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Measuring Surface Roughness

IP.com Disclosure Number: IPCOM000079004D
Original Publication Date: 1973-Apr-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Grimm, MA: AUTHOR [+2]

Abstract

This apparatus measures surface roughness by a noncontact method, which directs a laser beam incident to the surface at a small angle. The resulting reflected beam of laser energy is focused at the plane of an energy chopper. A photocell then detects the chopped laser beam. If the surface is smooth, the photocell output is a square wave. As surface roughness increases, the photocell output approaches a sine wave. Thus, the shape of the photocell output waveform is a measure of surface roughness.

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Measuring Surface Roughness

This apparatus measures surface roughness by a noncontact method, which directs a laser beam incident to the surface at a small angle. The resulting reflected beam of laser energy is focused at the plane of an energy chopper. A photocell then detects the chopped laser beam. If the surface is smooth, the photocell output is a square wave. As surface roughness increases, the photocell output approaches a sine wave. Thus, the shape of the photocell output waveform is a measure of surface roughness.

Surface 10 represents the surface whose roughness is to be measured, e.g., the magnetic oxide surface of a rigid magnetic disc or flexible magnetic tape. A low-power source of visible laser energy 11 is directed through beam expander 12 and long-focal length imaging lens 13. The converging beam 14 is incident on surface 10 at an angle 15, which is less than the angle at which specular reflection occurs, and is focused at the plane of a beam chopper 16.

The chopper is driven at a constant speed by motor 17. Chopper 16 includes alternating opaque and transparent regions of equal arc length, uniformly spaced about the circumference thereof. The number of opaque regions and the speed of chopper rotation is such that the reflected laser beam is chopped at a frequency of from 500 to 1,000 Hz.

The chopped, reflected beam passes to large-area photodiode 18. The photodiode output waveform is AC coupled to preamplifier 19, where it is amplified and passed on to spectrum analyzer 20 and visual displ...