A class of optoelectronic noncontact computer-controlled scanning surface profilometers is undergoing development for use in automated or semiautomated in- spection of nominally flat surfaces. When fully developed, these profilometers would generate three-dimensional maps of the scanned surfaces for characterization of such defects as pits and scratches.

A profilometer of this type includes a two-dimensional (x,y) horizontal-translation stage, on which the object to be inspected is mounted with the surface of interest facing upward toward a stationary optical head. The optical head contains a laser diode aimed to project a beam of light downward at an angle (22-1/2° in the prototype profilometer) to the perpendicular to the surface of interest. The height of the optical head is set at a nominal vertical distance [0.25 in. (6.35 mm) in the prototype] above the surface of interest. When the vertical distance between the optical head and the surface equals this nominal distance, the spot of light formed on the surface by the laser beam is at a nominal central horizontal position. When the surface lies at a different height, the spot of light is formed at a different horizontal position.

The optical head contains optics that focus the light reflected from the laser-illuminated spot to a corresponding spot on a one-dimensional position-sensitive detector (PSD), which is a photodetector with electrodes at opposite ends of a line. The deviation of the position of the spot of light along this line from a nominal central position depends on the deviation of the illuminated spot on the surface of interest from the nominal vertical distance below the optical head. The spot of light focused onto the PSD gives rise to photocurrents in the two electrodes; the relative values of these photocurrents depend on the position of the spot of light along the line between the electrodes and thus indicate the local deviation of the height of the surface of interest.

Under computer control, the translation stage is actuated to scan the optical head across the surface. For each increment of horizontal position (x,y), the optical head generates photocurrents indicative of the local deviation of the surface from the nominal vertical distance. The computer builds up a three-dimensional map of the surface from the ensemble of vertical-deviation data for all increments of horizontal position.

The prototype profilometer has been tested in scans of several objects, including a shiny new dime and specimens of diffusely reflective materials. The results of the tests suggest that surface deviations could be routinely resolved to within 10–4 in. (5 μm).

This work was done by Jeffrey A. Hooker and Stephen M. Simmons formerly of I-NET for Kennedy Space Center.

NASA has granted Laser Technology, Inc., an exclusive license for this technology. Inquiries concerning the commercial use of the “Noncontact scanning Surface Profilometers” should be addressed to:

John Newman, President
Laser Technology, Inc.
1055 W. Germantown Pike
Norristown, PA 19403
Tel. No.: (610) 631-5043

Refer to KSC-11759

NASA Tech Briefs Magazine

This article first appeared in the October, 2002 issue of NASA Tech Briefs Magazine.

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