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Image Scanner Basics
By  Geoff Priest
Updated Jan 2008
Points to consider before purchasing a scanner:
Bit Depth
Refers to the number of colors offered by the scanner. 24-bit color (16.7 million colors [2 to the 24th power]) is the maximum for most graphics boards today and is required for high-quality image manipulation. If you plan to scan color art or color photographs into your computer then be sure to purchase a scanner with a bit depth of at least 24, or even higher if you plan to scan artwork regularly. Though most scanners with bit depths higher than 24 send only a final 24-bit image to your computer, they are better at estimating colors in dark areas. Nowdays, a good scanner covers up to 48 bit color.
Resolution (Optical, Mechanical, or Interpolated?)
Measured in DPI (dots per inch), resolution determines the detail and clarity of an image. A scanner's optical resolution or 'true resolution', usually the smaller number given by the manufacturer, is the most important to consider. Since a resolution of 300 dpi is essential for high-quality prints, an optical resolution of at least 300 dpi is recommended. Less is needed however if the only concern is web/monitor graphics. As of 2008, even the most inexpensive flatbed scanner has more than 2400 DPI.
Some notes on interpolated resolution
Don't mind the DPI's of the 'Interpolated Resolution' since it is just a marketing buzz word. It indicates the scanner can resize and upscale the image for you before it reaches the computer (usually to ridiculously high DPI's) - the same effect can be achieved in any decent paint program.
'Manufacturers often claim interpolated resolutions as high as 19,200 ppi; but such numbers carry little meaningful value, because the number of possible interpolated pixels is unlimited.' - Wikipedia
Scan Area Size
If you plan to scan larger works, papers, etc a bigger scan area is to prefer. Most european scanners cover A4 or somewhat bigger, but if you have the bucks and desk space; go for A3!
Bundled Software
Most scanners today comes bundled with type of software for aquiring images and do basic operations with them, including USB drivers. If you use Mac or Linux - make sure its compatible!
Watch out for expensive software bundles which you may not need, such as the full version of Adobe Photoshop - unless you find some use for it. Usually manufacturers will offer two different packages in such cases.
OCR?
Optical character recognition, usually abbreviated to OCR, is the mechanical or electronic translation of images of handwritten, typewritten or printed text (usually captured by a scanner) into machine-editable text. Sometimes, scanners comes bundled with software for this task, which can be very useful when you're ready to put some larger printed text into your computer. Today, typical accuracy rates exceed 99%, although certain applications demanding even higher accuracy require human review for errors. Handwriting recognition, including recognition of hand printing, cursive handwriting, is still the subject of active research.
Type (Flatbed, Sheet-Fed, or Hand-held?)
Flatbed scanners are the way to go if you plan to scan a variety of images rendered on various materials. Sheet-fed scanners are only suited to single sheets of loose paper and/or photos. Hand-held scanners or 'half-page' scanners typically provide too little quality for too much work, but are very uncommon nowadays.
Connection (USB, SCSI, Parallel Port)
Make sure your computer can accomodate a USB based scanner. If your computer supports USB 2.0 (hi speed USB), get a scanner that also supports this interface; it will significantly speed up image transfer from the scanner to the computer.
Really old scanners (if you buy some old model off eBay, etc) may require a SCSI card in which case you will need an available slot. SCSI scanners are faster than scanners running through a parallel port, but provides no advantages over USB ones.
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