspixels per inch, thus the resolution isdecreased. Although the pixels appearbigger, their description in the file isidentical, so the file storage requirementremains constant.

When images are enlarged too much,individual pixels become clearly visible.The resulting staircasing or aliasing indiagonal lines is particularly disturbing(jaggies).

The opposite happens when an image isreduced in size without removing pixels.Pixels become smaller, so the resolutionincreases. Visually this is not a problembut the resolution could becomeunnecessarily high when compared withoutput requirements. Keeping imageresolution in the correct relationship tothe intended output device minimises filesizes and ensures efficient processing andprinting. Appropriate resolutions areindicated in the following “Line-art,Greyscale and Colour resolution”sections.

If an original will need to be resized, thescanning resolution should be adaptedaccordingly. For example, a 5 cm x 5 cmphotograph has to be scanned andenlarged to a size of 20 cm x 20 cm. Thisis a sizing factor of 4 (20 cm / 5 cm),meaning that the adapted scanningresolution needs to be four times as highas the desired final image resolution.When a final resolution of 200 ppi isrequired, the original photograph must bescanned at 800 ppi (200 x 4).

Some scanner interfaces allow output sizeand resolution to be specified, avoidingthe need to calculate sizing factors.

Original size

Pixels remain thesame size as those

in the originalimage.

Original size

Enlargement without resampling

Interpolation programs determine where new pixels must be addedthroughout an image to achieve an increased resolution. They thennormally use one of three methods to decide what colour the newpixels should be.

Nearest neighbour interpolation is the quickest but least accuratemethod where each new pixel takes on the colour of the closest pixel.Bilinear interpolation averages the colours of two pixels either side ofthe new pixel, giving a more accurate result. The most accurate buttime-consuming method is bicubic interpolation. In this case, all pixelssurrounding each new pixel are averaged to determine its colour.

Interpolation

Enlargement with resampling

Reduction with down-sampling

Adaptedscan

resolution=

Originalscan

resolution

Sizingfactorx

Scale: 200%Pixels have four

times the area of those in theoriginal image.

Original size Pixel size remains thesame but the image

becomes smaller.

1716

OUTPUT BASICS

This section briefly covers output techniques, in order toexplain their relationship with scanning resolution. Furtheroutput information is available in Agfa’s “Digital ColourPrepress” guides.

Scanning resolution is determined by the number of samplesor pixels per inch (ppi) that are recorded. Output devicesproduce a hard copy from digital image information, eitherby applying small dots of pigment to a substrate such aspaper, or by using an intermittent light source to expose dotsin a light-sensitive emulsion. The resolution of an outputdevice is the number of dots it is able to reproduce within aninch (dpi). In most cases, scanning resolution is not the sameas output resolution, so the bitmapped image is sampled toproduce a new output grid.

Line art consists of black and white pixels, which are easilyreproduced by adjoining dots of pigment or exposedemulsion. If line-art output resolution is too low, staircasingoccurs on angled edges and fine details are lost.

One method of outputting the 256 tonal levels in an 8-bitgreyscale image is to produce a grid or raster of varying sizedots. This is also known as a halftone screen. Viewed from adistance, halftone dots and white substrate merge to createdifferent grey tones. The larger the dots, the darker the tonebecomes. Screen ruling or screen frequency is the distancebetween lines of halftone dots, which is normally quoted inlines per inch (lpi) or lines per centimeter (lpcm).

Since most output devices use a fixed dot or “spot” size,varying numbers of these spots are grouped together toproduce larger halftone dots. The spot size of a 2400-dpiimagesetter is around one hundredth of a millimetre.Imagesetter resolution may be quoted in recorder elements or rels per inch (rpi) instead of dpi. A minimum of 64 greylevels are needed to print smooth tonal gradations on a laserprinter. This means that each halftone dot must comprise atleast an 8 x 8 matrix of spots (64 spots). Using a 400-dpi laserprinter, a screen ruling of 50 lpi is the maximum that willallow an 8 x 8 matrix (400 dpi / 8 spots = 50 lpi). Specifying a 100-lpi screen ruling for this device reduces the possible greylevels to 16, creating tonal banding in gradations. Variationsin screen angle change this ruling calculation slightly.

In summary, the maximum resolution of halftone outputdevices limits both the clarity of details and the number ofgrey levels that can be reproduced from bitmapped images.

Colour images can be printed by creating separate halftonescreens for the CMY primary pigment colours. The additionof a black halftone increases contrast and reduces thequantities of more expensive CMY pigments. Each halftonescreen must be printed at a specific angle to prevent adisturbing interference pattern called moiré occurring.Adjacent or overlapping CMYK halftone dots mergetogether when viewed from a distance, creating a wide rangeof colours.

The recently introduced stochastic orfrequency modulated (FM) screeningtechnique uses much smaller CMYKhalftone dots than the traditionalmethod, retaining more image detail.Tonal variations are obtained bychanging the number rather than sizeof dots. Scans to be output with FMscreening can be of lower resolutionthan those made for traditionalscreening, increasing productivitywithout loss of quality.

An alternative to the halftonereproduction of greyscales is theapplication of translucent pigments indifferent thicknesses to producecontinuously variable tones withoutseparate dots. This continuous tone, orcontone printing is achieved in ink jetprinters by changing the CMYK spraydurations at any given position to varyink densities. Thermal sublimationdevices incorporate an array of minuteheating elements, which vaporizevarying quantities of wax-basedpigment from a carrier film, depositingit smoothly onto a special substrate.Although the 300-dpi resolution ofmany of these devices seems low, theblending of pigment between dots givesthe impression of a much higherresolution.

Photographic methods of reproducingimages allow much higher resolutionsthan pigment-based systems.Imagesetters employ one or more finebeams of intermittent light to exposeadjoining dots or spots. Film recordersexpose the red, green and bluecomponents of an image in threeseparate passes. Resolutions up to 5000 dpi are used by some filmrecorders, allowing high-quality secondoriginals to be created.

Line art

Halftonegreyscale

Contonegreyscale

Halftone colour

Contonecolour

300 dpi 1200 dpi

Low screen ruling: 50 lpi High screen ruling: 175 lpi

Contone printer: 300 dpi Film negative

Low screen ruling: 100 lpi High screen ruling: 175 lpi

Contone printer: 300 dpi Film positive

Original Low resolution High resolution

18 19

LINE-ART RESOLUTION

Note: Although not definitive, Agfa has established these rules through practical experience.

When an original image consists of lines and solid areas of flat black or dark tones, it can be scanned as line art. Anycolour or grey tones present will be reduced to pure black and white, creating a bilevel image. Pen or pencil drawingsscanned as line art may be converted to contours for furthermanipulation in drawing programs (vectorisation). Objectsdrawn with a single flat colour, such as logos, are also referredto as line art.

Four factors are important when scanning line art: sizingfactor between original and output formats; output resolution;sharpening; black-white threshold value. The examples givenbelow assume one-to-one sizing. Scanning resolutions must bemultiplied by a sizing factor if the output format differs fromthe original.

The resolution at which line art is scanned depends upon itsintended application. If it is to be converted into contours foruse in a drawing program, the highest resolution available onthe scanner could be used. When line art will be outputwithout conversion, the scanning resolution should match theoutput device resolution, unless this is higher than 1200 dpi. It can be seen from the output samples made at differentresolutions that there is minimal visible difference betweenscans made at 1200 ppi and 2400 ppi. There is, however, aconsiderable difference in file size, which makes handlingmore difficult and storage more costly. No benefit will begained by scanning an image at 1200 ppi instead of 300 ppiwhen output will be made with a 300-dpi laser printer. If themaximum resolution of a scanner is not high enough, agreyscale scan could be resampled, sharpened and thenconverted to line art in an image-editing program.

The conversion of grey tones to either black or white isdetermined by reference to a threshold value. Pixels that arelighter than the threshold value will be converted to white,and darker pixels will be changed to black. To retain details ina greyscale original, the threshold should be set at around themiddle of the tonal range present. Sharpening prior toconversion may also improve results. Different thresholdsettings for line-art scanning may allow line thicknesses to bevaried.

Image resolution: 2400 ppiFile size: 2130 Kb




Some scanners allow varying levels ofsharpening to be applied to an image,increasing the definition of details in darkand light areas, before conversion to pureblack and white. This is important whenedges or fine details are slightly blurred,since these may otherwise be lost.

The threshold setting determines how muchdetail is retained in light areas and whetheror not small highlights in shadow areas fill in.

