Charge Coupled Devices

Electronic cameras and camcorders use charge coupled devices CCDs - a type of semiconductor that sensitive to light consisting of a 2-D array of elements, essentially a capacitor able to store a small amount of electric charge.

The charge stored by the CCD is created when photons of light strike the semiconducting material and liberate electrons. More photons liberate more charge, creating a charge proportional to light intensity. This happens for each CCD in the array. To make an image, all these charges in the array must be read. Each individual CCD consists of three transparent polysilicon gates over a buried channel of doped photosensitive silicon that generates the charge, flanked by a pair of channel stop regions to stop the charge leaking.

To read and digitize a particular CCD's charge, the voltages of the three gates are cycled which causes the charge to migrate down the channel to the next gate, then to the next pixel, and ultimately down the row until it reaches the end column, where it's read out into a serial register and ultimately sent to an analog-to-digital converter. This charge transfer occurs with an efficiency close to 100%. The sequence of moving the charge from one gate to the next is called coupling.

The CCDs are only sensitive to light intensity. One way to capture a colour image is to use three CCD arrays, each covered by a filter that passes one of the three primary colours - red, green or blue. Onboard camera electronics merge these primary components into a color pixel. Alternatively a special color grid, known as a Bayer pattern, is placed over the imaging array. This pattern ofalternating red-green and green-blue filters enables a single CCD

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