Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Digital Imaging: CCDs Imaging Science Fundamentals

Imaging Science Fundamentals Chester F. Carlson Center for Imaging Science

Digital Imaging: CCDsDigital Imaging: CCDs

Imaging Science Fundamentals


Charge Coupled Charge Coupled Device (CCD)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

Light Sensitive Area


Response of CCDResponse 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

Log H

Den

sity

Response of photographicnegative

Exposure

Dig

ital

Cou

nt

Response of CCD


Spectral Response (sensitivity) Spectral Response (sensitivity) of a typical CCDof a typical CCD

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

300 400 500 600 700 800 900 1000

Incident Wavelength [nm]

RelativeResponse

Visible Light IRUV


Goal of CCDGoal of CCD

Capture electrons formed by interaction of photons with the silicon

Measure the electrons from each picture element as a voltage

CCDPhotons Electronic Signal


Spatial SamplingSpatial Sampling

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.

Scene Grid over scene Spatially sampled scene


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


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


CCDs as SemiconductorsCCDs as Semiconductors

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.

ConductorInsulator


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


ReadoutReadout

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


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)


Bits and BytesBits 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

Decimal012345

Binary011011100101


BitsBits

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


QuantizationQuantization

Let’s say our 8 bit ADC accepts input voltage range of 0 to 10v.

ADC

6.8 volts

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).

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

6.8v 174.08

Volts DC

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

172

173

174

175

176

255

00v

6.8 volts/0.0390625 volts per DC = 174.08

10v

Binary equivalent of 174 is 10101110.


QuantizationQuantization

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

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

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Imaging Science FundamentalsChester F. Carlson Center for Imaging Science Digital Imaging: CCDs Imaging Science Fundamentals