Charge-coupled device

A charge-coupled device, or 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.

Applications

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 This application of CCDs has now been mostly superseded by digital delay lines.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 rows or grids of capacitors are used in digital cameras, optical scanners and video cameras as light-sensing devices. 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.

A very interesting astronomical application is to use a CCD in a telescope that is not tracking, i.e. it is not following the motion of the sky, but instead transferring the charges in the CCD and reading them in a direction parallel to the motion of the sky, and at the same velocity. In this way, a region of the sky much larger can be imaged.

CCDs are typically sensitive to infrared light, which allows infrared photography[?], Night-vision devices, 0 lux (or near 0 lux) video-recording/photography.