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Printers

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Types of Printers

• Impact– Dot matrix

• Inkjet– Bubble jet– Piezoelectric

• Dye-sublimation

• Laser

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Resolution

• Printer resolution is generally reported in dots per inch (dpi).

• The horizontal and vertical dpi do not have to be the same.

• If the horizontal and vertical dpi are 300, then there are 300300=90,000 dots per square inch.

• Dpi can be misleading – especially when comparing different types of printers

• The dot’s size, shape, and placement as well as the toner or ink’s characteristics can affect print quality.

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Metric

• 600 dpi

• 1 inch = 2.54 centimeters = 0.0254 meters

• 0.0254 m / 600 = 0.0000423 meters per dot (the diameter) – 0.0423 millimeters– 42.3 micrometers or microns

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Resolution Enhancement

• Resolution Enhancement (Technology) REt is a set of techniques that can make a printer appear to have a higher resolution that it does. – One technique will allow for variable sized

dots. The decision will be based on the surrounding dots. The idea is to give a shape a sharper (less pixilated) edge.

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Two HP printer specs

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Speed

• There are various issues in measuring printing speed. – Normal mode versus draft mode (resolution)– Black speed versus color speed– Text versus graphics

• Character printing (cps: characters per second) versus page printing (ppm: pages per minute)

– First page versus bulk printing

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Speed

• Different companies test in different ways. So comparisons between companies based on their published specs is difficult. – Some test from applications.

– Some test on pre-rasterized files.

– Some start the clock when the paper feeder starts.

• Article on printer speed– http://www.pcworld.com/news/article.asp?aid=16386

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“Raster” definition

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Cost

• Another factor to consider is, of course, cost. There is not only the cost of the printer itself but the cost of printing – measured in cents per page. – Some printing requires special paper, so paper

costs might need to be factored in as well. – Some companies will lose money on the sale of

a printer, knowing that they will recover the loss and then some on the ink cartridges.

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Impact

• Impact printers are so named because they make physical contact with the surface on which they are printing.

• The standard impact printer is the dot matrix printer. – Striking pins force a ribbon into contact with page– Quality varies with number of pins: 9 - 24 – Often “legacy” printer– Good if there are forms with carbon papers for

duplicate or triplicate copies.

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Dot Matrix Specscpi: characters per inch

cps: characters per second

ips: inches per second

mtbf: mean time between failures

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Ink Jet• An ink jet printer sprays ink through nozzles onto

the page. – In this context ink refers to a liquid as opposed to toner

which is a powder.

• There are two standard methods for spraying: – Thermal bubble used by Canon and Hewlett Packard– Piezoelectric used by Epson

• They use different inks – the thermal bubble ink will be heated and requires different thermal properties. – If you are refilling cartridges, make sure it is the correct

type of ink.

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Thermal Bubble Jet

Heating causes a bubble to form, that pushes ink out of the nozzle. When the bubble bursts more ink is pulled in from the reservoir.

Bubble

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“Piezoelectricity” definition

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Piezoelectric Jet

Piezoelectric means that there’s a relationship between a substance’s mechanical properties (size, pressure, etc.) and its electrical properties (charge, current, etc.). The electric signal causes the crystal to swell,

pushing liquid from the nozzle.

Swelling crystal

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Print Head

• The print head houses hundreds of nozzles for spraying the ink.

• In some printers, this comes with the cartridges and is disposable.

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Stepper Motor

• A stepper motor, belt and stabilizer bar allow the print head to be placed at a precise horizontal position.

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“Stepper Motor” definition

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Rollers

• Another stepper motor controls the rollers move the paper from the paper tray or feeder to print head, giving control over vertical positioning.

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And the rest

• A printer also has a power supply, a circuit board to house memory and control circuitry.

• The data comes through a port – typically a parallel port or a USB.

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Ink Jet Printing Steps

1. Application sends data to printer driver.2. Driver converts data to form printer

understands and establishes a connection to the printer.

3. Data is sent. 4. Received data is placed in printer’s buffer.5. (May check that nozzles are clean and

ready.)

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Ink Jet Printing Steps (Cont.)

6. Check for paper and start feed. 7. Stepper motor positions print head. 8. Ink is sprayed. This is multiple steps in color

printing – the cyan, magenta, yellow and black inks are applied.

9. Another stepper motor moves the rollers –positioning it for the next row of printing.

10. Park the print head and move the paper to the output tray.

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The Quality of Paper

• While the ink dries quickly, there can be some spreading (“feathering”) before it dries. Having a high nozzle resolution is meaningless if the ink spreads significantly. High quality ink jet paper has a waxy finish or coating that keeps the ink from spreading.

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The Quality of Paper (Cont.)

• Images on paper are formed by reflection. • White light (a mixture of all colors) hits the

paper, and in a given spot certain colors are absorbed and others reflected.

