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Methods for the Reduction of Paper Noise in a Bar Code Scanner

IP.com Disclosure Number: IPCOM000034464D
Original Publication Date: 1989-Feb-01
Included in the Prior Art Database: 2005-Jan-27
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Dickson, LD: AUTHOR [+2]

Abstract

A holographic bar code scanner can provide a large effective depth of field by having individual facets on the holographic disk focus at different distances from the disk. To achieve satisfactory performance throughout the full depth of field of the scanner, the laser beam diameter at the holographic disk must be large enough to yield acceptably small focussed beam diameters at the longest focal lengths. In a large-depth-of-field scanner, this will result in extremely small beam diameters at the shortest focal lengths. This creates a problem when scanning bar codes on rough textured surfaces, such as normal paper, as the small scanning spot will "see" the surface texture. This adds "paper noise" to the light reflected from the bar code, lowering the probability of getting a correct scan and decode.

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Methods for the Reduction of Paper Noise in a Bar Code Scanner

A holographic bar code scanner can provide a large effective depth of field by having individual facets on the holographic disk focus at different distances from the disk. To achieve satisfactory performance throughout the full depth of field of the scanner, the laser beam diameter at the holographic disk must be large enough to yield acceptably small focussed beam diameters at the longest focal lengths. In a large-depth-of-field scanner, this will result in extremely small beam diameters at the shortest focal lengths. This creates a problem when scanning bar codes on rough textured surfaces, such as normal paper, as the small scanning spot will "see" the surface texture. This adds "paper noise" to the light reflected from the bar code, lowering the probability of getting a correct scan and decode. This article discusses three solutions to this problem. 1. The first solution is to introduce a controlled amount of astigmatism for the short focus facets. This can be accomplished, for example, by introducing an orthogonal pair of cylindrical lenses into the object beam path when constructing the master holographic elements for these facets. The separation of the lenses can be used to control the amount of astigmatism introduced. When this astigmatic facet is illuminated by the laser beam in the scanner, it will focus astigmatically. The beam will focus to a vertical line at the x-axis focal point and a horizontal line at the y-axis focal point. In between these two focal points, the caustic of rays will converge to a "circle of least confusion." By selecting the two focal points properly, one can make the circle of least confusion large enough so that the scanning "spot" will not see the paper texture and will, therefore, not introduce paper noise in the reflected laser light. This separation of x-axis and y-axis focal lengths can be different for each facet so that the effect can be optimized at each focal length. At the focal points, the elliptical "focal-lines" will also act to average out the paper noise. The net result will be that the paper noise will be reduced throughout the total depth of field for each facet. 2. The second solution uses an elliptical, but stigmatic, beam to produce an effect that is similar to Solution 1 at the focal point of the facet. This solution has an advantage over the astigmatic solution in that the beam diameter will increase more slowly to either side of the focal point. If the reference laser beam incident on the holographic scanning disk is elliptical in cross-section, with the long axis of the...