Sharpening and threshold

Good highlight detail

Good shadow detail

Highlights too dense

Lacking shadow detail

Lacking highlight detail

Shadow detail too open

Scan res.* = Output device res. x Sizing factor

*1200 ppi is the upper scanning resolution limit (assuming no resizing),since the improvement gained by higher resolutions is insignificant.

Resolution rule

2120

GREYSCALE RESOLUTION

Screen ruling: 133 lpi / Image res.: 266 ppiFile size: 263 Kb



Stochastic screening technique Image res.: 300 ppi / File size: 273 Kb



To scan greyscale images for halftone reproduction, four itemsmust be considered: output screen ruling (frequency); sizingfactor between original and output formats; correct tonal range;sharpness. Output device resolution instead of screen ruling isimportant for contone printing.

When a greyscale image is converted to a halftone, grey pixelsare changed to black dots of varying size or number. In the casewhere scanning resolution is similar to screen ruling, pixelpositions may not always coincide with dot positions, causingincorrect dot densities to be chosen. If more than one pixel isavailable to define the dot density at any position, better resultsare obtained. Scanning resolution is therefore equal to outputresolution multiplied by a quality factor (also known ashalftoning factor). When scanning images to be converted intohalftones of above 133 lpi, a quality factor of 1.5 is required.Images containing geometrical subject matter, including straightlines or repeated patterns and textures, benefit from an increasedquality factor of 2. Halftones of 133 lpi and below need a qualityfactor of 2, because incorrect output dot densities will be morenoticeable at low screen rulings. Restricting the quality factor to a minimum keeps file sizes small whilst ensuring the bestpossible printed results. An image to be output using a 175 lpi at 75% of its original size should be scanned at 197 ppi (Scan res. = Screen ruling x Quality factor x Sizing factor = 175 lpi x 1.5 x 0.75 = 197 ppi).

Reflective contone printing devices give best results when imageresolution is the same as the device (after any resizing).Scanning resolutions required for output to film recorders arecalculated as described in the following “Colour resolution”section.

The full range of tones present in an original should be capturedin the scanned image, and these must be correctly distributedbetween black and white to ensure good contrast and brightness,whilst retaining maximum image detail. These aspects areexplained fully in the sections covering scanner density controls,histograms, tone curves, linear and non-linear tone corrections.Sharpening applied either during the scanning process or in animage-editing program will increase apparent image detail byheightening the contrast at object edges. Further information isgiven in “Sharpen your image”.

Resolution rule: Conventional halftone printing

Scan res. = Screen ruling x Quality factor (qf) x Sizing factor*qf = 2 if screen ruling ≤ 133 lpi qf ≥ 1.5 if screen ruling > 133 lpi

Resolution rule: Stochastic halftone printing

Scan res. = Comparable screen ruling x Quality factor (qf) x Sizing factor*For stochastic screening, a scanning resolution equal to a conventionalhalftone screen ruling gives comparable print quality.qf ≥ 1 for stochastic screening

Resolution rule: Contone paper output

Scan res. = Output device res. x Sizing factor*

*Sizing factor =Desired size

Original size

2322






The four criteria described for greyscale images apply also toscanning colour originals, but two additional items should beconsidered. Colour and grey balance vary from one image toanother, often requiring some modification. RGB scans needto be correctly converted to CMYK separations for halftoneoutput.

Just as for greyscale images, the scanning resolution forprinting four-colour halftones at above 133 lpi shouldemploy a quality factor of 1.5. Images containing geometricalsubject matter, including straight lines or repeated patternsand textures, benefit from an increased quality factor of 2.Halftones of 133 lpi and below need a quality factor of 2.Scanned RGB files require three times the storage spaceneeded for greyscale images and CMYK files use four times asmuch space. It is therefore even more important to keep thequality factor to the lowest effective level. Scanningresolutions must be multiplied by a sizing factor if the outputformat differs from the original.

Sharpening of both colour and greyscale images increasesapparent detail by heightening the contrast at object edges.Further information is given in “Sharpen your image”.

Colour and grey balance are affected by the automatic ormanual selection of the darkest and lightest neutral tones inan image, known as black and white point setting. Colourcorrections can be made either at scan time or later in animage-editing program. It is advisable to obtain the bestpossible result during the scanning process, since more data isavailable at this stage. If the scanned image does not containthe necessary information, it is very difficult to create it later.The sections dealing with scanner density controls,histograms, tone and colour corrections explain how toobtain a well-distributed tonal range with correct colourbalance.

Reflective contone printing devices give best results when images are scanned at identicalresolution, taking into account the sizing factor. The resolution of film recorders is typically quotedas being 2K, 4K, 8K or 16K. This measurement refers to the maximum number of addressable pixels that can be exposed on a film, regardless of its format. The number of pixels per inch must be calculated from this to indicate scanning resolution. For example, when a 4K recorder (4096 pixels) is used to produce a 35mm slide (defined as 1.5” x 1”), the resulting resolution is 2731 ppi (4096 / 1.5). A 4”x 5” slide produced on an 8K recorder (8192 pixels) requires a scan resolution of 1638 ppi (8192 / 5).

COLOUR RESOLUTION

Stochastic screening technique Image res.: 300 dpi / File size: 1060 Kb

Contone film output (digital photo-imaging)

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Resolution rule: Conventional halftone printing

Scan res. = Screen ruling x Quality factor (qf) x Sizing factor*qf = 2 if screen ruling ≤ 133 lpi qf ≥ 1.5 if screen ruling > 133 lpi

Resolution rule: Stochastic halftone printing

Scan res. = Reference screen ruling x Quality factor (qf) x Sizing factor*For stochastic screening, a scanning resolution equal to a conventionalhalftone screen ruling gives comparable print quality.qf ≥ 1 for stochastic screening

Resolution rule: Contone paper output


Resolution rule: Contone film output


Output device res. = Maximum addressable pixelsLongest side of output film

*Sizing factor =Desired size

Original size

24

HISTOGRAMS AND TONE CURVES

The histogram of an 8-bit greyscaleimage contains 256 vertical bars (0 to255), each representing a specific greylevel. Bar heights are proportional to thenumber of pixels per grey level. For RGBimages, a combined histogram indicatesoverall brightness but separate histogramsfor each primary colour may also beviewed.

The distribution of pixels in a histogram,especially at its extremities, provides aguide for tonal corrections. The top-leftscanned image is low in contrast, havingvirtually no pixels at the black (0) andwhite (255) ends of the histogram.Stretching the data out to fill thehistogram, as shown in the top-rightimage, increases the contrast but causesgaps to open up. This absence of pixels ina number of consecutive grey levelscreates posterisation or tonal banding,which may only become obvious whenadditional corrections are applied.

Using incorrect highlight and shadowsettings to scan an image that has a widertonal range will result in a histogramcontaining very high values at both ends.Shadow detail (a) in the middle-leftimage has been forced or clipped to blackand highlights (b) are clipped to white, asindicated by the circles.

Scanners that provide automatic densitycontrol create internal histograms after aprescan, from which they determinecorrect shadow and highlight settings.The final scan then captures the fulltonal range without posterisation orclipping.

An unevenly distributed histogram doesnot necessarily mean that the image isincorrect. The intentionally high-keyimage at the bottom-left contains fewshadows, as indicated by its histogram.Conversely, the histogram of the low-keyimage is weighted towards the dark end.Redistribution of these histograms woulddestroy the intended effect. The earlier“Judging your originals” section shouldhelp in assessing the need for corrections.

Histogram

Low contrast Posterisation

Clipping Correct tonal range

High-key image Low-key image

0 255

(a) (b)

A histogram shows the distribution of pixelsthroughout the tonal ranges of an image,

highlighting irregularities.

25Modifying histograms with tonecurvesOne method of redistributing tones in ahistogram is to use tone curves, whichallow smooth changes to be applied tospecific tonal ranges. A single curvemodifies overall brightness levels incolour images, whilst separate tone curveschange primary colours individually.

Normally, the horizontal axis of a tonecurve indicates the tonal ranges of animage prior to changes (input values) andthe vertical axis shows the effect of tonecorrections (output values).

Tone curve modifications cause thevertical line from a specific input value to cross the curve at a new point. A horizontal line from this point showswhat the output value will become.

The top-left image is lacking in shadowdetails, indicated by densely packedhistogram bars of similar height in thatarea. Drastically raising three-quarter andmidtones expands input shadows across awide range of output grey levels,amplifying subtle tonal variations (top-right image). This compresses inputhighlight and quarter tones, losing detailsin these areas. Excessive expansion of 8-bit data in an image-editing programcauses posterisation.

The bottom-left image has a moremoderate lift of three-quarter tones, witha slight drop of highlight quarter tones,improving shadow detail without undulysacrificing highlight detail or producingdisturbing posterisation. Automaticcorrection in some scanners applies thistype of curve.

