ECEG287 Optical Detection Course Notes Part 15: Introduction to Array Detectors

1. ECEG287 Optical Detection Course NotesPart 15: Introduction to Array Detectors Profs. Charles A. DiMarzio and Stephen W. McKnight Northeastern University, Spring 2004 Charles A. DiMarzio, Northeastern University

2. x A -x’ A’ s s’ Imaging Detectors • Goal: • Measure I(x,y,t) • Or perhaps E(x,y,t) • Other Variables • z, l, etc. • Approaches: • Scanning • Arrays • Combinations Charles A. DiMarzio, Northeastern University

3. 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 Charles A. DiMarzio, Northeastern University

4. 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 Charles A. DiMarzio, Northeastern University

5. Scanning Systems Charles A. DiMarzio, Northeastern University

6. Array Detector Concept Charles A. DiMarzio, Northeastern University

7. Pixelation and Digitization Count 255 0 “Brightness” Charles A. DiMarzio, Northeastern University

8. Digitization and Dynamic Range 2N-1 Saturation Maximum Signal Minimum Signal Step Size Pedestal Signal Voltage 0 Dark Charles A. DiMarzio, Northeastern University

9. 1 0.8 0.6 Output Voltage 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 Input Voltage Linearity and AGC • Automatic Gain Control (AGC) • Feedback • Control G • Based on... • Peak Signal • Average Signal • Peak in a Region • Not Desirable for Quantitative Work Charles A. DiMarzio, Northeastern University

10. Clock Voltage 1 0.9 2 0.8 4 0.7 0.6 m10057_1.m Figure 1 6 Row Number 0.5 0.4 8 0.3 10 0.2 0.1 12 0 0 0.5 1 1.5 2 2.5 3 time, Clock Cycles CCD Charge Transfer V One Line V Clock Signals time Charles A. DiMarzio, Northeastern University

11. Formats Frame Transfer Line Transfer Collection Frame Transfer Frame Charles A. DiMarzio, Northeastern University

12. Computer Interfacing • Analog Camera and Frame Grabber • Digital Camera Digital Camera Analog Camera Computer Computer with Frame Grabber Computer Monitor Analog Monitor Computer Monitor Charles A. DiMarzio, Northeastern University

13. Signals and Noise 10057p1-3 and 4 on this page (Gaussian) Charles A. DiMarzio, Northeastern University

14. 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 Charles A. DiMarzio, Northeastern University

15. Quantitative Calculations Difficult Subject to Change Calibration Standards Light Level Reflectance Sources of Variation Light Source Camera Sensitivity Filter Losses Geometry Atmosphere? Other? Quantitative Imaging Charles A. DiMarzio, Northeastern University