Colour scanners

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.

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When a scanner … something into digital form, it looks at the image pixel by pixel and records what it sees. That part of the process … simple enough, but different scanners … different amounts of information about each pixel. How much information a given scanner records … … by its bit-depth.

The simplest kind of scanner only records black and white, and … sometimes … as a 1-bit scanner because each bit … only express two values, on and off. In order to see the many tones in between … … …, a scanner needs to be at least 4-bit (for up to 16 tones) or 8-bit (for up to 256 tones). The higher the scanner's bit-depth, … accurately it can describe what it sees when it looks at a given pixel. This, in turn, makes for a higher quality scan.

Дата добавления: 2015-07-20