Ins and Outs of Scanning - An Introduction

Flatbed scanners, which somewhat resemble a copy machine in their functionality, are the most common type of desktop scanner. Documents are placed onto a glass plate which the head moves underneath. Flatbed scanners are versatile. They have the ability to scan flat originals of various sizes and can even scan small three-dimensional objects. Flatbeds usually give the best combination of quality and flexibility.
Sheetfed scanners resemble a fax machine more than a copier. This is because a sheetfed moves the paper being scanned past the scanning heads instead of the other way around. Sheetfed scanners tend to scan pages less clear than a flatbed scanner because of the difficulty of moving the sheet of paper without introducing distortions in the document. Sheetfed scanners still work well for handling paperwork (mainly text).
Photoscanners, as their name implies, scan photos to turn them into digitized images. This type of scanner is usually fairly small and made especially for scanning color photos.
Resolution - is how many pixels a scanner can see in a given image. With a higher resolution you get a sharper image. With a lower resolution you would get a lower quality image. Higher resolution scanners tend to cost more but provide the best results. How much resolution do you need? If you will be printing your scans on a traditional printing press, and working with several different sized images, get the highest resolution you can afford. If you will be displaying items primarily on screen then a 300 dpi scanner should give you the resolution you are looking for.
Bit Depth is also another consideration. Different scanners record different amounts of an item scanned. These amounts are measured in bit depth. The higher the bit, the more details and color your scanner will record. Most color scanners are 24 bits. What does this mean for you? It means that they capture 8 bits of each of the primary scanning colors (red, blue, & green), and can capture over 16 million colors.
Dynamic Range is very similar to bit depth in that it measures how wide of range of tones the scanner can catch. The scale of measurement used for this ranges from 0.0 being a perfect white to 4.0 being perfect black. The dynamic number given for a scanner tells how much of that range it can distinguish.
The Scanning Method is also important to consider when choosing a scanner. In part, the scanned image's overall quality is determined by how the scanner collects data. Flatbed scanners take two different approaches:
CCD (charged coupled devices) scanners pass a light source over a document on a scanning bed and use mirrors and also a lens to focus the item on the chip.
CIS (contact image sensor) scanners have a single row of sensors placed very close to the document. The result of this is a scanner that is more slim and cheaper then CCD scanners, but does not offer the quality of images that the CCD's do.
Scanning Area should also be a consideration when buying a scanner. Most flatbed scanners have a range of 8.5" x 11" to 8.5" x 14".
Scanner Speed - How often you scan will determine how fast you need your scanner to scan. Generally if you scan a couple items a day, a slow scanner will probably not matter. But if you scan large numbers of documents, then you are going to want more speed.

www.apogeephoto.com/july2000/scanner.shtml - 6 steps to buying a scanner
desktoppub.about.com/compute/desktoppub/library/weekly/aa031097.htm - online info on buying a scanner
www.zdnet.com/zdhelp/stories/main/0,5594,2242674,00.html

last modified by jstratton November 4, 2002
thanks to www.grsites.com for the graphics