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Ambient Light Rejection for Laser Scanners

IP.com Disclosure Number: IPCOM000083423D
Original Publication Date: 1975-May-01
Included in the Prior Art Database: 2005-Mar-01
Document File: 2 page(s) / 26K

Publishing Venue

IBM

Related People

Soderstrom, RL: AUTHOR

Abstract

Scanner 10 employs a laser 11 and a deflector 12 to project light beam 13 in a scan pattern onto an object plane 14. Diffuse reflections 15 are collected in photomultiplier tube (PMT) 16. Scanner 10 operates in a high ambient-light environment. To improve the signal-to-noise ratio, a multilayer narrow-band filter 17 is placed over the face of PMT 16.

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Ambient Light Rejection for Laser Scanners

Scanner 10 employs a laser 11 and a deflector 12 to project light beam 13 in a scan pattern onto an object plane 14. Diffuse reflections 15 are collected in photomultiplier tube (PMT) 16. Scanner 10 operates in a high ambient-light environment. To improve the signal-to-noise ratio, a multilayer narrow-band filter 17 is placed over the face of PMT 16.

It is common practice to center the filter bandpass L(o) at the laser wavelength, for example L=632.8 nm. But filter center wavelengths vary with the angle of incidence a according to the relation: L(a) =L(o) X the square root of 1- sin/2/ a/n/2/ where n is the index of refraction of the filter material. For a maximum angle a=20 degrees and the largest practicable index n=2.0, the filter center wavelength is shifted as much as 9.4 nm at the edges 18 of its field. The actual bandwidth must be twice 9.4 nm, plus an allowance for tolerances, or about 25 to 30 nm.

To reduce the filter bandwidth, and thus the amount of ambient light entering PMT 16, its center wavelength is raised by half the wavelength shift. In the above example, L(o)=632.8 + (9.4/2)=637.5 nm. The filter bandpass may then be reduced by a factor of about two, to 12 nm. The filter bandpass is centered about the laser wavelength L=632.8 nm at an incidence angle a=14 degrees. The transmission of the laser light 13 may be maintained nearly constant over angles up to a=20 degrees by using three layers (or cavities),...