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

March 2004 Charles A. DiMarzio, Northeastern University Imaging Detectors Goal: –Measure I(x,y,t) –Or perhaps E(x,y,t) Other Variables –z,  etc. Approaches: –Scanning –Arrays –Combinations A’ A ss’ x -x’

March 2004 Charles A. DiMarzio, Northeastern University 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.

March 2004 Charles A. DiMarzio, Northeastern University Image Orthicon ~ 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.

March 2004 Charles A. DiMarzio, Northeastern University Scanning Systems

March 2004 Charles A. DiMarzio, Northeastern University Array Detector Concept

March 2004 Charles A. DiMarzio, Northeastern University Pixelation and Digitization “Brightness” Count 0 255

March 2004 Charles A. DiMarzio, Northeastern University Digitization and Dynamic Range Signal Voltage 2 N -1 0 Pedestal Saturation Step Size Dark Minimum Signal Maximum Signal

March 2004 Charles A. DiMarzio, Northeastern University Linearity and AGC Input Voltage Output Voltage Automatic Gain Control (AGC) Feedback –Control G –Based on... Peak Signal Average Signal Peak in a Region Not Desirable for Quantitative Work

March 2004 Charles A. DiMarzio, Northeastern University CCD Charge Transfer Clock Signals time One Line V V

March 2004 Charles A. DiMarzio, Northeastern University Formats Collection FrameTransfer Frame Frame Transfer Line Transfer

March 2004 Charles A. DiMarzio, Northeastern University Computer Interfacing Analog Camera and Frame Grabber Digital Camera Analog Camera Computer with Frame Grabber Analog Monitor Computer Monitor Digital Camera Computer Monitor

March 2004 Charles A. DiMarzio, Northeastern University Signals and Noise 10057p1-3 and 4 on this page (Gaussian)

March 2004 Charles A. DiMarzio, Northeastern University 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

March 2004 Charles A. DiMarzio, Northeastern University Quantitative Imaging Quantitative Calculations –Difficult –Subject to Change Calibration Standards –Light Level –Reflectance Sources of Variation –Light Source –Camera Sensitivity –Filter Losses –Geometry –Atmosphere? –Other?