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

Appendix ASPOT Digital CamerasIn order to provide you with an understanding of what happens when you take a picture, the followingsections highlight some of the basic operating principles of SPOT cameras.Pixel DigitizationFollowing each exposure, SPOT camera systems read the CCD chip and digitizes each pixel data as it istransferred off of the CCD chip, and before it is sent through the cable into the computer. This techniqueminimizes noise and maintains the lack of distortion that is inherent with CCD chip geometry. The SPOTInsight cameras use two methods of pixel digitization, one for live imaging, and one for image capture:• Live imaging (RT cameras and Insight cameras)To provide the fastest possible frame rate, SPOT RT and Insight cameras use two separate circuits todigitize the left and right halves of the CCD simultaneously.Pros and Cons: This method provides an extremely fast display when using SPOT’s Live Imagefeature. The challenge comes in achieving a good brightness match between the two image halves.Our current algorithm achieves a good match with bright images. However, with dim images,mismatches can occur, resulting in a “split” appearance, where one half of the image is brighter thatthe other half. A “split” Live Image view is not indicative of what your image capture will look like.• Image capture (all SPOT cameras)To provide the most accurate image captures, SPOT cameras use a single circuit to digitize the entireCCD chip.Pros and Cons: Because a single circuit is used to digitize the entire CCD chip, all pixels areinherently matched. However, this method provides a slower download than the two circuit methodused for live imaging.GainNormally an auto exposure routine in a digital camera sets the exposure time so that the brightest pixel onthe CCD chip is completely “filled” with electrons, or is “full well” (see the Digital Photography section).This simple technique works well for bright images, but, as an image gets dimmer, exposure times can getuncomfortably long.With the SPOT camera you can shorten exposure times by filling the brightest pixel on the CCD chip toless than full well, and then using this lower level as full scale. The inverse fraction of full well that thebrightest pixel on the CCD is filled to is known as the gain. For example, filling a pixel to half of full wellcuts the exposure time in half. Since the exposure time is cut in half, we call this a gain of 2.The following table illustrates the relationship between gain, the fraction of full well that the brightest pixelis filled to, and the effect of gain on exposure time:Gain Fraction of Full Well Exposure Time (sample)1 1 80 seconds2 _ 40 seconds4 _ 20 seconds8 1/8 10 seconds16 1/16 5 secondsUser Guide to the SPOT Insight Camera 315