charge-coupled device

charge-coupled device (CCD) A silicon chip containing an array of light-sensitive diodes, used for capturing images. The photodiodes, arranged in an array of rows and columns, become charged when light falls on them. The amount of charge depends on the amount of light, which may be built up over time. These charges are read out column by column to provide an analogue signal of the image on the array, which is then converted to digital form for display and storage on a computer. CCDs are widely used both by professional and amateur astronomers as they are more sensitive to light than a photographic emulsion, give an output in almost constant proportion to the amount of light falling on them (i.e. a linear response), and have no reciprocity failure. The image can be displayed almost immediately after the end of the exposure, and image processing can be used to enhance it. However, the detector area is much smaller than a photographic plate or film, which can be of any size required and provides much finer resolution.