Illumination Device for Inspecting Window Surfaces

John F. Kennedy Space Center, Florida

A special-purpose fiber-optic illumination device can be attached temporarily to a window to help an inspector examine window surfaces for scratches, pits, small-scale roughness, and dirt. This device was developed to replace an older fiber-optic illumination device that provided intense illumination to an area not much larger than the cross section of the fiber-optic light guide, had to be scanned manually over the entire window surface to complete an inspection, and often caused bright light to be reflected directly into the inspector's eyes during maneuvers ancillary to scanning. The present device illuminates a much larger area, does not shine bright light into the inspector's eyes, and leaves the inspector's hands free.

The basic illumination strategy is to launch the light into the window at an angle for which the light is totally internally reflected at smooth, flat, parallel window surfaces. Because of the total internal reflection, such surfaces appear dark when viewed from outside the window. However, surface imperfections scatter incident light, so that they appear bright when viewed from outside the window. Thus, small surface imperfections in a large area can readily be identified because they appear bright against a dark background.

The unique aspect of the present device lies in a fixture that aligns the output end of the fiber-optic light guide at the required angle and couples the light into the window (see figure). A transparent acrylic block holds the light guide at an angle of about 60° from the perpendicular to the surface of the window. The light emerges from the end of the light guide and travels through the block, through a thin interfacial layer of clear material, and into the window.

The interfacial layer can be made of clear rubber attached to the acrylic block by a clear glue. Alternatively, a liquid (e.g., water, an alcohol, an oil, or a glycerine compound) can be used as the interfacial material. In either case, it is necessary that the interfacial material fill the entire gap between the window and the block and have an index of refraction approximately equal to the indices of refraction of the acrylic and window materials, to suppress reflections at the window/block interface.

The block is mounted in a frame that can be attached to the window by use of suction cups. The depth of mounting within the frame can be adjusted so that when the suction cups are pressed against the window, the clear rubber sheet is also pressed tightly against the window.

As an unavoidable consequence of the fan-out of light from the end of the fiber-optic light guide, some of this light travels at angles that are not within the range for total internal reflection. This portion of the light is partially reflected and partially transmitted at the window surface. In principle, one could incorporate a lens to reduce the fan-out, but in a typical case, the small increase in performance would not be worth the added complexity.

Because of small differences among the indices of refraction of the block, window, clear rubber, and glue, a small portion of the incident light is reflected from the interfacial region. This reflection could distract the inspector. Therefore, the exposed optical surfaces of the acrylic block are painted black to obscure the interfacial region from view.

This work was done by Barry Braden of Kennedy Space Center,Robert C. Youngquist and Robert B. Cox formerly of I-NET, Inc., and Charles R. Floyd of United Space Alliance.

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

the Technology Programs and Commercialization Office
Kennedy Space Center
(407) 867-6373

Refer to KSC-11966.

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