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

Appendix AColor ImagesBecause a CCD chip is inherently monochromatic (black and white), color filters must be used to extractthe color information from the image. There are three basic design methodologies.• Single CCD design using color masked pixels (Insight Color Mosaic camera; RT KE ColorMosaic)Digital cameras designed in this manner have one CCD chip with color filters physically bonded toeach pixel on the chip. Some pixels get red filters, some pixels get green filters and some pixels getblue filters. Since each pixel is only able to measure the intensity of one color, the intensity of themissing two colors for this pixel must be estimated, based on nearby pixels that have precisemeasurements of the missing colors. For example, a pixel that has a green filter will have an exactmeasurement of the green color value, but the red and blue values must be estimated, based on thenearest red and blue pixels. This estimation is known as interpolation.Pros and Cons: A digital camera designed in this way is inexpensive and can freeze moving imageswith a single exposure. However, because they interpolate brightness values, such cameras cannotprovide as high of a resolution as that provided from a non-filtered CCD chip.• Three CCD design using color beam splitterThis design uses three CCD chips. The incoming image first goes through a color beam splitter whichdirects the red light to one chip, the green light to a second chip and the blue light to a third chip. Thechips are very accurately aligned so as to achieve near perfect registration between the three chips. Thered, green, and blue values for each point on an image are measured by the corresponding pixel oneach of the three chips.Pros and Cons: This design produces an image that retains the high resolution of the individual chips.This type of camera can also freeze moving images with a single exposure. However, because most ofthe cost in a high resolution CCD camera is in the CCD chip itself, three chip cameras are far morecostly than their single chip counterparts.• Single CCD design that takes three pictures (three pass method) (RT 3-shot and Slider cameras,Enhanced Slider camera and Insight 3-Shot camera)Cameras designed in this way expose a single CCD chip three times, once to red light, once to greenlight, and once to blue light. One variant in this type of camera is how the chip is exposed to thedifferent colors of light. Some cameras switch individual red, green and blue glass filters in front ofthe chip for each exposure. Others use a liquid crystal filter that changes from red to green to blue asdifferent voltages are applied to it. By exposing the chip three times, each cell on the CCD is able tomeasure all three color values.Pros and Cons: This technique attains the high resolution of the three CCD design without the highcost associated with using three chips. However, because these cameras require three exposures, theycannot freeze moving images.314 User Guide to the SPOT Insight Camera