Barry Rice

- doing science on a terrestrial planet -

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Digital Detectors

Most modern remote sensing devices do not use photographic film, but instead use digital detection systems. The most commonly used digital device is called a CCD chip. These chips consist of millions of tiny electronic detectors arranged in a square array. Imagine a microscopic checkerboard, where each square of the board is a distinct photon detector, and you have a good conceptual model for a CCD chip. Some digital detectors are CMOS devices. The differences between CCD and CMOS detectors is beyond the scope of this paper, and in any event will not be something you will probably have to consider in your work.

Like photographic emulsions, cameras incorporating digital detectors have bandwidths set by their physical characteristics. CCD cameras are typically sensitive well into the infrared, and are usually used with filters to block unwanted, long-wavelength infrared radiation.

It is easy to visualize how black and white images are made by CCD cameras. The image from the camera lens is focused on CCD chip, where it is sampled by the matrix of millions of little detectors. The assembly of color images is a little trickier. A color CCD camera has a complicated filter overlaid upon its detector chip. This filter consists of repeating rows (and columns) of green, red, and blue micro-filters, where each micro-filter is as big as an individual detector. The filter array equips 1/3 of the CCD pixels with green filters, 1/3 with red filters, and 1/3 with blue filters. When an exposure is obtained, the "green" data, "red" data, and "blue" data are read out separately, resulting in three separate images corresponding to the three bands. Using interpolation software, the camera overlays and combines the three images.


4 February 2009