Specifications

As with other types of scanners, reflected light is used to start the imaging process, but flatbed scan-
ners require a more precise design than hand or sheetfed scanners because the light that reflects off
the document has a long way to go afterward (and even before because scanning colored images
requires the light to go through red, green, and blue filters first). See Figure 9.
The light that bounces off the document is reflected through a series of mirrors to light-sensing
diodes that convert the light into electricity. The electricity is sent to an analog-to-digital converter
(ADC) that converts the electricity into digital pixels that can represent black-and-white, gray tones,
or color (if the original was scanned in color). The digital information is sent to the computer, where
your application determines its future as text, graphics, or a bad scan that must be redone.
Pivoting mirror
Pivoting mirror
Cover glass
Light source and mirror
Light-sensing diodes
and A/D converter
Interface (SCSI, Parallel, or USB)
Lens
Drive mechanism
Light Path from Light Source to A/D Converter
Figure 9 The path from the light source to the A/D converter is shown here.
Advantages of Flatbed Scanners
■ Flexible media handling. Even the simplest flatbed scanners, such as flatbed copiers, can handle
documents of varying sizes, from small scraps of paper and wallet-sized photos to letter-size
documents and books. More sophisticated scanners with automatic document feeders can even
handle legal-size 8.5”×14” documents. Add a transparency adapter, which shines light through
the item to be scanned, and your scanner can also handle negatives, slides, and filmstrips of
varying sizes.
Several ways of handling transparency media are available. The least expensive way is an
adapter that will reflect light from the normal scanner mechanism behind and through the
media. This unit, which might be capable of handling a few 35mm slides or a single strip of
negatives, is intended for casual use. More serious users will opt for a true transparency adapter.
Some models have a lid that doubles as a transparency adapter for up to 4”×5” slides, whereas
others use a device that replaces the normal scanner cover and contains its own light source
and mirror mechanism. The scanner driver is used to select this adapter as the scanning source,
and the normal light source inside the main scanner body is turned off. The most sophisticated
variation found in very high-priced scanners is a media drawer that slides open and enables
transparencies to be slid into the body of the scanner. Of these three, only the media drawer
technology provides for true high-resolution scanning of small 35mm slides and negatives.
■ High resolution made even higher through interpolation. Unlike simpler types of scanners, flatbed
scanners actually have two resolutions: optical and interpolated. Optical resolution refers to the
actual resolution of the scanning optics—the hardware. To achieve higher resolution, flatbed