The facility to download user-defined tonecurves to a scanner by means of its softwaredriver, allows tone corrections to be made onsuper-sampled data at greater than 8-bitdepth, avoiding posterisation and retainingsmooth tonal gradations in the final image(bottom-right).

Tone curve

Acceptable tone correction Improved tonal correction usingwith reduced posterisation scanner tone curve

No correction Excessive tone correction with posterisation

Shadows

Input values

Out

put v

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s

3/4 tones

Midtones

1/4 tones

HighlightsInput values prior to changes are shown onthe horizontal axis. Modified output valuesare read from the vertical axis. Dark and lightquarter tones are indicated (3/4 and 1/4)together with the midtone position (1/2).Some programs show reversed black andwhite positions. The 45° line leaves outputvalues unchanged. Any other curve causestonal changes, which are evident when inputand output greyscale wedges are compared.

26

LINEAR AND NON-LINEAR TONE CORRECTIONS

Linear tone correctionsSelectively modifying points in a tonecurve to affect some output ranges morethan others is known as non-linearcorrection. A more basic variety is linearcorrection where overall changes to imagebrightness and contrast are made bysimply changing the position of a straighttone curve.

Both linear and non-linear tonecorrections to prescanned files throwaway some grey level information in orderto expand or move other areas. Linearcorrections remove data less intelligentlythan non-linear ones, so they should beused with caution. Multiple tonalcorrections reduce information at eachstage. This is another reason for makingcorrections during the scanning process.

BrightnessDarkening an image causes the 45° lineartone curve to be shifted to the right, orwhite end of the input axis. Input shadowdetail is completely lost because valuesare clipped to black. No white or brighthighlights will be present in the imagenow, meaning that overall contrast hasbeen reduced.

Brightening the image moves the tonecurve to the left, or black end of theinput axis, sacrificing all highlight detailby clipping it to white. Black and darkshadows are totally removed, resulting ina smaller tonal range.

ContrastWhen the overall contrast of an image isincreased, its tone curve is rotated so thatthe midtone input is expanded to fill thecomplete output range. Any inputshadow detail is clipped to black andhighlights are clipped to white.Posterisation may be visible if the curve istoo steep.

Reducing overall contrast rotates thetone curve in the opposite direction,compressing the full input range into onlythe output midtones, whilst removinglight and dark tones.

Original image

Brightness adjustments

Contrast adjustments

Linear darkening of animage throws awayshadow details byclipping them to black.

Increasing brightnesslinearly removes highlightdetails by clipping them towhite.

Increasing contrastlinearly clips highlightdetails to white andshadow details to black.

Linear reduction of contrastcompresses the full inputrange into a smaller outputrange.

Highlights 1/4 tones

MidtonesShadows

3/4 tones

27Non-linear tone correctionsGamma correction is another term usedfor non-linear tone corrections. Derivedfrom the photographic industry, highgamma specified a film with highcontrast. Image-manipulation programsuse different methods to edit gamma ortone curves in a non-linear fashion. Someoffer a freehand drawing tool, which isdifficult to control precisely, althoughsmoothing may be possible. Others allowthe curve to be split into a number ofcontrol points, which can be moved tonew positions manually, or in some casesnumerically. Sliders may also be provided.

In colour images, tone corrections arenormally carried out before any colourcorrections are made. Tone curves cansometimes be saved for use with otherimages. The ability to download tonecurves to the scanner allows the extra,oversampled data to be utilised duringtone corrections that involve theexpansion of tonal ranges, such as shadowareas.

A dark original is improved by lifting thequarter tones and midtones, increasingshadow details whilst brightening theimage. Highlight details are virtuallyabsent, so compression of this area is nota problem.

Lowering the shadow three-quarter toneand raising the highlight quarter toneproduces an S-shaped curve that will givea low-contrast image more “snap”.Midtone contrast and detail is increased,whilst highlight and shadow details arecompressed but not lost entirely.

A high-contrast image containing fewmidtones may be improved by lifting theshadow quarter tone and dropping thehighlight quarter tone, expanding theseareas into the midtone output range.Compression of the few midtones presentis a worthwhile sacrifice.

The high shadow density oftransparencies is normally expanded bygamma correction to retain additionaldetail.

Original image Highlight/shadow detail

Original image Increased midtone contrast

Original image Increased shadow detail

Shadow detail is lacking in the originalimage. Raising midtones and quarter tones

emphasises decoration in the poorly litchurch dome.

The midtone area of interest is lacking in contrast. By lowering the shadow

three-quarter tone and raising the highlightquarter tone, details in the bone

are more pronounced.

Raising the shadow three-quarter tone andlowering the highlight quarter tone increases

detail at both ends of the tonal range.Patterning in the dark church door and

texture in the light walls become evident.

28 During a low-resolution prescan, someflatbed and drum scanners employautomatic density control to calculatespecific exposure settings for varyingdensity originals, prior to making the finalscan. The lightest tone (Dmin) anddarkest tone (Dmax) are automaticallylocated, indicating the density rangepresent. This automatic setting is good fororiginals with bright, neutral highlightsand dark shadows but some originals willbenefit from alternative light and darkpoints being selected elsewhere in theimage. The manual selection of a newwhite point and black point can be used toretain the intentionally limited tonalranges of high-key and low-key originals,or to correct contrast and brightness levels.These modifications have a similar effectto the linear tone corrections described inthe previous section.

Automatic white point location will betoo dark in a low-key image, causing darkshadows to be lightened unrealistically. Ina high-key original, the automatic blackpoint will be too light, resulting in anunwanted darkening of the image. Whenimages contain a restricted tonal range, itmay be necessary to scan a greyscale tonewedge next to the image to enable whiteand black points to be set correctly. Somescanner interfaces include a tone wedge forthis purpose.

If an image contains direct light sources orspecular highlights such as reflectionsfrom metal or glass surfaces, these shouldremain as pure white. An automatic whitepoint will generally be placed incorrectlyin specular highlights, causing otherhighlights and midtones to be darkened.The aim should be to set a white point ina bright, neutral highlight where somedetail is still visible. Setting the whitepoint to light grey causes all lighter greysto be burnt out to white (clipping),washing out remaining tones in theprocess. Black point setting is less criticalthan the white point. The darkest neutraltone is selected where detail should stillbe visible. Any darker tones will then beclipped to pure black, removing all details.

On-screen densitometer

SCANNER DENSITY CONTROLS

Specular highlights

Incorrect automatic settings Manually-adjusted settings

Scanning artwork

Automatic location of the white point in specular highlights may cause other highlights andmidtones to be too dark. In this image, the white point has been moved to a highlight thatcontains unwanted tone. When this is pulled to white, the image is lightened, and specularhighlights are burnt out correctly.

Many interactive scanner interfaces provide an on-screen densitometer, allowing exact pixelcolour values to be displayed. A few provide CMYK values in addition to RGB, giving anindication of results after separation. These readings identify any colour casts, which may nototherwise be noticed. A neutral grey normally contains equal quantities of magenta and yellow,with a slightly higher percentage of cyan.

To ensure that correct colours are retained when reproducing artwork, a colour reference card isoften included in photographs, which can be measured during the printing stage. The pictureshown above has been scanned directly on a flatbed with a colour reference and tone wedgeplaced beside it. These enable white and black points to be set precisely, and colour casts to beavoided.

29

GLOBAL AND SELECTIVE COLOUR CORRECTIONS

Global colour correction Global colour correctionsUnreal colour casts in originals can also beremoved (or added for artistic reasons) bythe appropriate choice of white and blackpoints. To remove the overall magentacast from the lake photograph, the whitepoint has been repositioned in a magentahighlight. All colour present at theselected point is removed, pulling it towhite. This lightening process is applied indiminishing quantities towards the blackpoint, reducing the red and blue(magenta) values more than the green toeliminate the magenta cast. Tonalimbalance at the black point is less visible,so an on-screen densitometer may beprovided to view precise colour values. Ifthe black point is placed in a dark shadowwhere red and blue (magenta) arepredominant, green will undergo thegreatest darkening when all three coloursare pulled to black, reducing the magentacast in diminishing proportions as thewhite point is approached. The RGBcolour space has been used for thisdescription, but densitometer readings inCMYK could also be available.

In some cases, one or more additional greypoints may be indicated to provide greatercontrol over irregular colour casts. Anycolour imbalance at the chosen grey pointis averaged to a neutral tone and thischange blends with those introduced bysetting white and black points.

