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Image Scanner Basics
By  Thomas F Abrahamsson
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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