Browse Prior Art Database

Polygonal Mirror Scanner With Uniform Return Signal

IP.com Disclosure Number: IPCOM000037088D
Original Publication Date: 1989-Nov-01
Included in the Prior Art Database: 2005-Jan-29
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Cato, RT: AUTHOR [+2]

Abstract

This disclosure proposes a method for obtaining a uniform return signal from a rotating-polygon laser beam scanner used in a retroreflective system.

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Polygonal Mirror Scanner With Uniform Return Signal

This disclosure proposes a method for obtaining a uniform return signal from a rotating-polygon laser beam scanner used in a retroreflective system.

To obtain the maximum scan angle multiplication (degrees of beam motion per degree of polygon rotation) from a rotating mirror scanner, the polygon mirror facets are oriented parallel to the polygon axis of rotation and the incident laser beam is directed perpendicular to the axis of rotation. This configuration results in a 2:1 scan angle multiplication.

When the resulting scan line is used to read a bar code, scattered light from the bar code is reflected from the polygon face and is directed to a photodetector. The strength of the return signal depends on the effective area of the polygon face. The effective area of the polygon face depends upon the rotational position of the polygon and is equal to the actual area multiplied by the cosine of the rotation angle. The result is a variation in collected signal strength across the scan line.

The disclosed system uses the power modulation capabilities of diode lasers to make the return signal uniform across the entire scan line. The figure shows a typical laser output power vs. drive current curve. The drive current starts at i0 at the start of the scan line and decreases toward i1 as the polygon facet approaches its central position, where collection efficiency is maximized. As the polygon continues to rotate, the dr...