24-bit color scanners: InfoWorld evaluates nine flatbed units for PC and Macintosh. (Hardware Review) (includes related articles on rating image quality, portable scanners, how the scanners were tested, alternative output devices and the Polaroid CS-500i color scanner and its SprintScan scanning software) (Evaluation)

Color scanners were introduced a few years ago, providing a method to transform hard copy color images into digital form. However, they were very expensive, bulky, and usually relegated to professional art departments or service bureaus.

As scanner technology evolved and prices fell, businesses began to jump into the scanner market, first with low-cost monochrome or gray-scale models and then slowly, as color output devices became less expensive, 8-bit (256-color) scanners gained broad appeal.

Twenty-four-bit scanners are still a rarity, but where only a couple of years ago a 300-dot-per-inch (dpi) gray-scale scanner cost more than $2,000, today you can buy many full-color scanners for well under that amount.

Most scanners have a standard 300-dpi resolution. This standard evolved mainly as output device resolution increased from 72 dpi to a maximum of 300 dpi. However, an increasing number of vendors are pushing resolutions up to 400 dpi, 600 dpi, and even higher. This may seem like a tremendous leap in technology to provide much more for a relatively small price increase. But if you dig deeper, most midrange scanner manufacturers can't offer "hardware" or "optical" resolution of more than 400 dpi. Therefore, in order to increase the effective dpi up to 1,600, they use built-in software to "interpolate" dots, enhancing resolution.

By using interpolation, the scanner uses a mathematical algorithm to "fill in the gaps" of an image, smoothing curves that would otherwise appear jagged and thereby giving you double or sometimes triple the real optical resolution. However, 1,200 dpi or even 1,600 dpi isn't quite good enough for high-quality commercial reproduction. For that you will need a scanner that costs well up in the five-figures range and is based on a different technology than the standard flatbed scanners tested in this comparison.

In this comparison we review nine units, some in both PC and Macintosh versions (but with different software). The PC/Mac group is AVR's 8000/CLX, Epson ES-300C, Hewlett-Packard ScanJet IIc, Microtek ScanMaker 600Z, and Sharp JX-320. In PC-only interface, we evaluate Digital's MD30C, SIIG AV800, and Umax UC630. The lone Mac- only product is Seiko SpectraPoint.

Despite the conception that scanners are highly complicated devices, they are very similar to the standard copying machine. A fluorescent light source is mounted on a carriage that moves along the length of the scanner's "bed" (glass plate). The light is reflected off the image and onto a charge-coupled device (CCD), a photosensor that translates the reflected light into variable amounts of electronic voltage. The voltage from each cell is then converted into digital values that the computer can interpret.

Color scanners work basically the same way as gray-scale scanners, with the essential difference that the light source passes through three color filters: red, green, and blue. A 24-bit color scanner processes 8 bits of information for each of the three colors, for a total of 16.7 million possible colors.

Generally, less expensive scanners make three "passes" at the image, once for each of the three color filters. Some of the most expensive units use three different colored lights, as opposed to filters, so that only a single pass is required. This technology is more efficient and usually faster than the three-pass method.

When you are choosing a scanner, there are many factors to consider. If you are going to scan documents only for optical character recognition (OCR), a 400-dpi monochrome scanner is your best bet. However, most people never use a scanner exclusively for one task, and the cost of a monochrome scanner is very close to, or sometimes even higher than, a color scanner. Thus a color scanner may be your best bet if you suspect your needs might include color; the price difference won't be large enough to scare you off.

Once you have decided on a color scanner, there are many other options you should consider: effective resolution, speed, bundled software, and compatibility. The actual quality of the scans can be variable depending on the platform (Macintosh or PC), driver software, scanner settings, and software used to scan the image.

Scanner resolution, like printer resolution, is measured in dpi. Most of the scanners in this comparison range from 300 to 1,600 dpi (using interpolation). However, scanning images at 300 dpi and above will produce files that use up an incredibly large amount of storage without improving the quality on any of the most common output devices. For example, a 5-by-7-inch photograph scanned in 24-bit color at 300 dpi and saved as a TIFF file is larger than 8 megabytes. The QMS color PostScript printer's highest resolution is 300 dpi, the RasterOps dye-sublimation printer is 300 dpi, and the Canon Bubble-Jet is 150 dpi. If you scan a file at a higher resolution, these devices have to interpret the greater amount of information, and the image can come out muddy. Thus, if you plan to use standard color output devices, scanning in 24-bit at 300 dpi should be more than adequate; only if you plan to go to a service bureau that has a Linotronic 1200 or 2400 will scanning at a greater resolution produce a sharper image.

If you have varying output needs, it is important to consider what resolutions a scanner is capable of producing. The more steps available, the more easily

you can fine-tune the inevitable compromise between image quality and file size.

Bundled Software:

Most of the scanners in this comparison are bundled with some type of image editing software. The quality of the software, and the features it offers, can greatly affect the "real" cost of the scanner. It's not going to be to your advantage to save a few hundred dollars on a scanner if you have to purchase additional image editing software, which runs around $800. (For a complete evaluation of high-end image editing packages, refer to our recent comparison, August 10.)

Compatibility:

Each image editing package is compatible with a wide variety of scanners, and there is no real standard for scanner drivers. However, this problem is rapidly disappearing as the TWAIN standard for input devices is established. (See sidebar.) So if you have a particular application you want to use, you will want to make sure the …