| 摘要 |
In order to control laser beam propagation, working range and beam cross section in a bar code scanner, diffractive optics are used to modify the amplitude and/or phase distribution of the beam in the scanner. The beam is diffracted by a mask which has a plurality of apertures in the form of segments which are arranged across the beam cross section in the vicinity of a converging lens. The segments are sized (A, a) and their transmission functions selected (one segment having for example 100 % transmission (A) and the other having 50 % transmission (a)) such that bar cedes close in (dwa1) and far out (dwa2) from the scanner are in regions of far field diffraction of different segments, or where such regions overlap, thereby extending the working range (w) of the scanner where bar codes of high density can be resolved.
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