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The bit depth of a scanner is basically how precise the scanning element is in distinguishing between various shades of colors. The higher the bit depth, the more exact each individual color will be. Also, a higher bit depth will create a smoother transition between colors will be in the final file.
The minimum acceptable depth for color scans is 24 bits, which means that the scanner can distinguish between 16.8 million colors. For black and white and grayscale scanning, 8 bits are more than sufficient.
A scanner's resolution is how many dots or pixels per inch (listed as either DPI or PPI) a scanner reads from a document. The higher the number the more exact the scan will be and the better the final image. Resolution is sometimes listed with two numbers shown being multiplied by each other (600 x 300, for example). The lower number is the actual scanning resolution and the one that should be used in deciding the scanner's quality.
There are usually two types of resolution listed with a scanner: Optical and Interpolated. Optical resolution is the actual physical resolution that a scanner can read and the more accurate of the two. The interpolated resolution is how large a scan can be "blown up" using an included software algorithm. This should only be of interest to people who plan on enlarging their documents significantly.
For the average user, a minimum of 300 dpi should be more than sufficient for most uses. This will produce good image quality for both web and print presentations. As with everything, though, the higher quality you buy the more flexibility you will have later on.
Similar to bit depth, optical density is a measurement of the range of tones a scanner can see. It is measure from 0.0 (perfect white) to 4.0 (the blackest black). The higher the number the more accurate the tonal range will be.
A good tonal range for a flatbed is at least 2.8. For Film and Slide scanners you should shoot for at least 3.2. In either case, the higher the better. Going for more initially could save you a lot of time and trouble later.
A scanner's sensor is basically its eye. Right now there are two primary types of scanner sensors: CCD and CIS. CCD (Charge-Coupled Device) is by far the most common and most reliable of the two. CIS (Contact Image Sensor) is a new, smaller chip that allows for scanners with a much more compact design.
In general, a CCD is still the better way to go in that it is more accurate and much more reliable. A CIS based scanner is a good option for those who don't require the best image quality and want to maximize space.
Optical Recognition Software (OCR)
To produce editable text from a scanned document you need OCR. When an image is scanned, it is usually turned into a single static image file. Any text in the document is treated as an image and can't be changed. OCR software "reads" the text in a scan and turns it into something easily editable in a word processing program, like Microsoft Word.
There are a number of different ways to connect a scanner to your computer. The oldest, and slowest, is through a parallel port connector. Most new scanner, though, tend to connect via either USB or SCSI.
USB (Universal Serial Bus) is the most common and easiest connection. It is reasonably fast and a snap to connect. In fact, most USB scanners feature Plug and Play installation, which mean you just plug them in and they set themselves up with minimum input from you. A newer standard, USB 2.0, has recently come out that is much faster, but requires an adapter card or a computer that supports the standard.
SCSI (Small Computer System Interface) is an extremely fast high end connection used by many graphics professionals. While much more complicated to install, and requiring a special interface card on the computer, the SCSI standard is lightning fast and extremely reliable. SCSI is primarily reserved for graphics professionals and high-end users.
FireWire (IEEE 1394) is another new standard worth looking into which combines the speed of SCSI and the ease of use of USB. Unfortunately, it's still a new standard and still relatively expensive.