Although scanner specifications are valuable for assessing the performance of a particular model, they don't always reveal whether the manufacturer has used the best components available. Cost and other considerations often require that manufacturers make compromises during the design process, and many of these compromises are related to the choice of various key components. What follows is a quick review of some of a scanner's key components, with notes on what to look for in each.

Most scanners capture image data by reading light that's been reflected off, or passed through, the item being scanned. This process obviously requires a light source, and the quality of that component can have a dramatic impact on the quality of the resulting scan.

When desktop scanners were first introduced, many manufacturers used fluorescent bulbs as light sources. While good enough for many purposes, fluorescent bulbs have two distinct weaknesses: they rarely emit consistent white light for long, and while they're on they emit heat which can distort the other optical components.

For these reasons, most manufacturers have moved to "cold-cathode" bulbs that deliver whiter light and less heat. Fluorescent bulbs are now found primarily on low-cost units and older models. Contact image sensor (CIS) scanners employ dense banks of red, green and blue LEDs to produce white light.

To direct light from the bulb to the "eyes" that read light values, CCD scanners use prisms, lenses, and other optical components. Like eyeglasses and magnifying glasses, these items can vary quite a bit in quality. A high-quality scanner will use high-quality glass optics that are color-corrected and coated for minimimum diffusion. Lower-end models will typically skimp in this area, using plastic components to reduce costs.

Single-pass CCD 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 colors, 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 colors 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.

At the heart of any scanner is an electronic component that processes the analog signals from its sensors into digital data for the computer it's connected to. This is a sensitive process, and one that is susceptible to electrical interference and noise in the system.

In order to protect against image degradation, the best scanners on the market use an electrically isolated analog-to-digital converter that processes data away from the main circuitry of the scanner. However, this introduces additional costs to the manufacturing process, so many low-end models include integrated analog-to-digital converters that are built into the scanners' primary circuit board.