In 1669 Danish scientist Erasmus Bartolin discovered that calcite
crystals could polarize light by a phenomenon called double refraction,
now commonly called birefringence. In the mid-1960s there was
speculation that Bartolin was predated by more than half a millennium
by some sailors, Vikings, who used the crystals to locate the sun
through cloudy arctic skies for navigation.
Regardless of who discovered it, used it first, or explained it,
birefringence has important applications for ophthalmic dispensers. The
basis of these applications lies in the fact the stress on some
transparent materials causes birefringence or a partial depolarization
of polarized light passing through the lens. Unfortunately only a few
dispensers today are making use of this practical and easily mastered
tool.
Remember air-tempered glass lenses?
The uneven pressure within the lenses gave rise to a birefringence that
can be easily detected with a colmascope or polariscope (Fig. 1.) As
air tempering became obsolete and plastic replaced glass, colmascopes
were eliminated from the tools of many dispensers.
BUT WHAT ABOUT TODAY?
Lens stress can still provide important clues to help dispensers
avoid problems with new eyewear and you can easily use birefringence as
a tool for detection, analysis, and to guide you in correcting problems
and potential problems. Also, if you don’t have a colmascope, it is
easy enough to devise one (Fig. 2).
MECHANICAL PROBLEMS
Visualized stress can help predict lens chipping and fracturing. A
quick colmascope enables you to identify lens areas that will be prone
to chipping and fracturing. A lens “too tight” in the frame can chip
when even a small amount of twist or bend is applied. The area that is
chip-prone will show up as a light area in a colmascope and the
brighter and more concentrated the area of light, the more chip-prone
that area will be. Usually, simply loosening the eyewire screw a
half-turn or so will relieve the stress and prevent chipping during
dispensing. Chipping most often occurs if the temples are not splayed
enough for the patient’s head at the outset. Once the frame is
completely aligned, the eyewear screw can be safely tightened—an
important step that should not be overlooked.
Drill-mounts are most likely to chip around these holes when the
birefringence patterns are sharply defined (Fig. 3). A little practice
will tell you when to be especially careful as you adjust the
drill-mounted eyewear.
VISION PROBLEMS
Lenses too tightly clamped in an eyewire may develop warped areas
that can make adaptation difficult. Often this clamping causes stress
that can be easily seen on a colmascope. If you suspect a patient with
adaptation problems may have a warpage problem, the lens must be
removed from the frame, sized down slightly, heated gently in warm (AR
lenses) or hot water and reinserted. You should do a colmascope check
for stress before going through all this. Warped areas, once
identified, are often best seen by reflected rather than transmitted
light. Lenses with very thin edges or very thin centers are especially
prone to warpage.
POLARIZING LENSES
Polarizing
lenses represent one of the absolute best of today’s ophthalmic
technologies. They selectively eliminate useless reflected glare and
give an easily perceptible improvement in sunwear performance.
If polarizing sun lenses arestressed by being too tightly mounted
in the frame, they will show “patchy” transmission of polarized light
because of birefringence. A colmascope is not needed to view this
decreased performance. Simply placing the lenses in front of a
polarizing light source and rotating them for maximum darkness will
quickly reveal any patchy lightened areas. It is important to recognize
that small, peripheral lightenedpatches will be of no practical
significance, but a large or central lightened area can seriously
affect lens performance.
If you have a polarizing demonstrator, you may be able to us it to
detect areas of birefringence by just holding the polarizing sun
eyewear in front of the unit. An even easier method is to hold a plano
polarizing plastic (not poly) blank in front of the lens you are
testing, rotate the blank until it appears to be dark when viewing
through the sun lens you are testing. Any light or patchy areas will be
apparent (Fig. 4). This test should be done after the patient’s new
lenses have been mounted in the frame.
ABOUT POLYCARBONATE
Most polycarbonate lenses have internal stress that will give a
colorful birefringence pattern in the colmascope. This internal stress
does not affect vision nor does it prevent locating chip prone areas.
Birefringence is an easy-to-use dispensing tool that can help you
avoid chipping. It can warn you of warpage and it can assure that your
patient’s get top-notch performance from their polarizing sunglasses.
The Vikings used it, Erasmus Bartolin challenged the wave theory of
light with it and today’s dispensers can and should use birefringence
to make their jobs a little easier
and their patients a little happier.
Palmer R. Cook, OD, is director of professional education for Diversified Ophthalmics in Cincinnati.
