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Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester 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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
““Grain” of Film and PaperGrain” of Film and Paper
Electron Photomicrographs of Emulsion Grains
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
What is Silver Halide?What is Silver Halide?
Silver (Ag)
Halide group
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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™!
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Silver Halide GrainsSilver Halide Grains
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
What determines how dark film What determines how dark film becomes?becomes?
THE GRAINS! Size Shape Chemical composition Distribution
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
““Grain” of Film and PaperGrain” of Film and Paper
Electron Photomicrographs of Emulsion Grains (n.b. Measurement Bars
indicate scale)
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
D-Log H Curve and ContrastD-Log H Curve and Contrast
Log H
D
Log H
D
More contrast Less contrast
Film response
Image
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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)
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Additive Color MixingAdditive Color Mixing
Red Green
Blue
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Subtractive Color MixingSubtractive Color Mixing
Cyan Magenta
Yellow
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Traditional Traditional vs.vs. Digital Photography Digital Photography
Chemical processing
Detector: Photographic film
Digitalprocessing
Detector: Electronic sensor (CCD)
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Charge Coupled Device (CCD)Charge Coupled Device (CCD)
CCD chip replaces silver halide
film
No wet chemistry processing
Image available for immediate
feedback
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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.
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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-
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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)
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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’):
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
RGB Color ImagesRGB Color Images
=
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
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)
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Summary - DetectorsSummary - Detectors
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
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Summary - DetectorsSummary - Detectors
New technologies are being developed to improve performance and reduce cost
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science
Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science