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.

Vidicon Tubes

The key component in the vidicon tube is the photo conductive target. The lens focuses the the scene to be viewed on the target, and the target stores the scene in the form of an electrical charge. The electrical conductivity of the target varies with the amount of light absorbed. Where there is minimum light, the material has a high electrical resistance. The points on the target with considerable light have a lower resistance.

The remainder of the tube consists of an electron gun with surrounding magnetic coils. The cathode of the electron gun generates a stream of electron focused into a narrow beam to strike the inside of the target. Other magnetic coils surround the vidicon tube to produce horizontal and vertical deflection of the electron beam. Signals applied to these coils cause the electron beam to scan from left to right and top to bottom across the inside of the target. This causes a current to flow between the target and the transparent signal electrode. This current is proportional to the light amplitude. This is the video output signal.

For scanning the target, many methods are used, but the most widely used method is the standard NTSC pattern of 525 total scanning lines. When the electron beam scans the target, an electrical signal is produced. They varying amplitude in the signal represents brightness variations along one of the scan lines.

As soon as one line is scanned, the vertical deflection signals move the electron beam a small distance farther down, and another horizontal scan line is developed. Special synchronization pulses designate the beginning and end of each horizontally scanned line. The horizontal sync pulse is approximately 5 microseconds wide and occurs at a frequency of 15,750 Hz.

The total number of horizontal scanning lines is 525, but the way in which the target is scanned is unusual. The scanning takes place in two separate fields of 262 1/2 lines, each occurring every 1/60th of a second. The scan lines of the two fields are interlaced to produce a completely scanned image every 1/30th of a second. Of the 525 total lines, only about 480 of them are actually designated to the scene itself. The remaining 22 1/2 lines per field occur but they are not seen. This provides time for the electron beam to re-trace from bottom to top. Special sync pulses included in this interval keep the lines properly synchronized and meshed.

The electrical output signal has an approximate voltage range of .7 volts to 1.5 volts. The analog video signal between the sync pulses, represents the light intensity variations on one scan line. It is this voltage that will be fed to the analog-to-digital converter for translation into binary numbers.

Video Cameras

Two devices are most commonly used in computer vision cameras to convernt light into an electrical signal are the vidicon tube and a CCD array. The vidicon tube has been around for many years and is still the primary device used in commercial television cameras.

For computer vision systems, charged coupled devices (CCD's) are far more widely used. These semi conductor devices offer smaller size, greater light sensitivity, and lower power operation than vidicons. Both the CCD's and vidicons are still used.