Digital%20Imaging:%20CCDs

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Digital Imaging CCDs

• Imaging Science Fundamentals

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Charge Coupled Device (CCD)

• CCD replaces AgX film
• Based on silicon chip
• Disadvantages vs. AgX
• Difficulty/cost of CCD manufacture large arrays
  are VERY expensive
• Young technology rapidly changing

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Response of CCD

• The response of CCD is linear (i.e., if 10,000
  captured photons corresponds to a digital count
  of 4, then 20,000 photons captured yields a
  digital count of 8)
• Linearity is critical for scientific uses of CCD

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Spectral Response (sensitivity) of a typical CCD
UV
Visible Light
IR
Relative Response
300
400
500
600
700
900
800
1000
Incident Wavelength nm

• Response is large in visible region, falls off
  for ultraviolet (UV) and infrared (IR)

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Goal of CCD
CCD
Photons
Electronic Signal

• Capture electrons formed by interaction of
  photons with the silicon
• Measure the electrons from each picture element
  as a voltage

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Spatial Sampling
Scene
Grid over scene
Spatially sampled scene

• When a continuous scene is imaged on the array
  (grid) formed by a CCD , the continuous image is
  divided into discrete elements.
• The picture elements (pixels) thus captured
  represent a spatially sampled version of the
  image.

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Basic structure of CCD
Divided into small elements called pixels
(picture elements).
Shift Register
Image Capture Area
Rows
Voltageout
Columns
preamplifier
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Magnified View of a CCD Array
Individual pixel element
CCD
Close-up of a CCD Imaging Array
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CCDs as Semiconductors
Insulator
Conductor

• Conductors allow electricity to pass through.
  (Metals like copper and gold are conductors.)
• Insulators do not allow electricity to pass
  through. (Plastic, wood, and paper are
  insulators.)
• Some materials are halfway in between, and are
  called semiconductors.

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Basic structure of a pixel in a CCD
Metal gate
Oxide Layer
Silicon base
One pixel

• 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.

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Photon/Silicon Interaction
e-
e-
Silicon

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

• Electron wanders around randomly through the
  matrix.

• Electron gets absorbed into the silicon matrix
  after some period.

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Collection stage
Voltage

• 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.

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Collection stage
Voltage
e-
e-

• Electron formed in the silicon matrix by a
  photon.

• Electron wanders around the matrix.

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

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Collection
Light
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-
e-

• 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.

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Readout

• Now that the electrons are collected in the
  individual pixels, how do we get the information
  out?

Alright! How do we get the electrons out?!
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Readout

• How do you access so much data efficiently? (i.e.
  a 1024 x 1024 CCD has 1,048,576 pixels!)
• Possible solutions
• 1. Have output for individual pixels.
• Too many wires
• 2. Somehow move the charges across the CCD array
  and read out one by one.
• Bucket Brigade

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Bucket Brigade

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

e-
e-
e-
e-
e-
e-
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Bucket Brigade

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

100 transfers
200 transfers
Shift Register
100 pixels
100 transfers
1 transfer
100 pixels
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Converting 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

ADC
6.18 volts
01100101 (117)
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Bits and Bytes

• In the digital domain, there are only two
  possible numbers in a digit 0 or 1.
• This numbering system is called a binary system.
• Each digit is called a bit (Binary digIT).
• Byte is 8 bits

Decimal 0 1 2 3 4 5
Binary 0 1 10 11 100 101
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Bits

• Bits dictate how fine the quantization levels
  are.
• An n bit system can represent 2n numbers.

1 bit system 21 2 levels (Black or White)
8 bit system 28 256 levels
12 bit system 212 4096 levels
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Quantization

• Lets say our 8 bit ADC accepts input voltage
  range of 0 to 10v.

ADC
Volts DC
10v

• Since there are 256 discrete levels in an 8 bit
  system, each level will be 10v/256 or 0.0390625
  volts per analog-to-digital unit (ADU).

6.8 volts
174.08
6.8v

• So, if the input voltage was 6.8 volts . . .

0v

• Since ADU are stored as binary integers, the
  decimal must be truncated (to 174).

• Binary equivalent of 174 is 10101110.

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Quantization
Spatially sampled scene
Numerical representation

• Spatially sampled image can now be turned into
  numbers according to the brightness of each
  pixel.