Selective colour correctionsThere are some occasions when it is usefulto apply local corrections to specific colourranges in an image, either to increaseimpact or to totally change colourrelationships. Numerous methods ofmaking selective colour corrections exist,the more complex ones involving maskingtechniques. A simple way is to select somepixels within the range to be changed, andthen to specify how many additionalcolours should be affected by setting thewidth of the range. These corrections arenormally carried out in the CMYK colourspace to observe the impact on printingprocesses.

The shirt of the saxophone player was violet in the original photograph. Most of the magenta hasbeen removed, and the cyan has been lightened to simulate blue denim.

Magenta has been added to the guitar so that it appears more orange, whilst the red shirts of thepeople in the back row have had yellow removed and magenta increased.

The magenta (red and blue) cast in this originalis removed by setting white, grey and blackpoints in areas where magenta is predominant.The tone curves show how red and blue arelightened in relation to green.

Red Green Blue

Selective colour correction: adding impact

Selective colour correction: changing emphasis

30

0

255

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9880

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981047480

SHARPEN YOUR IMAGE

Unsharp originals are given the impressionof being sharper by the application ofunsharp masking (USM). This processdoes not add detail but heightens thecontrast at edges of objects to make themmore visible.

Image-manipulation programs and mostmodern scanners carry out USM bymodifying scanned pixels completely insoftware, although a few drum scanners usea fourth PMT for this purpose. The USMtechnique is similar to traditionalphotographic methods in that it combinesthe unsharp image with an even moreblurred copy (mask) to produce thesharpening effect.

For each stage of the USM process, anenlarged view of the text on a ship’s hull isshown next to the original image. Greylevel charts indicate the changes thatoccur at the edge of one of the paintedcharacters. The first chart shows that thereis a tonal jump of 18 grey levels betweenthe background colour and the lightcharacters. This jump is unsharp because itis blurred across 2 intermediate pixels.

A copy of the unsharp image is furtherblurred by a defocusing filter such as aGaussian blur filter. Tonal changes atobject edges are now spread across 8 pixels(9 steps), as shown in the second chart.When the tonal values in the blurred copyare subtracted from those of the unsharporiginal, the result is a set of positive andnegative tonal values, represented by thegreen areas in the third chart. When thesegreen areas are rearranged along ahorizontal line, a sharpening mask (c) isproduced.

The final stage, shown in the fourth chart,is to add the mask values to the unsharporiginal. This produces two tonal blips orpeaks, increasing the original tonal jump.Observed closely, these peaks will appearas adjacent light and dark lines aroundedges of objects.

Greater control is provided by software-based USM than the photographicmethod. Peak width determines thenumber of modified pixels. Kernel size andradius are terms used for peak width.When the width is too great, disturbinghaloes are created around objects,modifying or destroying image details inthe process.

This chart shows the original step in greylevels between the edge of a light characterand the dark ship’s hull in the above images.

The vertical arrow (a) in the chart indicatesthe tonal jump, which needs to be larger thanthe threshold setting for USM to be applied.

The first step in USM is to make a copy ofthe original and to apply a blurring filter to it,causing the tonal jump to be spread acrossadditional pixels.

The horizontal arrow (b) shows the kernelsize, which determines the number of pixelsaffected by USM.

The mask (c) is in reality a series of positiveand negative tonal values obtained bysubtracting the blurred copy from the unsharporiginal. Negative tonal values cannot bevisualised, so the zero line has been shifted tothe midtone grey level (128) in order toillustrate the contents of the mask.

Positive and negative tonal values in themask (c) are added to, or subtracted from theoriginal image. The light and dark peaks thusformed exaggerate the original tonal jump,making it appear sharper.

(c)

(b)

(a)

(c) Grey levels

Grey levels

Grey levels

Grey levels

31The height of the peaks or amount oflightening and darkening is changed by astrength setting. When the strength is settoo high, tonal peaks reach the limits ofpure black and white, producing anartificial appearance.

USM is only applied when tonal jumps aregreater than the number of grey levelsspecified by a threshold setting. A highthreshold restricts USM to large jumps intone such as the white text on the ship’shull, preventing it from being applied toareas that should remain smoothgradations.

Most scanned images contain subtle tonalvariations in flat areas of colour. If USM isapplied to these variations, an unpleasanttexture called mottling occurs. Darkshadows sometimes contain a few isolatedlighter pixels, caused by noise duringscanning. USM emphasises these pixels,producing speckling. Mottling andspeckling are avoided by raising thethreshold setting. Some scanner interfacesallow USM to be switched off in shadowareas or in specific colours such as skintones.

The USM process exaggerates thestaircasing or aliasing along angled edges.This will only be apparent when the imageresolution is too low in relation to the finaloutput resolution.

Applying USM to images that containfine textures or patterns may produceunexpected results. These subject-relatedproblems are more difficult to control.

Descreening halftone originalsSharpening will definitely not improvescans of halftone originals because haloeswill occur around each dot. Conversely,they must be blurred by a software filter orby defocusing the scanner optics to avoidmoiré patterning and colour shifts duringoutput.

Unwanted effects of sharpening

Halo Mottling

Subject-related problems Speckling

Without descreening With descreening

Unsharpmasking: anotherviewpoint

The action of USM atobject edges is simply explained by a

3-D view of the combined grey level charts from theprevious page. The purple surface represents the original, unsharp

tonal step. This is blurred to produce the red surface. Tonal values lying between theoriginal purple and blurred red surfaces are flipped to the outside of the purple surface to createthe sharpened green surface. By this means, the light tones in the original have been lightenednear the object edges, and the dark tones have been darkened, increasing the overall tonal jump.

Haloes occur when kernel size is too great. Mottling and speckling are avoided by raising the USMthreshold setting. Sharpening fine textures or patterned contents may produce unacceptableresults. Screened originals may suffer from moiré patterning and colour shifts when reprinted.Defocusing during the scan, or blurring after the scan will help avoid this.

3332

0 0.1

0.1

0

0.2

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700-750

650630

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450460

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400-380

COLOUR DEFINITIONS

The ability to precisely measure anddefine colours is essential in thereproduction of images. All visiblecolours can be defined by the threefactors described below. Alternativeterms are shown in brackets.

Hue - the colour perceived when one or two of the three RGB colours of lightpredominate (colour).

Saturation - the extent to which one or two of the three RGB colourspredominate. As quantities of RGBequalise, colour becomes desaturatedtowards grey or white (chroma, purity,intensity, vividness).

Lightness - the strength or amplitude ofthe RGB wave forms activating the eyes’receptors (luminance, brightness, value,darkness).

Frequently associated terms for thesethree factors are HSV (hue, saturation,value), HSL (hue, saturation, lightness),and HVC (hue, value, chroma).

These characteristics can be illustrated bya three-dimensional model consisting ofstacked “disks”. Circular movementaround each disk varies the hue. Upwardsmovement from one disk to anotherincreases the lightness. Radial movementfrom the centre of each disk outwardsincreases saturation. The model isirregularly shaped because the eye is moresensitive to some colours than others.

The standard observerIn 1931 the “Commission Internationalede l’Eclairage” (CIE) precisely definedthree primary colours, or tristimulusvalues, called X (red), Y (green) and Z(blue) from which all other coloursvisible to a “standard” observer could becreated. More recently, the CIE Yxycolour model was introduced.

All colours having the same lightness lieon a roughly triangular flat plane. Thehorizontal x axis in the illustration of theCIE Yxy model shows the redness ofcolours and the vertical y axis indicatesthe amount of green in colours. The Yaxis representing the value or lightness ofcolours can only be shown in a 3-D viewof the CIE Yxy model, since it comes outof the page.

Pure wavelengths of light lie on thecurved edges of the triangular gamut ofvisible colours. The lower straight edgerepresents the colours obtained by mixingred and blue wavelengths from both endsof the spectrum. Although distancesbetween colours in this model do notcorrespond to perceived colourdifferences, it allows us to indicate therelative gamuts of RGB monitors, anddifferent sets of printing inks.

Inks in the Pantone Matching System(PMS) provide a much larger colourgamut than CMYK process inks, such asthe Standard Web Offset Press set(SWOP). A special fifth ink is sometimesused to extend the CMYK gamut.

The non-linear CIE Yxy colour model wasmathematically transformed in 1976 tothe uniform CIE L*a*b* model, in whichdistances between colours more closelymatch those perceived. All colours of thesame lightness lie on a circular flat plane,across which are the a* and b* axes.Positive a* values are reddish, negative a*values are greenish, positive b* values areyellowish and negative b* values arebluish. Lightness varies in the verticaldirection.

