Depending on the configuration of the machine, a scanner can produce images that are black and white or color. In a color scanner the digital color image consists of three gray-scale images. These images are often called layers. One layer defines which areas will be green in the final color image. The other two layers do the same for red and blue.
To create the differently colored layers, the scanner must be able to distinguish those parts of the color image that are blue, red or green.
The three colors can be created optically. Here, the scanner's light is shown through a red, green or blue color filter before it strikes the item that is being scanned. An image is scanned three times, once with each filter, to create the three layers. The actual process of shining the light through a filter is simple. But, overall the method is relatively time-consuming because each image must be scanned three times. As well, problems can arise if the three scans are not aligned precisely. For example, if one of the scans is misaligned by even a pixel of less, discoloration and other problems can occur in the final image.
Instead of using filters, some scanners use three different light sources to create red, green and blue light. The lights turn on and off in a regular and coordinated way for each line of pixels scanned. The advantage with this approach is that the three images are obtained in a single scan of the document or photograph. Thus, the three images are aligned correctly.
A third approach uses a single white light. However, filters separate the red, green, and blue wavelengths of the visible light spectrum after the light reflects from the image being scanned. Each color of light is then focused onto its own row of light sensors. The three colors are read at the same time during one scan.
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