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. 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.