The Automated Window Inspection Device (AWID) is a prototype apparatus for computer-controlled, noncontact inspection of windows, with reproducible positioning and electronic recording of data on the locations, sizes, and depths of flaws. The AWID was developed to accelerate and facilitate (1) postflight inspections of the windows on the space shuttle for damage by micrometeorites and (2) the recording, analysis, and retrieval of records of such inspections. The AWID could also be adapted to industrial inspection of window glass prior to cutting, and to inspection of windows on aircraft and land vehicles.

The AWID includes a scanner housing on a two-dimensional (x-y) translation mechanism on a portable frame that mates with posts on the window frame for proper location with respect to the window to be inspected. The translation mechanism comprises x and y drives, each actuated by a stepping motor under computer control for automated or manual scanning. The scanner housing contains two optical inspection instruments; a polariscope for locating both surface and subsurface flaws, and a refocus microscope with focus depth adjustable by use of a third (z-axis) stepping motor for measuring the depths of selected flaws.

The Polariscope in the AWID includes a liquid-crystal polarization rotator that enables operation in two modes: (1) illumination and imaging polarizations parallel for detecting surface flaws, and (2) illumination and imaging polarizations orthogonal for detecting subsurface flaws.

The polariscope can reveal otherwise invisible subsurface damage ("bruises") via the effects of subsurface stresses on polarized light. The polariscope includes an illumination and an imaging module (see figure). A lamp in the illumination module consumes a power of 3.8 W and features a nearly-square-appearing tungsten filament. A lamp lens of 6-mm diameter and 6-mm focal length projects an image of the filament onto an illumination lens. The image of the filament fills the aperture of the illumination lens, providing nearly uniform illumination across the entire field. The illumination lens projects a magnified image (about 20-mm diameter) of the lamp lens onto the window.

The polariscope exploits the approximately 4 percent of the illumination that is reflected from each surface of the window. By use of a beam splitter and an imaging lens, light from an 11-by-15-mm portion of the 20-mm-diameter illuminated area is focused into a miniature color video camera. On its way to the camera, the light passes through a fixed linear polarizer. As explained below, the polariscope can operate alternately in a surface-damage-detection mode and a subsurface-damage-detection mode.

In addition to passing through the lamp lens, light from the lamp passes through a fixed linear polarizer and a liquid-crystal variable polarization rotator. The polarization axes of the two linear polarizers are orthogonal. In the surface-damage-detection mode, a square wave with a frequency of 2 kHz and an amplitude between 2 and 3 volts is applied to the polarization rotator, causing the plane of polarization of the illuminating light to be turned about 90° and thus lie approximately parallel to the polarization axis of the polarizer in front of the camera. As a result, an image of surface flaws is formed in the camera in reflected parallel-polarized light, in the same manner as in the case of ordinary unpolarized light.

For operation in the subsurface-damage-detection mode, no voltage is applied to the polarization rotator, causing the plane of polarization of the illuminating light to remain perpendicular to the polarization axis of the polarizer in front of the camera. In this mode, the illumination reflected (without change of polarization) from the front surface is polarized perpendicularly to the axis of polarization of the polarizer in front of the camera and is thus prevented from reaching the camera. However, the stress field in the interior of the window causes the polarization of light reflected from the rear surface to rotate by an amount that depends on wavelength. As a result, any subsurface damage is seen as a colored image on a darker background. The characteristic time for switching between surface and subsurface polarization modes is 30 to 50 ms.

After a scan has been performed to locate surface and subsurface flaws in xand y, the refocus microscope is used to locate flaws in z. The microscope is mounted in the scanner housing next to the polariscope and is accurately positioned in x and y over each flaw to be inspected. The refocus microscope has an "extend-retract" feature to prevent any interference with the polariscope during x -y scans. The microscope is equipped with miniature strain gauges, which act as limit switches to prevent damage in collisions between the microscope and other objects near or on the window undergoing inspection.

Two desktop computers are used for control, analysis, and operator interface. Dedicated image-processing circuit boards relieve the computers of much of the analysis effort.

This work was done by Matthew J. Verdier and Frederick W. Adams of KennedySpace Center and Stuart M. Gleman, Stephen W. Thayer, Carl G. Hallberg, Joseph E. Kachnic, Curtis M. Lampkin, and Terry D. Greenfield of I-NET. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Physical Sciences category,or circle no. 157 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

Inquiries concerning rights for the commercial use of this invention should be addressed to

the Patent Counsel
Kennedy Space Center; (407) 867-2544.

Refer to KSC-11889.


NASA Tech Briefs Magazine

This article first appeared in the February, 1998 issue of NASA Tech Briefs Magazine.

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