• Light can be reflected “specularly” or “diffusely” depending on the smoothness or roughness of the paper.

• This is referred to as the paper’s brightness.

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Paper Brightness

Brightness number

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“Specular” definition

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“Diffuse” definition

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Laser Printer

• There are many misconceptions about laser printing. – Some think that “laser” implies something

about the speed of the printer. – Some think that the laser burns the image into

the paper. – The laser is used mainly because it is easy to

collimate as opposed to the laser’s intensity which need not be that great.

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“Laser” definition

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Photoconductors

• Actually the key is not laser light but the photoconductivity.

• In a semiconductor, electrons are not free to move around (conduct) unless they are “promoted” (given enough energy to move) from the valence band to the conduction band.

• The valence band and conduction band are separated by the gap.

• In a photoconductor an electron can get the required energy from light.

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“Photoconductivity” definition

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Laser printing

• A drum is made from a photoconductor.

• The primary corona wire places positive charge on the drum.

• It’s dark so the charges can go anywhere (the drum is a poor conductor in the dark)

Corona wire

Note: two of my resources disagree on what is positive and negative.

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Laser Printing (Cont.)

• The laser is directed onto the drum, scanning across, turning on and off.

• Places where the laser light hit can conduct and the positive charge leaves the drum surface.

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Laser Printing (Cont.)

• Positively charged toner is applied to the drum.

• It is repelled by parts of the drum that still have a positive charge (like charges repel) but is attracted to places where the laser has shone.

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Toner is plastic and pigment.

Pigment to add color to the page.

Plastic to melt into the page and hold the pigment in place.

Some of the print head (parts that are more likely to wear out) comes along with the toner cartridge.

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Laser Printing (Cont.)

• Next, negatively charged paper is introduced.

• It attracts the toner more strongly than the neutral parts of the drum.

• After picking up the toner the paper is discharged so that the paper won’t cling to the drum.

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Laser Printing (Cont.)• At this stage the toner is just

sitting on the page. • Next it passes through the

fuser (heated rollers). • When a laser printer is

warming up – the fuser is literally warming up.

• The toner is melted into the page.

• The fuser is made of Teflon, so the toner doesn’t stick to it.

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Laser Printing (Cont.)

• The drum is exposed to light, the erase lamp, so that now all of the charge can leave it.

• This “erases” any image written onto the drum by the laser.

• It is ready for the next page.

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Two Power Supplies

• A laser printer has two power supplies. • Recall that part of a power supply is a transformer

that takes the 120 V (rms) power supplied and can raise (step up) or lower (step down) the voltage. – The primary power supply handles the motors, control

circuitry etc.

– The high-voltage power supply handles the primary corona wire.

• This high-voltage power supply is particularly dangerous.

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System Board

• A laser printer has a system board housing the required RAM and ROM.

• Some of the processing needed to display an image can be done by the printer itself.

• One can add to the printer’s RAM in the same way one can add RAM to a computer’s system board.

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Dye Sublimation

• A dye sublimation (a.k.a. thermal dye transfer) printer is a very high-end color printer.

• While other laser printers achieve their color by having nearby dots of cyan, yellow, magenta and black (called dithering), in the dye sublimation process these pigments actually mix.

• For each color there is a film (like cellophane), the print head heats the film which turns into a gas. The gas is absorbed by the nearby paper and re-solidified.

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Sublimation: solid gas

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Printers understand ASCII

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Some ASCII characters are instructions for the printer

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PostScript• PostScript is a Page Description Language developed

by Adobe mainly for laser printers. • In PostScript, the graphics are vector or object-oriented

rather than bitmapped. • Because the printer is given a description of a graphic

rather than precise directions of dot placement. This makes the graphics more scalable since the printer is responsible for turning the graphic into dots it can readily change the size of the graphic before rasterizing it into a bitmap.– RIP: raster image processor

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Variations

• Encapsulated PostScript (EPS) is a mixture of PostScript directions and some bitmapped graphics.

• What PostScript files (extension: ps) are for printers, Portable Document Files (extension: pdf) are for displays.

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PostScript

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PCL

• Printer Control Language (PCL) was developed by Hewlett Packard.

• PCL is more advanced than ASCII but not quite a true Page Description Language like PostScript.

• Less portable than PostScript.

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Windows GDI

• Windows Graphical Device Interface (GDI) is part of the operating system that handles graphics for display or printing.

• It does the rasterizing to bitmap instead of having the printer’s control board do it.

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Windows GDI

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References

• http://computer.howstuffworks.com/inkjet-printer.htm/printable

• http://computer.howstuffworks.com/laser-printer.htm/printable

• All in One A+ Certification, Meyers and Jernigan

• http://www.webopedia.com