You may want to scan photo negatives and transparencies rather than documents. Many flatbed scanners can accommodate an optional transparency adapter unit that replaces the lid. Very simply, this device shines a bright light above whatever you place on the scanning plate so that the CCDs underneath can pick up the transmissive light passing through. Bear this in mind when you look at prices, because transparency adapter units can be very expensive when bought as an afterthought.

Flatbed scanners are not well-designed for transparencies, except at the high-end. Because of the small size and transmissive nature of the originals, you can never obtain as great a resolution or color match as with flat artwork and photo prints. This is one of the best kept secrets of the scanner industry. Professional scanners sidestep this limitation by providing a special high-power lens mode which doubles the resolution and increases the lamp brightness across a narrow strip down the scanning plate.

If you want to scan transparencies in earnest, invest in a slide scanner instead. These devices are small and compact, usually much smaller than a printer, and can scan anything from negative strips to mounted slides using a holder. They are more expensive than a typical entry-level flatbed scanner, but produce far superior results, at good resolutions of 1200dpi and higher.

A few tips:

1. Always try to get as large of films to scan as possible. A flatbed will do much better scanning a 2 1/4" piece of film than a 35 mm slide even at the same file size. This is because of the optics of the scanner and is not dependent on scanners resolution.

2. Remove any slide from the any mount and tape it to the glass. This is what they do with drum scanners because any bend in the film will cause strange rainbow color effects. You should do it with flatbeds.

3. Trans units run at different Gamma settings than flatbeds. Do some test to adjust for the best results and save those settings.

4. Select a white point and black point manually. The negative border fools auto color correction routines.

5. Buy a dedicated slide/transparency scanner.