march 2004 charles a. dimarzio, northeastern university 10464-15-1 eceg287 optical detection course...

March 2004 Charles A. DiMarzio, Northeastern University 10464-15-1

ECEG287 Optical Detection Course Notes

Part 15: Introduction to Array Detectors

Profs. Charles A. DiMarzio

and

Stephen W. McKnight

Northeastern University, Spring 2004


Imaging Detectors

• Goal: – Measure I(x,y,t)

– Or perhaps E(x,y,t)

• Other Variables– z, etc.

• Approaches:– Scanning

– Arrays

– Combinations

A’A

s s’

x

-x’


Nipkow Disk, 1884

• The Nipkow disk was a device which its inventor, Paul Nipkow, thought that could be used to transmit pictures by wire. The disk had a spiral of holes cut into it. These holes were positioned so that they could scan every part of an image in turn as the disk spun around. The light coming from each point would then be turned into an electrical current. This electrical signal would light up a second light at the other end of the wire. The second light would flicker because the amount of current it received would depend on the brightness of the image being scanned. The light from this light bulb passing through a second disk spinning at the same speed, would then project the picture onto a screen.

http://www2.fht-esslingen.de/telehistory/nipkow2.html


Image Orthicon ~1940-60

• The front of the Image Orthicon contains a screen called a photocathode that releases electrons when light from the camera lens strikes it. Bright parts of the scene knock out more electrons than dim parts do. Another screen behind the photocathode, called the target, attracts the released electrons, and a positively charged electronic image of the scene forms on the target. The image consists of highly and weakly charged spots that correspond to the bright and dim areas of the scene. A beam of electrons then scans the target, which absorbs electrons from the beam in proportion to those knocked out by the image.

http://www.acmi.net.au/AIC/IMAGE_ORTHICON.html


Scanning Systems


Array Detector Concept


Pixelation and Digitization

“Brightness”

Count

0

255


Digitization and Dynamic Range

Signal Voltage

2N-1

0

Pedestal

Saturation

Step Size

Dark

MinimumSignal

MaximumSignal


Linearity and AGC

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Input Voltage

Ou

tpu

t Vol

tage

Gxy

21

5.0

• Automatic Gain Control (AGC)

• Feedback– Control G– Based on...

• Peak Signal• Average Signal• Peak in a Region

• Not Desirable for Quantitative Work


CCD Charge Transfer

Clock Signals

m10057_1.mFigure 1

time, Clock Cycles

Row

Num

ber

Clock Voltage

0 0.5 1 1.5 2 2.5 3

2

4

6

8

10

120

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

time

OneLineV

V


Formats

Collection Frame Transfer Frame

Frame Transfer Line Transfer


Computer Interfacing

• Analog Camera and Frame Grabber

• Digital Camera

Analog CameraComputer withFrame Grabber

AnalogMonitor

ComputerMonitor

Digital Camera

Computer

ComputerMonitor


Signals and Noise

10057p1-3 and 4 on this page

(Gaussian)


Some Standard and Extreme Parameters

• VGA Frame Size: 640 by 480– Up to 4k Square?

• Standard Update Rate: 30 Hz. Interlaced– Up To few kHz.

• Standard Digitization: 8 Bits– Up To 12.

• Pixel Size: 10 micrometers.• Color Camera: 3 Channels, 8 Bits Each


Quantitative Imaging

• Quantitative Calculations– Difficult

– Subject to Change

• Calibration Standards– Light Level

– Reflectance

DarkWhite

DarkTarget

R

• Sources of Variation– Light Source

– Camera Sensitivity

– Filter Losses

– Geometry

– Atmosphere?

– Other?

march 2004 charles a. dimarzio, northeastern university 10464-15-1 eceg287 optical detection course...

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