Civil Engineering Reference
In-Depth Information
4.14
SPECIAL PURPOSE LIGHTING FOR INSPECTION AND
QUALITY CONTROL
Special types of illumination can be used to detect faults in manufacturing. For example,
to make surface scratches on glass or plastic visible, it is common to use edge lighting
that is directed from the side. There are many other special types of light, including
polarized lights, cross-polarization, spotlights, convergent lights, and transillumination
(Faulkner and Murphy, 1973). The information given in Table 4.4 is adapted from
Eastman Kodak (1983), where more complete information is given. The second column
in the table describes special purpose lights or other aids, and the last column describes
how the techniques work.
Table 4.4
Special-Purpose Lighting for
Inspection Tasks
Desired
Enhancement in
Inspection Task
Special-Purpose
Lighting or Other
Aids
Technique
Enhance surface
scratches
Edge lighting can be used
for a glass or plastic plate
at least 1.5 mm thick
Internal reflection of light in a transparent
product; use of a high-intensity fluorescent or
tubular quartz lamp
Spotlight
Assumes linear scratches of known direction;
provides adjustability so that they can be aligned
to one side of the scratch direction; uses louvres
to reduce glare for the inspector
Dark-field illumination
(e.g., microscopes)
Light is reflected off or projected through the
product and focused to a point just beside the eye;
scratches diffract light to one side
Enhance surface
projections of
indentations
Surface grazing or
shadowing
Collimated light source with an oval beam
Moiré patterns (to
accentuate surface
curvatures)
Project a bright collimated beam through parallel
lines a short distance away from the surface,
looking for interference patterns (Stengel, 1979);
either a flat surface or a known contour is needed
Desired Enhancement
in Inspection Task
Special-
Purpose
Lighting or
Other Aids
Technique
Spotlight
Adjust angle to optimize visualization of these
defects
Polarized light
Reduces subsurface reflections when the axis of is
transmission parallel to the product surface
Brightness
patterns
Reflection of a high-contrast symmetrical image on
the surface of a specular product; pattern detail
should be adjusted to product size, with more detail