Samples from the CIE L*a*b* colourspace (more simply known as CIE LAB)are used to create industry standard IT8 transmissive and reflective referencecharts, against which the gamuts of inputand output devices can be compared and calibrated by use of the ColourManagement Systems (CMS) described in the next section. Spectrophotometersor colorimeters make precise colourmeasurements, normally quoting valuesin both CIE Yxy and CIE LAB colourmodels.

Pantone® Monitor SWOP-CMYK

3-D colour model

CIE Yxy model Colour gamuts

CIE L*a*b* model

HueVisible spectrum

Monitor SWOP-CMYK Pantone®

Lightness Saturation

IT8.7/2 colour reference

WhiteL*

Yellow+b*

Blue-b*

Green-a*

Red+a*

Black

CMS

34

COLOUR MANAGEMENT

Matching colours in printed output tothose in scanned originals is no simpletask, due to the number of variablefactors in the reproduction chain. Image-capturing devices output different valueswhen reading the same original.Adjustments to monitor controls causewide colour variations. Gamut differencesbetween monitors and printing processesmean that unprintable colours can beintroduced during image retouching. The conversion of scanned RGB data to CMYK separations differs from oneprogram to another. Proofing devices vary wildly in colour rendition, due topigment and substrate characteristics.Viewing proofs and printed matter under non-standard lighting conditionsintroduces errors of judgement. Ink-basedpress adjustments permit wide variationsin ink densities. Alternative ink sets andpaper types affect colour rendition. Papercoating and texture affect dot gain, whichmodifies colours.

Attempting to compensate for all thesecolour variations by trial and error is tooexpensive in time and materials. Colourmanagement systems (CMS) solve thecolour mismatch between input andoutput devices. These systems vary intheir method of application but ideally,the gamut of each device in the colourreproduction chain is related to astandard colour space such as CIE LAB.Variations from the chosen standard arerecorded in a device-specific tag orprofile. Future input or output from eachdevice is then matched by use of itsprofile, resulting in device independenceor portable colour.

The input characterisation processrequires industry standard colour referencetargets of transmissive (IT8.7/1) andreflective (IT8.7/2) varieties to be scannedusing normal settings. These referencetargets contain 264 patches of colour andneutral greys, representing the completegamuts for the media used to create them. The CMS relates scanned readings foreach patch to colorimetric readings of theIT8 reference, which have been measuredby a spectrophotometer.

Output device characterisation isachieved by printing an IT8.7/3 referencefile, which contains more colour patchesthan the IT8.7/2 used for input devices.

Colour mismatch between devices

Input and output devicesproduce unique colour gamuts.Lack of compensation for thesedifferences causes unreliableresults.

Scanned IT8 values related to reference IT8 values

provide a device-specific profile.

Measurements of printed IT8 values related to reference IT8 values create output device profiles.

Device-specific profile

Device-specific profile

Using the device-specific profiles,colour matching between alldevices in the colour chain isguaranteed by the underlyingCMS. Final prints reliably matchoriginal images.

Device characterisation

Matching colours throughout the colour chain

A

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A B C D E F1 2 3 4 5

1 2 3 4 5 6 7 8 9 10 11 12 13

1 2 3 4 5 6 7 8 9 10 11 12 13

12

34

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1 2 3 4

35

IT8 colour reference targets

The results are accurately read with aspectrophotometer or a colorimeter andare fed back into the CMS to createunique profiles. A range of profiles may bemade for a device that uses more thanone pigment or paper type. Varying levelsof dot gain on ink-based systems can alsobe profiled.

Having made colour profiles for scanners,proofers and printing devices, final resultsshould now reliably match originalimages, assuming that no colours arejudged and corrected on a monitordisplay. The final link in the chain isaccurate monitor calibration. Profiles forspecific brands of monitor may be createdand supplied as digital data by the CMSmanufacturer. These allow approximatecalibration but settings vary betweenmonitors. On-screen colours may bevisually matched to standard colourpatches, although some monitors nowinclude their own calibration sensor,providing automatic adjustment to matchCMS profiles. Monitor calibrationcompletes the chain, permitting reliableon-screen colour corrections to be carriedout.

The success of any CMS relies upon thecolour stability and correct calibration ofall devices in the reproduction chain.Keeping printing press results identical totheir measured reference targets is adifficult task, although modern ink-basedpresses are fairly stable once correctcolour balance for a specific job has beenachieved.

Due to increasing demand for low-cost,high-quality colour printing,manufacturers of computers, input andoutput devices are working together withsoftware developers to implement highly-beneficial colour management facilitiesthroughout the design and productionchain. Computer operating systems havealready been modified so that all residentcolour-aware programs can access acommon CMS.

Matching output on different devices

Skin tones and other frequently

occurring colours in nature

Midtones CMYKcolours

RGBcolours

Highlighttones

The industry standard IT8.7/3 reference target for outputcharacterisation

The industry standardIT8.7/2 referencetarget for inputcharacterisation

Original Thermal transfer process

Sublimation process CMYK offset reproduction

Shadow tones

High total inkamount (TIA)patches to checkink trapping

Saturatedcolour patcheswith no black

Saturated colourpatches with20% black

Solid CMY patchesto check density

CMYK densitywedges toestablish dotgain

Neutral greys printedwith CMY (and K insome cases) to checkgrey balance

Shadow tones

3736

FILE FORMATS AND FILE STORAGE

Image files may be stored in a wide variety offormats. The choice depends largely uponhow and where images will be used. Will theyneed to be imported into other programs forpage layout or image manipulation? Do theyneed to be saved in a compressed format toreduce storage space or transmission time viaa network or telephone lines? Would a loss inimage quality caused by high compressionfactors be noticed in the chosen outputmedium? The tables on this double-pagespread list the resolution requirements forvarious printing applications, together withalternative file formats and corresponding filesizes.

Native formats that are proprietary to oneprogram manufacturer may be extremelyefficient in their own environment but theyprovide little or no compatibility with othersystems.

A few image formats have become universallysupported by the graphics industry, due totheir versatility and openness. The TIFFformat is capable of describing bilevel,greyscale, RGB and CMYK images with morethan ten compression techniques available.This versatility has one main drawback.Programs designed to read TIFF files need tobe equally versatile to understand any datacontained within them, which isunfortunately not always the case.

The EPS format is more comprehensive thanTIFF, being able to describe both vector andimage data, together with page-layout details.Its greater complexity results in larger filesthan the TIFF format but image compressiontechniques are also available. EPS files areintended to be included in other PostScriptfiles so they normally contain a low-resolution, bit-mapped image for fast on-screen manipulation. Both RGB and CMYKcomposite EPS files exist.

In order reduce the quantity of CMYK dataloaded into page-layout programs, theDesktop Colour Separations (DCS) formatwas developed. Also known as EPS 5, thisconsists of four files containing full-resolutionCMYK data and an additional fifth masterfile, which contains a low-resolution previewimage. Only the master file is loaded intolayout programs to reduce memoryrequirements and increase operation speeds.At output time, the high resolution files areautomatically used in place of the master file.

The PICT 2 format, developed by Apple, also caters for vectorand bit-mapped data but it has little support on other platforms.

Numerous methods of file compression exist to reduce storageand transport problems. Compressed files need to bedecompressed before they can be edited, although this processmay be carried out automatically when a file is loaded.Compression techniques fall into two categories - lossy and non-lossy. Lossy compression means that data is permanentlyremoved during the compression process, causing a loss inquality. When coarse screen rulings and low-quality paper arebeing used this loss is not noticeable, allowing considerablereductions in file size. LZW compression is a non-lossytechnique, which is particularly effective when repeated patternsof pixels are present in an image. JPEG compression offers bothlossy and non-lossy versions. The lossy version is very efficient,requiring only one bit per pixel instead of eight to reproduce animage that is virtually indistinguishable from the original. A special utility may be needed in addition to standardprograms, to enable decompression of JPEG files.

File sizes and storageTraditional photographic reproduction methods require thestorage of films and plates for possible future use. Digital storagemethods take up less space and the data can be repeatedlyreproduced with identical quality.

In most cases, images will need to be transferred between systemsin order to be manipulated and eventually output. The use of alocal area network (LAN) to frequently transfer large image filesmay prove impractical because it will slow down other networkactivities. Modem transfers via a normal telephone line areimpractical unless files are heavily compressed, since the transferrate on a reasonably good line (9600 baud) would be about 15 minutes per megabyte (Mb). One solution to file transport is to install devices that use removable storage media on allsystems needing to share images.

