FLAT BED SCANNERS

Flat Bed Scanners are devices designed to image any flat art,  manuscripts,  photos, maps, film positives/negatives or flat glass plates. Items are placed face down on  a flat bed of clear glass. A light source illuminates the side to be captured, and a trilinear array passes under the material and scans it.

Resolution is usually defined as the number of active sensors on the array. However, there must be  an adequate  number of pixels on the array to cover a specific geometric area. This value is usually referred to as PPI or Pixels Per Inch. Flat beds are always limited to the size of the physical bed. Therefore, works over 11"X17" are usually considered "oversized" and require large units, both in terms of the size of the bed and the sensor, since a larger scan area will require greater sampling to achieve standards from 300-1200  PPI. At that point, the use of Planetary Scanners (digital scan backs mounted on copy stands) may prove to be more economical.

As with any sensor, bit depth and dynamic range is also a consideration.