Identifying Glass Fiber, Why Polarized Oblique Illumination
Glass fiber can only be accurately identified using a polarized light microscope and by being aware of the possible
interferences. The images below provide examples demonstrating the importance of polarized light and natural substances
that can be confused with man-made mineral fiber. It also shows the need for oblique illumination when assessing
environmental samples. Environmental samples are in a fixed mounting medium. By using a permanent mounting medium in
the range of 1.49 to 1.51 oblique illumination will quickly separate opaline materials, phytoliths and sponge spicules,
from glass fiber.
Bird Feather Barbules, Need for Polarized Light
Bird feather barbule fragments are very common in indoor environments. Their sources include bedding, clothing, birds as pets, and wild
bird debris common in the outdoor environment that enters the indoor environment as part of the normal air exchange. At a scanning
magnification of about 100X many of these fragments look like glass fibers if polarized light is not used. With polarized light it is
quickly apearant that the fragments are birefringent and not glass. The series below show first the image without polarized light and then
with off-crossed circular polarized light.
Plastic Fiber
This photograph was taken with off-crossed circular polarized light. The two vertical fibers on the left of the image are plastic fibers.
They are birefringent and are brighter than the background, while the glass fibers are isotropic and about the same brightness as the background.
Phytoliths, Need for Oblique Illumination
The phytoliths shown here are the opaline silica phytoliths common in grasses. These are isotropic but have a lower refractive index
than man-made glass fiber. All of these photographs were taken with oblique illumination and the shadowing on the fibers show them to be
lower in refractive index than the 1.486 medium they are mounted in. The first few photographs in this series contains a glass fiber and a
phytolith. The difference in the shadowing of a man-made glass fiber and a phytolith in this mounting medium can be seen here.
Diatoms, etc., Need for Oblique Illumination
Diatoms generally have enough distinct morphology to identify them as other than glass fiber. However, diatom fragments, radiolaria
fragments, silicoflagellate fragments, and rotifer fragments can look like glass fibers. These are all opaline silica and will have low
refractive indices.
Sponge Spicules, Need for Oblique Illumination
Opaline silica sponge spicules are another source of glass fiber look-a-likes. These also have low refractive indices. This type of
particle is rare in indoor samples but does occur.
Insect Hair (Setae)
Some insect setae may be mistaken for glass fiber during a scan at 100X. These fibers are generally low in birefringence but their
birefringence is evident if crossed polars are used. Oblique illumination often makes characteristic morphology visible at 100X so that
these particles can be correctly identified.
Detection Problems, Need for Oblique Illumination
Without oblique illumination glass fibers can be easily missed. To assess exposure in environmental settings large areas of surface must
be scanned. The only efficient way of creating sufficient contrast for rapid detection of glass fibers is to use oblique illumination. The
photographs below illustrate the problem.
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