For back-up purposes, tape drives are ideal because of their largemedia capacity (8 gigabytes in some cases) and relatively lowmedia cost. Access to files is slow, however, and they must beloaded onto a disk before they can be used. Removable harddisks of the Syquest variety allow images to be modified in situ.Access to data is not quite as fast as a fixed hard disk. These are available in a variety of capacities. Magneto-Optical Disks(MOD’s) are gaining popularity because of their robustness andlow cost. Small diameter disks contain 128 Mb, whilst the largerformat disks hold 650 Mb (325 per side), 1.3 Gb or 2 Gb. Accesstimes for MOD’s are currently slower than fixed hard disks. Newdata formats and capacities are being introduced, as technologyrapidly chagnes. Those mentioned within this section are just aselection of currently available media.

EPS saved with 1-bit preview

(1) There is minimal visible difference between scans made at 1200 ppi and the higher resolution of 2400 ppi.

(2) Stochastic screening quality is comparable with conventional screening when scanning resolutions matchconventional screen rulings. These do not exceed 300 lpi so a maximum scan resolution of 300 ppi gives excellentresults for stochastic screening (assuming no sizing factor).

(3) Contone output devices normally give best results when scanning resolution is the same as output deviceresolution. This example uses a 300-dpi dye sublimation printer.

100 2

133 2

150 1.5

175 1.5

Stochastic screening(2)

300(3)

1600(4)

Newspaper

Magazine

Magazine

Art magazine

Art magazine

Paper print (proofing)

Film positive (second original)

200 5818 2821 7989 5324 1677 208

266 10283 4739 13943 9172 2951 351

225 7358 3491 10043 6669 2087 260

263 10056 4622 13637 8996 2776 345

300 13085 6097 17674 11525 3985 436

300 13085 6097 17674 11525 3985 436

1600 146500 27510 198778 131154 6737 2228

dpiContone output

ppi Size in Kb

Size in Kb

Image TIFF TIFF EPS PICT JPEG JPEGresolution Normal Compr. Composite Composite Non-lossy Lossy

Screen Qualityfrequency factor

Original: 16 x 20 cmOutput: as above

Reproduction quality

EPS saved with 8-bit preview in binary format

lpi qfHalftone

(4) Both input and output sizes are 4"x 5" (for this example only). The “Colour resolution”section (“contone film output”) explains how the resolution is calculated.

Image quality: excellent

Compression: fair

Image quality: good

Compression: good

Image quality: good

Compression: good


Resolutionrequirements


300 560 446 594 513 n/a n/a

600 2195 842 2228 1749 n/a n/a

1200 8729 2129 8762 5825 n/a n/a

2400 35726 5965 37924 32586 n/a n/a

Laserprint

Laserprint

Print

Print

dpi ppi Size in Kb

300

600

1200(1)

2400(1)


lpi qf Size in Kb


170 1403 842 1607 1353 462 231

266 3515 1636 3722 3303 835 364

225 2516 1171 2664 2364 598 260

263 3437 1599 3639 3230 816 356

300 4373 1832 4637 3993 957 463

Newspaper

Magazine

Magazine

Art magazine

Art magazine

85 2

133 2

150 1.5

175 1.5


ppi




EPS saved with 8-bit preview

200 7755 4550 9000 2581 429

266 13715 7599 14800 4433 712

225 9815 5610 11000 3179 520

263 13410 7300 14500 4121 676

300 17440 9393 18400 5623 865

300 17440 9393 18400 5623 865

Newspaper

Magazine

Magazine

Art magazine

Art magazine

Paper print (proofing)

100 2

133 2

150 1.5

175 1.5


300(3)

dpi

DCS saved in binary encoding, with 8-bit (72-dpi)preview in the separate CMYK master file

ppi Size in Kb

Image TIFF TIFF DCS JPEG JPEGresolution Normal Compr. Non-lossy Lossy



lpi qfHalftone

Contone output


Halftone

CMYK colour

RGB colour

Greyscale

Line art

ppi

ppi

Size in Kb


Compression: fair

Image quality: good

Compression: good


Compression: fair

Image quality: good

Compression: good


Compression: fair

38

GLOSSARY

A/D converterA device used to convert analoguedata to digital data. Analogue data iscontinuously variable, whilst digitaldata contains discrete steps.

additive primariesRed, green and blue are the primarycolours of light from which all othercolours can be made.

aliasingVisibly jagged steps along angledlines or object edges, due to sharptonal contrasts between pixels.

analogueContinuously variable signals or data.

batch scanningSequential scanning of multipleoriginals using previously-defined,unique settings for each.

baudBits per second. A measurement usedin data transfers via telephone lines.

bilevelA type of image containing onlyblack and white pixels.

binary number systemA counting system used in computersconsisting of only 1’s and 0’s.

bitBinary digit. The smallest unit ofinformation in a computer, a 1 or a 0.It can define two conditions (on oroff).

bit depthThe number of bits used to representeach pixel in an image, determiningits colour or tonal range.

bitmapA digitised image that is mapped intoa grid of pixels. The colour of eachpixel is defined by a specific numberof bits.

black pointA movable reference point thatdefines the darkest area in an image,causing all other areas to be adjustedaccordingly.

byteA measurement unit equal to 8 bits ofdigital information. The standardmeasurement unit of file size. See alsokilobyte, megabyte and gigabyte.

CCDCharge-coupled device. An integrated, micro-electronic light sensing device built into someimage-capturing devices.

CIEThe “Commission Internationale del’Eclairage”. An organisation that hasestablished a number of widely-usedcolour definitions.

clippingThe conversion of all tones lighterthan a specified grey level to white, ordarker than a specified grey level toblack, causing loss of detail. This alsoapplies to individual channels in acolour image.

CMSColour management system. Thisensures colour uniformity across inputand output devices so that finalprinted results match originals. Thecharacteristics or profiles of devicesare normally established by referenceto standard IT8 colour targets.

CMYKCyan, magenta, yellow and black arethe base colours used in printingprocesses. CMY are the primarycolorants of the subtractive colourmodel.

colorimeterA light-sensitive device for measuringcolours by filtering their red, greenand blue components, as in thehuman eye. See also spectrophoto-meter.

colour castAn overall colour imbalance in animage, as if viewed through acoloured filter.

compressionThe reduction in size of an image file.See also lossy and non-lossy.

contone (CT)An abbreviation for continuoustone. A colour or greyscale imageformat capable of illustratingcontinuously varying tonal ranges, asopposed to line art.

DCSDesktop colour separation. An imageformat consisting of four separateCMYK PostScript files at full-resolution, together with a fifth EPSmaster for placement in documents.

decompressionThe expansion of compressed imagefiles. See also lossy and non-lossy.

densitometerA measuring instrument that registersthe density of transparant or reflectivematerials. Colours are read as tonalinformation. See also colorimeterand spectrophotometer.

densityThe degree of opacity of a lightabsorbing filter, pigment or exposedphotographic emulsion.

descreeningRemoval of halftone dot patternsduring or after scanning printedmatter by defocusing the image. This avoids moiré patterning andcolour shifts during subsequenthalftone reprinting.

dichroic mirrorA special type of interference filter,wich reflects a specific part of thespectrum, whilst transmitting therest. Used in scanners to split a beamof light into RGB components.

digitalData or voltages consisting of discretesteps or levels, as opposed tocontinuously variable analogue data.

direct-to-plateDirect exposure of image data ontoprinting plates, without theintermediate use of film.

direct-to-pressElimination of intermediate film andprinting plates by the direct transferof image data to printing cylinders inthe press.

DmaxThe point of maximum density in animage or original.

DminThe point of minimum density in animage or original.

down-samplingThe reduction in resolution of animage, necessitating a loss in detail.

dpiDots per inch. A measurement ofoutput device resolution. See also lpi.

drum scanner (and recorder)An image scanning device in whichoriginals are attached to a rotatingdrum. Early drum scanners separatedscans into CMYK data, recordingthese directly onto film held on asecond rotating drum.

dye sublimationA printing process using smallheating elements to evaporatepigments from a carrier film,depositing these smoothly onto asubstrate.

EPSEncapsulated PostScript. A standardformat for a drawing, image orcomplete page layout, allowing it to be placed into other documents. EPS files normally include a lowresolution screen preview.

EPS 5Another term used for DCS.

film recorderUsed in reference to colourtransparency recording devices, andsometimes also to imagesetters.

flatbed scannerAny scanning device thatincorporates a flat transparent plate,on which original images are placedfor scanning. The scanning process islinear rather than rotational.

