Digital scanner - How it works

Desktop scanners include;
- a light source,
- a lens,
- a charged-couple device (CCD),
- one or more analog-to-digital converters (ADCs).

The CCD's elements are all in a row, with one element for each pixel in a line. If there are 300 CCD elements for each inch across the scanner, the maximum potential optical resolution is 300 pixels per inch, also referred to as dots per inch (dpi). 600 CCD elements per inch, gives a potential optical resolution of 600 dpi.

The image is scanned one line at a time. The light bounces off the target and goes though the lens to the CCD. Each CCD converts the light to an analog voltage (which varies with the intensity of the light) and indicates the gray level for one pixel. The ADC converts the analog voltage to a digital value, using 8, 10, or 12 bits per color.

In theory, a 24-bit scanner offers 8 bits, or 256 shades, for each of the three primary colors—red, green, and blue (RGB). A total of 16.7 million colors, the maximum most high-end graphics boards will display. A 30-bit scanner offers 10 bits, or 1,024 shades, for each color. A 36-bit scanner offers 12 bits, or 4,096 shades. Almost all 30- or 36-bit scanners use the extra bits internally only, sending just a final 24 bits to the computer.

The advantage of more bits per color is simple: More bits means smaller steps in the intensity of the hue levels, steps subtle enough to be imperceptible to the human eye. That, in turn, diminishes posterization—sudden shifts in color from one pixel to the next where there should be a smooth gradation. It also lets you see more detail in dark areas, depending on the quality of other components in the scanner.

Another important assumption is that you're getting all the bits the manufacturer claims. As already noted, the only bits that count are the ones that the scanner uses for gray scale. But that doesn't mean that some vendors don't count the calibration bits to pump up the specifications.

If you don't expect to scan photos very often, 24 bits should be enough. If you'll be scanning photos or artwork regularly, you'll want a high-quality 30 bits to give you better detail in dark areas. Don't bother with 36 bits unless you'll be working with lots of transparencies: Transparencies have a greater density range than prints and can benefit from the extra bits to show more detail in dark areas.