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More than 80 percent of aircraft inspection is
performed visually. Aircraft inspectors illuminate the exterior
surface and hard-to-access interior areas of a plane from a number
of different angles. Through visual examination, the trained eye
can detect glints or shadows that could indicate cracks, scratches,
corrosion or other underlying structural problems.
During close inspection, contrast between the background and the
target area being illuminated is critical. Often the structural
defect is obvious, but sometimes it is subtle. For example, wavy
surfaces — which are detected by grazing-incidence illumination
— may signal corrosion below the surface.
Aircraft inspectors typically use handheld, battery-powered flashlights
for local lighting. However, many commercial flashlights produce
poor beam quality (irregular shape and illuminance distribution)
because of the bulb filament and shape of the reflector. Sometimes
there are dark spots or holes in the beam. These factors may reduce
the useful area of the flashlight beam to 70 percent or more below
peak beam illuminance. The visual result is loss of contrast to
the human eye. An Inspector using an uneven beam may be looking
for a site that, at times, appears to be in a shadow. Inspection
efficiency and accuracy suffer.
A simple and low-cost improvement adaptable to most commercial
flashlights is the light-shaping diffuser from Physical Optics Corp.
in Torrance, Calif. As a substitute for the flashlight's clear lens
element, this lightweight, holographic diffuser provides even illumination
with up to 95% of the light transmitted to the target. A proprietary
holographic replication process enables mass production using similar
methods as those used to produce security holograms. |
| Figure 1. A standard flashlight
illuminates a surface unevenly. |
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My field trials have shown that, for close inspection work,
aircraft maintenance personnel from several airlines preferred
flashlights equipped with the diffuser over nondiffused flashlights.
Inspectors participating in the study readily adopted the
new technology, and the Federal Aviation Administration is
now encouraging it.
Light-shaping diffusers effectively solve the problem
of uneven illumination by homogenizing light, smoothing out
irregularities in the beam, and eliminating glare and hot
spots. By using a uniform, homogenized beam as the illumination
source, an aircraft inspector is more likely to concentrate
on the surface under observation than on the beam properties.
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Meet the author
Richard N. Shagam, PhD, is a senior member of the technical
staff of the Aging Aircraft Program at Sandia National Laboratories
in Albuquerque, N.M. This work was performed, in part, for
the Federal Aviation Technical Center under U.S. Dept. of
Transportation Contract #DTFA-03-91-A0018.
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| Figure 2. A flashlight covered
by a light-shaping diffuser improves the ability to
detect flaws. |
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