FPOFor Position Only. A low resolutionimage placed in a document toindicate where the final version is tobe positioned.

frame-grabbing systemA combination of hardware andsoftware, designed to captureindividual frames from video clips forfurther digital manipulation, orconsecutive replay on computerplatforms.

gamma correctionThe correction of tonal ranges in animage, normally by the adjustment oftone curves.

gamutThe limited range of colours providedby a specific input device, outputdevice, or pigment set.

gang scanningSequential scanning of multipleoriginals using the same previously-defined exposure setting for each.

gigabyte (Gb)1,024 megabytes, or 1,048,576kilobytes of digital data.

grey balanceThe balance between CMY colorantsrequired to produce neutral greyswithout a colour cast.

grey levelsDiscrete tonal steps in a continuoustone image, inherent to digital data.Most CT images will contain 256 grey levels per colour.

greyscaleA continuous tone image comprisingblack, white and grey data only.

halftoneAn simulation of continuous tones bythe use of black or overlappingprocess colour dots of varying size orposition.

halftoning factorSee quality factor.

haloA light line around object edges in animage, produced by the USM(sharpening) technique.

high keyA light image that is intentionallylacking in shadow detail.

highlightThe lightest tones in an image. A spectral highlight is a bright,reflected light source.

histogramA chart displaying the tonal rangespresent in an image as a series ofvertical bars.

39hueThe colour of an object perceived bythe eye due to the fact that a single orpair of RGB primary colourspredominates.

imagesetterA device used to record digital data(images and text) onto monochromefilm or offset litho printing plates bymeans of a single or multipleintermittent light beams. Colourseparated data is recorded as a seriesof slightly overlapping spots toproduce either solid areas of line-artor halftone dots for printingcontinuous tones.

interpolationIn the image manipulation context,this is the increase of imageresolution by the addition of newpixels throughout the image, thecolours of which are based onneighbouring pixels.

IT8Industry standard colour referencetarget used to calibrate input andoutput devices.

jaggiesSee aliasing.

JPEGJoint Photographic Experts Group.An organisation that has definedvarious file compression techniques.

kernel sizeThe number of pixels sampled as aunit during image manipulation andsharpening processes.

kilobyte (Kb) 1,024 bytes of digital data.

LANLocal Area Network. A wire oroptical fibre link between computersinstalled on a single site for datatransfers.

laser printerAlthough a number of devicesemploy laser technology to printimages, this normally refers to black-and-white desktop printers, whichuse the dry toner, xerographicprinting process.

line artImages containing only black andwhite pixels. Also known as bilevelimages. The term line art issometimes used to describe drawingscontaining flat colours without tonalvariation.

lossyImage compression that functions byremoving minor tonal and/or colourvariations, causing visible loss ofdetail at high compression ratios.

low keyA dark image that is intentionallylacking in highlight detail.

lpi/lpcmLines per inch or per centimetre.Units of measurement for screenruling.

LZWThe Lempel-Ziv-Welch imagecompression technique.

matrixThis often refers to a 2-dimensionalarray of CCD elements.

megabyte (Mb)1,024 kilobytes or 1,048,576 bytes ofdigital data.

midtoneThe middle range of tones in animage.

modemModulator/demodulator. A devicerequired to convert digital computerdata into modulated analogue datafor transfer via non-digital telephonelines.

moiréA repetitive interference patterncaused by overlapping symmetricalgrids of dots or lines having differingpitch or angle.

monochromeSingle-coloured. An image ormedium displaying only black-and-white or greyscale information.Greyscale information displayed inone colour is also monochrome.

mottlingA texture similar to orange peelsometimes caused by sharpening. It isparticularly visible in flat areas suchas sky or skin.

noiseIn the scanning context, this refers torandom, incorrectly read pixel values,normally due to electricalinterference or device instability.

non-lossyImage compression without loss ofquality.

OCROptical Character Recognition. Theanalysis of scanned data to recognisecharacters so that these can beconverted into editable text.

offset lithographyA high-volume, ink-based printingprocess, in which ink adhering toimage areas of a lithographic plate istransferred (offset) to a blanketcylinder before being applied to paperor other substrate.

optical resolutionIn the scanning context, this refers tothe number of truly separate readingstaken from an original within a givendistance, as opposed to thesubsequent increase in resolution (but not detail) created by softwareinterpolation.

PICT/PICT 2A common format for definingimages and drawings on theMacintosh platform. PICT 2 supports24-bit colour.

pixelPicture element. Digital images arecomposed of touching pixels, eachhaving a specific colour or tone. Theeye merges differently coloured pixelsinto continuous tones.

pixel skippingA means of reducing image resolutionby simply deleting pixels throughoutthe image.

PMTPhotomultiplier tube. The lightsensing device generally used in drumscanners.

posterisationThe conversion of continuous tonedata into a series of visible tonal stepsor bands.

ppi/ppcmPixels per inch or pixels percentimetre. Units of measurementfor scanned images.

primary colourA base colour that is used to composeother colours.

process ink coloursCMYK pigments used in printingprocesses, chosen to produce thewidest range of colour mixtures.

profileThe colour characteristics of an inputor output device, used by a CMS toensure colour fidelity.

quality factorA multiplication factor (between 1 and 2) applied to output screenruling to calculate scanningresolution for optimum outputquality. This is also known as thehalftoning factor.

quarter tonesTones between shadow and midtonesare known as 3/4 tones and thosebetween highlight and midtones areknown as 1/4 tones.

rasterA synonym for grid. Sometimes usedto refer to the grid of addressablepositions in an output device.

relRecorder element. The minimumdistance between two recorded points(spots) in an imagesetter.

resA term used to define imageresolution instead of ppi. Res 12indicates 12 pixels per millimetre.

resamplingAn increase or reduction in thenumber of pixels in an image,required to change its resolutionwithout altering its size. See alsodown-sampling and interpolation.

RGBRed, green and blue are the primarycolours of light perceived by the eye.

rpiRels (recorder elements) per inch. A measurement of the number ofdiscrete steps that exposure units inimagesetting devices can make perinch.

samplingThe process of converting analoguedata into digital data by taking aseries of samples or readings at equaltime intervals.

saturationThe extent to which one or two ofthe three RGB primariespredominate in a colour. Asquantities of RGB equalise, colourbecomes desaturated towards grey orwhite.

screen frequencyThe number of rows or lines of dotsin a halftone image within a givendistance, normally stated in lines perinch (lpi)or lines per centimetre(lpcm). A frequency of 200 lpi wouldonly be used in high-quality printing.

screen rulingAnother term used for screenfrequency.

second originalHigh-quality, contone reproductionof an image, intended to be identicalto the original.

secondary colourColour obtained by mixing twoprimary colours. Although known asprimary colorants, C, M and Y arethe secondary colours of light. Redplus green produce yellow forexample.

shadowThe darkest area of an image.

specklingIsolated light pixels in predominantlydark image areas, sometimes causedby incorrect readings or noise in thescanning device.

40 spectral highlightA bright reflection from a light sourcecontaining little or no detail.

spectrophotometerAn extremely accurate colourmeasurement device using adiffraction grating to split light intoits component wavelengths, whichare then measured by numerous lightsensors.

staircasingSee aliasing.

substrateThe base material used to carry orsupport an image, for example paperor film.

subtractive primariesAnother term for primary colorants.

supersamplingThe capture of more grey levels percolour than is required for imagemanipulation or output. Thisadditional data allows shadow detailsto be heightened, for example.

tagSee profile.

thresholdThe point at which an action beginsor changes. The threshold settingused in scanning line art determineswhich pixels are converted to blackand which will become white. The threshold defined in the USMprocess determines how large a tonalcontrast must be before sharpeningwill be applied to it.

thermal wax transferA printing process using smallheating elements to melt dots of wax pigment on a carrier film, which are then transferred to paper or transparent film by contact. This differs from the dye sublimationprocess in that individual dots do notfuse together, so thermal wax transferappears to be of a lower resolution.

TIFFTag Image File Format. A popularimage file format supported by themajority of image-editing programsrunning on a variety of computerplatforms.

tone curvesAlso known as gamma curves. Theseare used to smoothly adjust theoverall tonal range of an image, orthe individual tonal ranges of eachcolour channel.

USMUnsharp masking. A process used tosharpen images.

white pointA movable reference point thatdefines the lightest area in an image,causing all other areas to be adjustedaccordingly.

x < yx is less than y.

x ≤ yx is less than or equal to y.

x > yx is greater than y.

x ≥ yx is greater than or equal to y.

41

OTHER EDUCATIONAL AND REFERENCE PUBLICATIONS FROM AGFA

An Introduction to DigitalColour Prepress

A fundamental reference foranyone interested in PostScriptcolour. Basic concepts areexplained in a clear, objective, andhighly visual way. Now in anupdated fifth edition, with over300000 copies in print in eightlanguages. Also available as a scriptedslide presentation (English only).

