Imaging Science FundamentalsChester F. Carlson Center for Imaging Science

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science









Results of AnalysisResults of Analysis

• Image layer of the First Photograph is not a continuous layer, but rather has a random dot pattern.

• First Photograph was made using a pewter plate containing a high concentration of tin alloyed with lead, copper and iron.

• Nondestructive infrared analysis of the image layer of the First Photograph revealed a complex composition of bitumen and oil of lavender.

• The metal plate of the First Photograph does not have consistent thickness nor are its dimensions uniform.

• Evaluations of the First Photograph's earlier protective enclosure revealed the urgent need to design and build a new oxygen-free enclosure to protect the artifact.


““Grain” of Film and PaperGrain” of Film and Paper

Electron Photomicrographs of Emulsion Grains


What is Silver Halide?What is Silver Halide?

Silver (Ag)

Halide group


Structure of a Typical B&W FilmStructure of a Typical B&W Film

Film base Plastic

Antihalation backing Prevents light from reflecting back.

EmulsionSilver Halide Crystals

Suspended in gelatin, likefruits in Jell-O™!


Exposed AgX CrystalsExposed AgX Crystals

When a silver halide crystal is exposed to light, some of the AgX molecules break up into their constituents, one of which is metallic silver (“pure” Ag).

Exposure

AfterExposure


Silver Halide Process ChainSilver Halide Process Chain

A latent image is formed after exposure (invisible to human eye).

After processing, the latent image is turned into a visible, stable image.

Exposure Processing

Develop Stop FixLatentImage

Visible(Stable)Image


Processing Photographic FilmProcessing Photographic Film

Developer “amplifies” the atomic silver to visible silver strands.

Stop Bath stops the development process.

Fix dissolves the unexposed AgX crystals, making the film safe to expose to light.

Wash with water to rinse fix chemicals away.


Silver Halide GrainsSilver Halide Grains


Why does processed film look Why does processed film look “negative”?“negative”?

Silver strands formed by exposure of photographic film to light actually appear dark (they are NOT shiny).

So, where light hits the film during exposure, it turns darker.


What determines how dark film What determines how dark film becomes?becomes?

THE GRAINS! Size Shape Chemical composition Distribution


““Grain” of Film and PaperGrain” of Film and Paper

Electron Photomicrographs of Emulsion Grains (n.b. Measurement Bars

indicate scale)



What determines how dark film What determines how dark film becomes?becomes?

Consider the so-called “D-Log H” curve. Describes how film responds to light: Density (D) is how dark the film is. Log H is the exposure (H) in logarithmic scale.

Log H

D

More ExposureLess Exposure

Lighter

Darker


D-Log H Curve and ContrastD-Log H Curve and Contrast

Log H

D

Log H

D

More contrast Less contrast

Film response

Image


Color ImagesColor Images

In most cases, we also want to capture color information

The way that we capture, store, view, and print color digital images is based on the way that humans perceive color


Color PerceptionColor Perception

The eyes have three different kinds of color receptors (‘cones’); one type each for blue, green, and red light.

Color perception is based on how much light is detected by each of the three ‘primary’ cone types (red, green, and blue)


Additive Color MixingAdditive Color Mixing

Red Green

Blue


Subtractive Color MixingSubtractive Color Mixing

Cyan Magenta

Yellow



Traditional Traditional vs.vs. Digital Photography Digital Photography

Chemical processing

Detector: Photographic film

Digitalprocessing

Detector: Electronic sensor (CCD)



Goal of Charge Coupled Device (CCD)Goal of Charge Coupled Device (CCD)

Capture electrons formed by interaction of photons with the silicon

Measure the electrons from each picture element as a voltage

CCDPhotons Electronic Signal


Charge Coupled Device (CCD)Charge Coupled Device (CCD)

CCD chip replaces silver halide

film

No wet chemistry processing

Image available for immediate

feedback


Magnified View of a CCD ArrayMagnified View of a CCD Array

Individual pixel element

Close-up of a CCD Imaging ArrayClose-up of a CCD Imaging Array

CCD



Spatial SamplingSpatial Sampling

When a scene is imaged onto the CCD by the lens, the continuous image is ‘sampled’ and divided into discrete picture elements, or ‘pixels’

Scene Grid over scene Spatially sampled scene


QuantizationQuantization

The spatially sampled image is then converted into an ordered set of integers (0, 1, 2, 3, …) according to how much light fell on each element

Spatially sampled scene

0

0

0

0

0

0

0

0

0

0

0

0

0

0

25

40

0

0

25

40

40

25 25

2540 40

40 40 40

25 2540 40 40

40 40 25

40

40

40

64

64

64

64

64

64

64 64 64

64 64 64

97

97

97

97

97

150

97

97

97

0 0 0 0 0 0

0 0 0 0 0 0

0

0

0

0

0 0

0

0

Numerical representation


Basic structure of CCDBasic structure of CCD

Divided into small elements called pixels (picture elements).

preamplifier

Image Image Capture Capture AreaArea

Shift Register

Voltageout

Columns

Rows


Basic structure of a pixel in a CCDBasic structure of a pixel in a CCD

Silicon is a semiconductor. Oxide layer is an insulator. Metal gates are conductors. Made with microlithographic process. One pixel may be made up of two or more metal gates.

