Charge-coupled devices (CCD's) were invented in 1969 by Bell Labs in the United States. They are lightweight, low-powered, extremely sensitive image sensors, and they are about 15 times more sensitive to light than regular photographic film. The surface of a CCD is covered with a rectangular grid of microscopic light-sensitive elements. An image is created by a CCD when light falls on individual sensors creating an electrical charge proportional to the light intensity.
CCD's, which count almost every photon of light delivered to them by the telescope on the satellite, do have some disadvantages. They photograph a much smaller area than other image sensors, and they do not photograph in colour. The images, however, can be colourized later by combining images taken through different mono-chromatic filters. CCD's are ideally suited to take spy photographs, since they are usually of very small, detailed areas rather than panoramic views. CCD's can photograph items as small as grapefruits from a low Earth orbit.