Charge Coupled Devices

A charge coupled device is light sensitive integrated circuit designed to convert a rectangular visual image into a video electrical signal. A lens system focuses the scene onto a photo conductive substrate such as silicon. This device absorbs the light and stores it as a charge on thousands of tiny square or rectangular capacitors. The silicon forms a single common plate for all of these capacitors. The other plates are individual tiny metal electrodes. Separated from the silicon photo conductor by a thin layer of silicon dioxide insulator. Light falling on the substrate causes the capacitors to charge and discharge to a level depending upon the intensity of the light striking the substrate directly below the tiny capacitive plates.

Voltages are applied to the capacitor electrodes in sequence from right to left, to "read" the scenes. As the control voltage is applied, the capacitor is discharged and an analog signal is transferred to the common silicon substrate.

The tiny capacitive cells in the CCD are arranged in a rectangular or square array. Typical array sizes are, 64x64, 244x190, 488x380m and 512x512. The larger the number of capacitive storage cells, the greater the resolution. High resolution means good definition of detail.

The output signal from the CCD varies from approximately 0 to 1 volts, with the two extremes representing black and white respectively. Interlace scanning is not used. Each row of the CCD matrix is scanned in turn. Each row in the matrix represents one scan line of the scene focused on the light sensitive array.

CCD video cameras are preferred for computer vision. They are small, highly sensitive and very reliable. they are also more rugged than the delicate vidicon tubes. They do not require a filament voltage or the high voltage required by the vidicon.

Their light weight and low power consumption makes them extremely small and portable. These units are also less expensive than vidicon cameras.

Both CCD and vidicon cameras produce a black and white analog video signal output. These cameras do not recognize color and generate an output signal that represents gray levels between black and white. All computer vision systems are black and white.

Color systems can be created, but they are far more complex and expensive. Yet color adds information that makes it easier for a computer to recognize shapes, objects, background, and other characteristics of the scene.