Flatbed desktop units now are widely considered to be competitive with traditional drum scanners for many operations, with the points of differentiation have shifted to productivity, image size, and maximum resolution. Still, there are a number of attributes that can impact the buying decision.
Resolution is the standard measure of scanner quality.
For an accurate evaluation, though, its important to compare the optical, or mechanical, resolution of each device. The other resolution issue is that depending on the device's design, the maximum resolution may not be supported over the entire scan area. Lenses may be used to reduce the effective size of larger originals to fit the CCD array dimensions, thereby reducing the maximum capture resolution. Dynamic range is a logarithmic measure of the sensitivity of the scanning sensor, especially in the shadow areas. Product specs can be given in terms of Dmax or dynamic range.