Tech Briefs

Surface flaws can be scanned automatically and displayed in real time.

A scanning optoelectronic instrument generates the digital equivalent of a three-dimensional (X,Y,Z) map of a surface that spans an area with resolution on the order of 0.005 in. (≈0.125mm). Originally intended for characterizing surface flaws (e.g., pits) on space-shuttle thermal-insulation tiles, the instrument could just as well be used for similar purposes in other settings in which there are requirements to inspect the surfaces of many objects. While many commercial instruments can perform this surface-inspection function, the present instrument offers a unique combination of capabilities not available in commercial instruments.

Figure 1. The Camera Observes the Line of Light projected on the surface of a plate by a tilted laser. Any deviation of the line from its nominal position is indicative of a hole or bump on the surface.
This instrument utilizes a laser triangulation method that has been described previously in NASA Tech Briefs in connection with simpler related instruments used for different purposes. The instrument includes a sensor head comprising a monochrome electronic camera and two lasers. The camera is a high-resolution unit with digital output. The sensor head is mounted on a computer-controlled, servomotor-actuated translation stage at a fixed height above the nominal X,Y plane. Scanning is effected by using the translation stage to position the sensor head repeatedly at small, equal increments of Y until the entire surface has been traversed in the Y dimension.

Figure 2. The Electronic System of the Instrument includes an onboard processor that generates 3D data in real time, making it possible to display images of the scanned surface in real time.
Figure 1 depicts the basic optical layout for the laser triangulation. The camera is aimed downward (in the –Z direction). Each laser is equipped with optics to project an X-oriented line onto the nominal X,Y plane at a nominal Y position, and is tilted at a known angle of incidence. At each incremental position along the scan, the camera records the image of the laser-illuminated line on the surface. The camera is oriented so that pixel rows are X-oriented and pixel columns are Y-oriented.

The X coordinate of each surface point in the image of the line is obtained by direct correspondence between X and the pixel-column number. Any deviation of the laser-illuminated line from its nominal Y position (and, hence, its nominal pixel-row number) indicates a deviation of the surface from the nominal X,Y plane. The image is digitized and the depth (Z) of the surface at each point along the line is calculated from the Y (pixel-row) deviation by use of a standard triangulation equation. The Y position of each point along the line is obtained from a combination of (1) the known Y position along the scan, (2) the aforementioned Y deviation of the illuminated line, and (3) another standard triangulation equation to correct for the effect of Z on the apparent Y position. The process as described thus far is repeated at each increment of position along the scan. The data collected at all the increments of position are assembled to produce a three-dimensional (3D) map of the surface.

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