SPOT 4.1 Basic and Advanced Software Manual for SPOT Insight ...

Appendix AAppendix A: Electronic Imaging Theory and SPOTCamerasIntroduction....................................................... 313Digital Photography......................................... 313Color Images.................................................... 314SPOT Digital Cameras...................................... 315Pixel Digitization .......................................... 315Gain.............................................................. 315An Automated Exposure Process............. 316IntroductionThis appendix is a brief overview of the operational theory behind SPOT digital cameras. The next twosections, Digital Photography and Color Images, provide a short introduction to electronic imaging theory;the following section describes how this theory applies to the SPOT cameras.Digital PhotographyModern electronic imaging is based on the charged coupled device (CCD). All of today’s digital camerashave a CCD chip or chips. In digital cameras, the CCD chip/sensor replaces the film of traditional camerasas the means by which the camera records the image. The surface of the CCD chip is composed of lightsensitivecells arranged in a checkerboard pattern. Each cell of the checkerboard is known as a pictureelement, or more commonly, a pixel. The following is a simplified description of what happens when youtake a picture with a digital camera:1. The camera’s optical system forms images on the “checkerboard” of pixels.2. The CCD is exposed to the image for a period of time.During this period, each photosensitive cell receives photons of light, converts the photons toelectrons, and then stores the electrons in the cell. The process by which each cell accumulateselectrons can be compared to a well filling with water. As more light hits a cell, the electron level inthe well rises. The more electrons that are in the cell, the more voltage it will have when read by thedigital camera.3. Following the exposure, a digital camera does three things:a. It measures the voltage of each cell.b. It converts the voltage to a binary number.c. It transmits this number down a cable to your computer.4. The computer reconstructs the image by assigning a brightness value to each pixel in the final image.Each brightness value is proportional to the voltage of the corresponding cell on the CCD chip.User Guide to the SPOT Insight Camera 313