Colour scanners have three light sources, one for each of red, green and blue primary. Some scanning heads contain a single fluorescent tube with three filtered CCDs, while others have three coloured tubes and a single CCD. The former produce the entire colour image in a single pass, the target being illuminated by the three rapidly changing lights, while the latter have to go back-and-forth three times.
Single-pass scanners have problems with the stability of light levels when they’re being turned on and off rapidly. Older three-pass scanners used to suffer from registration problems along with being slow. More modern three-pass units are much improved and able to match some single-passers for speed. However, by the late 1990s most colour scanners were single-pass devices.
These scanners use one of two methods for reading light values: beam splitter or coated CCDs. When a beam splitter is used, light passes through a prism and separates into the three primary scanning colours, which are each read by a different CCD. This is generally considered the best way to process reflected light, but to bring down costs many manufacturers use three CCDs, each of which is coated with a film so that it reads only one of the primary scanning colours from an unsplit beam. While technically not as accurate, this second method usually produces results that are difficult to distinguish from those of a scanner with a beam splitter.