How It Works - Charge Coupled Device?

How does a videocamera, digital camera, or scanner create a digital representation? All make use of a CCD.

A CCD (Charge Coupled Device) is made up of a large number (several thousand) photoelectric cells that generate voltage in proportion to the amount of light striking them. The ability to measure the amount of reflected light is a very useful property because white and other light colors reflect most light and black and darker colors absorb most light. In a flatbed or hand-held scanner, the photoelectric cells are arranged in a line that is passed over the image to be copied. A light source in the scanner is directed at the image to be copied and the CCD measures the amount of light that is reflected. In a camcorder or still videocamera, the sensors are arranged in a grid rather than along a line and the image is focused on this grid. The more sensors that are packed into the grid or along the line, the better the quality of the image that is captured. Working with color requires an additional enhancement to this system. Color is captured by passing the light reflected from the image through colored filters (red, green, blue) before it strikes the CCD and independently capturing these three sources of information. These three data sources will later be recombined to reproduce the original image.

If the output from the CCD is to be digitized, a special hardware converter then takes the voltages generated by the individual elements of the CCD and rounds them off into the number of alternative values that will be used to represent the image digitally. If the image is to be represented as line art, the continuous signal generated by the CCD elements is rounded off to a value of 1 or 0. If 256 shades of gray or color are to be represented, the signal from each CCD element is converted to the appropriate value within this range. CCDs presently are capable of capturing thousands of colors.

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