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Charge-coupled device

A charge-coupled device (CCD), is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to one or other of its neighbours. CCDs are used in digital photography and astronomy (particularly in photometry and optical and UV spectroscopy).


CCDs containing a single row of capacitors can be used as delay lines. An analogue voltage is applied to the first capacitor in the array, and at regular intervals a command is given to each capacitor to transfer its charge to its neighbour. Thus the entire array is shifted by one location. After a delay equal to the number of capacitors multiplied by the shift interval, the charge corresponding to the input signal arrives at the last capacitor in the array, where it is amplified to become the output signal. This process continues indefinitely, creating a signal at the output that is a delayed version of the input, with some distortion due to sampling. A CCD used in this way is also known as a bucket-brigade delay line. This application of CCDs has now been mostly superseded by digital delay lines.

CCDs containing grids of pixels are used in digital cameras, optical scanners and video cameras as light-sensing devices. They commonly respond to 70% of the incident light, making them more efficient than photographic film, which captures only about 2% of the incident light. As a result CCDs were rapidly adopted by astronomers. Before CCDs, planetary observations relied on the memory of the observer because the time over which clear air would remain in front of a planet was shorter than the exposure time required for film.

An image is projected by a lens on the capacitor array, causing each capacitor to accumulate an electric charge proportional to the light intensity at that location. A one-dimensional array, used in line-scan cameras, captures a single slice of the image, while a two-dimensional array, used in video and still cameras, captures the whole image or a rectangular portion of it. Once the array has been exposed to the image, a control circuit causes each capacitor to transfer its contents to its neighbour. The last capacitor in the array dumps its charge into an amplifier that converts the charge into a voltage. By repeating this process, the control circuit converts the entire contents of the array to a varying voltage, which it samples, digitises and stores in memory. Stored images can be transferred to a printer, storage device or video display. CCDs are also widely used as sensors for astronomical telescopes, and night vision devices.

An interesting astronomical application is to use a CCD to make a fixed telescope behave like a tracking telescope and follow the motion of the sky. The charges in the CCD are transferred and read in a direction parallel to the motion of the sky, and at the same speed. In this way, the telescope can image a larger region of the sky than its normal field of view.

CCDs are typically sensitive to infrared light, which allows infrared photography, night-vision devices, and zero lux (or near zero lux) video-recording/photography. Because of their sensitivity to infrared, CCD's used in astronomy are usually cooled to liquid nitrogen temperatures. One other consequence of their sensitivity to infrared is that infrared from remote controls will appear on CCD based cameras.

Digital cameras

Digital cameras use a two dimensional CCD, usually with a color filter over each group of four pixels (2x2): red, blue and two times green (because the human eye is more sensitive to some colors). The result is that you have the luminance from four pixels and less in color resultion. Hence the effective resolution (Mega pixels) is less then the number of capacitors on the CCD.

Better color separation can be reached by three CCD devices (3CCD) and a prism, that splits the colors. Some semi-professional digital video camcorders (and all professionals) use this technique.

See also