How Does the CCD Work in a Copier?

While some designs of photocopiers and many older machines still use photosensitive belts or drums to capture images, many newer copiers and scanners use a technology similar to that in digital cameras. A charge-coupled device is a sort of image sensor, converting photons from light into electrical impulses. The copier then translates these impulses into image data, allowing the copier to print a duplicate of the scanned image.

A charge-coupled device is similar to a photovoltaic cell. CCDs use a thin layer of silicon, which becomes electrically reactive when light strikes the surface. A CCD-equipped copier uses a bright light to illuminate the item being copied, passing the light over the surface of the page alongside the CCD. The reflected light strikes the CCD, and the device transmits the resulting electrical impulses to the copier’s processing unit.

In the case of black and white copying, no further manipulation is necessary. Areas of the page that are white reflect a great deal of light, so the CCD absorbs more light as it passes over those areas, transmitting more electrons. Darker areas absorb light, reflecting less, so the CCD is less active on those areas of the page. The copier measures the amount of light absorbed per pixel of the image, and uses that data to recreate the copied image for printing or transmission.

Color copying requires more elaborate techniques than black and white copying. Originally, color CCD devices relied on a series of prisms to break incoming light into its component red, blue and green colors, capturing each color on its own distinct CCD panel. These devices were expensive, however, due to the necessary triplication of the active mechanism, and were prone to failure due to misaligned optics. Today, some high-end cameras and camcorders use tri-color CCD setups to produce images with extremely accurate color saturation.

Most copiers use a more inexpensive solution to the color scanning problem. These devices include a Bayer mask over the CCD, a mesh of red, blue, and green filters over the pixels. Each four-pixel area contains one blue, one red, and two green filters, and each pixel thus records the information about one color as well as the brightness. As the CCD moves across the original, each pixel in the CCD records information as it passes, so the CCD eventually picks up red, green, and blue information for every part of the scanned image. The copier then averages that information, producing red, green, and blue channel negatives, finally combining the three of them into the true-color image.