Silicon baseSilicon base

Metal gate

Oxide Layer

One pixel


Photon/Silicon InteractionPhoton/Silicon Interaction

Photon knocks off one of the electrons from the silicon matrix.

Silicone-e-

Electron “wanders around” randomly through the matrix. Electron gets absorbed into the silicon matrix after some period.


Collection stageCollection stage

Voltage applied to the metal gates produces a depletion region in the silicon. (depleted of electrons)

Depletion region is the “light sensitive” area where electrons formed from the photon interacting with the silicon base are collected.

Voltage


Collection stageCollection stage

Electron formed in the silicon matrix by a photon.

e-

Electron wanders around the matrix.

If the electron wanders into the depletion region, the electron is captured, never recombining with the silicon matrix.

e-e-

Voltage


CollectionCollection

The number of electrons accumulated is proportional to the amount of light that hit the pixel.

There is a maximum number of electron that these “wells” can hold.

e-

e-e- e-e- e-e-e- e-

e- e-

Light


Bucket BrigadeBucket Brigade

By alternating the voltage applied to the metal gates, collected electrons may be moved across the columns.

e-

e-e- e-e- e-e-e- e-

e- e-e-

e-e- e-e- e-e- e-

e- e- e-e-e-e- e-

e- e- e-e- e-e- e-e- e-e-e-

e- e- e-e- e-e- e-e- e- e-e-e-

e-e-e- e-e- e-e- e- e-

e-e-e-e-e-

e-e- e-


Bucket BrigadeBucket Brigade

Charge is marched across the columns into the shift register, then read out 1 pixel at a time.

100 pixels

100 pixels

1 transfer100 transfers

100 transfers200 transfers

Shift Register



Converting Analog Voltages to DigitalConverting Analog Voltages to Digital

Analog voltage is converted to a digital count using an Analog-to-Digital Converter (ADC) Also called a digitizer

The input voltage is quantized: Assigned to one of a set of discrete steps

Steps are labeled by integers Number of steps determined by the number of available bits

Decimal Integer is converted to a binary number for computation

ADC6.18 volts 01100101 (117)


Response of CCDResponse of CCD

The response of CCD is linear (i.e., if 1000 captured photons corresponds to a digital count of 4, then 2000 photons captured yields a digital count of 8)

Linearity is critical for scientific uses of CCD

Log H

Den

sity

Response of photographicnegative

Exposure

Dig

ital

Cou

nt

Response of CCD


CMOS DetectorsCMOS Detectors

(Complementary Metal Oxide Semiconductor)

- Uses same physical principles as CCD’s- Different architecture:

- No shift register – each pixel individually addressable- Uses on-chip electronics support- Smaller “fill factor” – area of chip used to sense photons


CCD vs. CMOSCCD vs. CMOS

CCD Better performance

Resolution Sensitivity Signal to noise

Hi-end applications

CMOS Cheaper Less power required Low-end applications

Historically:

Currently:

CMOS is starting to bridge the performance gap


RGB Color ImagesRGB Color Images

To capture a color image we record how much red, green, and blue light there is at each pixel.

To view the image, we use a display (monitor or print) to reproduce the color mixture we captured.

Q) How many different colors can a display produce? A) It depends on how many bits per pixel we’ve got.

For a system with 8 bits/pixel in each of the red, green, and blue (a ‘24-bit image’):



RGB Color ImagesRGB Color Images

=










Summary - DetectorsSummary - Detectors

Chemistry-based detectors have been around a long time

Modern films use grains of silver halides to record the intensity of light

Color films use three layers of emulsion, each sensitive to one color (RGB)



Solid state detectors have been around since the early 70’s

Two main classes: CCD and CMOS Same physics, different architecture Early cost & performance differences are

disappearing Color images are obtained by filtering

the light before it hits the sensor’s silicon base



New technologies are being developed to improve performance and reduce cost







Documents

Imaging Science FundamentalsChester F. Carlson Center for Imaging Science