Digital Colour Prepress –volume two

The essential complement to “AnIntroduction to Digital Colour Prepress”. Thisbooklet provides a more advanced look at thetopic of PostScript colour, with a specialemphasis on reproducing colour pages in print.Now in print worldwide in eight languages.Also available as a scripted slide presentation(English only).

Working With Prepressand Printing SuppliersDigital Colour Prepress – volume three

The latest volume in the acclaimed “DigitalColour Prepress” series (Spring, 1994), thisbooklet clearly explains key elements in theworking relationship between documentcreators and their most important serviceproviders. Contains useful time-saving tips tohelp ensure successful transition of projectsfrom design to film output to final print.

Halftoning Software

PostScript Imagesetters

Colour Management Systems

Photographic Prepress Systems

FOR PRICING AND ORDERING INFORMATION, CONTACT:

North America, Agfa Prepress Education Resources, P.O. Box 7917, Mt. Prospect, IL, 60056-7917, Tel.: 1-800-395-7007, Fax: 1-708-296-4805.United Kingdom, Agfa Prepress Education Resources, P.O. Box 200, Stephenson Road, Swindon, Wilts, SN2 5AN, Tel.: (0793) 707099, Fax: (0793) 705745.

In other countries, contact your local Agfa subsidiary.

Agfa also offers a complete range of electronic and photographicprepress solutions. For more information on these and other products,contact your local Agfa dealer or sales office, listed on the back of thispublication.

PostScript Process ColourGuide

This 52-page reference contains over17000 electronically created CMYKprocess colour combinations (on coatedand uncoated stock), intended to helppredict how colours on the screen willlook in print. Also includes productiontips, instructions for use, and specialcolour viewing templates. Available inU.S. (SWOP) and a multilingualEuropean print standard version.

AGFA and Agfa rhombus areregistered trademarks and AgfaCristalRaster is a trademark ofAgfa-Gevaert AG, Germany.FotoLook and SelectSet aretrademarks of Agfa-Gevaert N.V.,Belgium. Adobe, Photoshop,Illustrator and PostScript aretrademarks of Adobe Systems,Inc. which may be registered incertain jurisdictions. QuarkXPressis a registered trademark ofQuark, Inc. Pantone and PMS areregistered trademarks of Pantone,Inc., for colour reproduction andcolour reproduction materials.Apple and QuickDraw areregistered trademarks of AppleComputer, Inc. All trademarkshave been used in an editorialfashion with no intention ofinfringement.

Scanners

Film Recording Systems

PostScript Type on CD-ROM

PostScript Raster Image Processors

The complete picture.

Credits

Project managementMarc Pollaris, Jan Tas, Martine Vandezande(Agfa-Gevaert N.V.)

Technical directionJan Tas, Rudy Van Hoey (Agfa-Gevaert N.V.)Patrick Gypen(Image Building bvba, Antwerp, Belgium)

Art direction, design,illustration and prepress:Patrick Gypen, Bart Van Put, Jean Oppalfens (Image Building bvba, Antwerp, Belgium)

ScanningJan Tas, Mireille De Baere (Agfa-Gevaert N.V.)

Copywriting and glossaryTangent Design sc, Meerbeek, Belgium

PhotographyRichard Cox Roger DijckmansKarel FonteynePatrick Gypen (all above: Antwerp, Belgium)Korff & Van Mierlo, Eindhoven,The Netherlands

Water-colour paintingEver Meulen

Special thanksPaul De Keyser, Gaby Herken,Eugene Hunt, Dirk Kennis, Viviane Michels, Koen Van de Poel,Kris Vangeel, Paul Vinck

This publication copyright © 1994by Agfa-Gevaert N.V. All rights reserved.

No portion of this book may bereproduced in any form withoutexpressed written permission fromthe publisher.

Production notes

The images in this booklet werescanned on Agfa CCD scanners,using the FotoLook driver softwarefrom within Adobe Photoshop.Images were manipulated in AdobePhotoshop, and placed intoQuarkXPress as EPS/DCS files.Illustrations were drawn usingAdobe Illustrator, saved as EPS filesand placed into the sameQuarkXPress document. All pageswere output as film positives at150 and 175 lpi or with AgfaCristalRaster stochastic screeningon an Agfa SelectSet 7000PostScript imagesetter.

Agfa CristalRaster™ screening wasused for the following images: cover; p. 11 (all images); p. 16 (all"original" images except line art);p. 17 ("Contone" images); pp. 20-21 (background image and stochastic screening image); pp. 22-23 (background image andstochastic screening image); p. 35 ("Original"); All other imagesand line art were output using AgfaBalanced Screening Technology.The book was printed on Profistarpaper with CMYK colours plusPMS 421 and varnish.

Printed in Belgium (577/EM)Published by Agfa-Gevaert N.V., B-2640 Mortsel-BelgiumNCUAV GB 00 1994 04

Agfa-Gevaert N.V.Septestraat 27B-2640 Mortsel

Agfa subsidiaries English literatureArgentina, Agfa-Gevaert S.A., Tel.: 54-1-981-0200, Fax: 54-1-953-4304Australia, Agfa-Gevaert Ltd., Tel.: 613-264-7711, Fax: 613-264-7890.Denmark, Agfa-Gevaert A/S, Grafiske Systemer, Tel.: 45-43-96-6766, Fax: 45-43-96-3955.Finland, Oy Agfa-Gevaert Ab, Graafiset järjestelmät, Tel.: 358-0-88781, Fax: 358-0-8878278.Greece, Agfa-Gevaert A.E.B.E., Tel.: 30-1-53333-200208, Fax: 30-1-574-4900.Hong Kong, Agfa-Gevaert (H.K.) Ltd., Tel.: 852-5-55-9421, Fax: 852-5-55-2480.Ireland, Agfa-Gevaert Ltd., Tel.: 353-1-506733, Fax: 353-1-519613.Japan, Agfa-Gevaert Ltd., Tel.: 81-3-5704-3072, Fax: 81-3-5704-3083.New Zealand, Agfa Division, Tel.: 649-443-5500, Fax: 649-443-5487.Norway, Agfa-Gevaert A.S., Grafiske Systemer, Tel.: 47-2-76-8941, Fax: 47-2-76-0753.Portugal, Agfa-Gevaert Lta, Tel.: 351-1-419-5558, Fax: 351-1-419-8165.Singapore, Agfa Division, Tel.: 65-261-3389, Fax: 65-266-4866.South Africa, Agfa Division, Tel.: 011-921-5563, Fax: 011-921-5548.South Korea, Agfa Korea Ltd., Tel.: 82-2-275-7181, Fax: 82-2-275-7187.Sweden, Agfa-Gevaert AB, Tel.: 46-8-793-0100, Fax: 46-8-793-0171.Taiwan, Agfa Division, Tel.: 886-2-503-9123, Fax: 886-2-504-4819.U.K., Agfa-Gevaert Ltd., Business Group Graphic Systems, Tel.: 081-560-2131, Fax: 081-234-4957.U.S.A., Agfa Division, Miles Inc., Graphic Systems, Tel.: 1-800-685-4271, Fax: 1-508-658-4193.Literature in other languages Austria, Agfa-Gevaert GmbH, Tel.: 43-1-89112.0, Fax: 43-1-89112204.Belgium, Agfa-Gevaert N.V., Verkooporganisatie Benelux, Tel.: 32-3-4509711, Fax: 32-3-4509898.Canada, Agfa Division, Miles Inc., Tel.: 1-416-241-1110, Fax: 1-416-241-5409.Chile, Agfa-Gevaert Ltda., Tel.: 56-2-2383711, Fax: 56-2-2384507.France, Agfa-Gevaert S.A., Tel.: 33-1-40-99-7991, Fax: 33-1-40-99-7990.Germany, Agfa-Gevaert AG, Grafische Systeme, Tel.: 49-221-57170, Fax: 49-221-5717130. Italy, Agfa-Gevaert S.p.A., Tel.: 39-2-30741, Fax: 39-2-3074429.Mexico, Agfa Division, Tel.: 52-5-250-2055, Fax: 52-5-203-95227.Netherlands, Agfa-Gevaert B.V., Tel.: 31-70-3-110591, Fax: 31-70-3-903175.Spain, Agfa-Gevaert S.A., Tel.: 34-3-207-5411, Fax: 34-3-458-2503.Switzerland, Agfa-Gevaert AG/SA, Tel.: 41-1-823-7111, Fax: 41-1-823-7376.Venezuela, Agfa-Gevaert S.A., Tel.: 58-2-238-2922, Fax: 58-2-239-0477.

Documents

An Introduction to Digital Scanning