Talk Given By D. B. Whitney at Montgomery-Frost Dinner on
June 5, 1958
The history of plastic lenses at American Optical Company is a long
one. As far back as 1937, work was going on in an attempt to find a material
suitable for ophthalmic lens application. At that time, the only available
material having satisfactory optical properties were molded Methyl Methacrylate.
This material has two drawbacks; first, its scratch resistance was poor,
and secondly, it was a thermoplastic material. By thermoplastic, we mean
that the material would soften when heated, and this is a disadvantage
for ophthalmic lenses since such lenses are sometimes submitted to rather
high temperatures, such as when left in a closed automobile.
Shortly prior to World War II, a new material became available which
showed some promise. This material was a Copolymer of Methacrylate. A Copolymer
is actually a mixture of resins, and in this case, the mixture was Methacrylate
and Methacrylic Anhydride. This material was used by American Optical Company
for a period of time, and while a definite improvement over Methacrylate,
was not ideal partly because of color. The material did show promise, but,
unfortunately, its production was abandoned by DuPont, its sole maker,
and so, again, straight Methyl Methacrylate was about all that was available.
During World War II, a brand new material was developed by the Columbia
Southern Chemical Division of the Pittsburgh Plate Glass Company. This
material, named Allyl Di-Glycol Carbonate, differed from Methyl Methacrylate
in two important respects. First, its scratch resistance was considerably
improved - under some test conditions, it had a scratch resistance of some
thirty to forty times that of Methacrylate. Secondly, it was a thermo-setting
rather than a thermoplastic material. By this is meant that once the material
has polymerized, or become solid in the manufacturing process, it will
never again soften to any significant extent, no matter how high the temperature
is raised. Initially, its color was poor, but as time went on, this was
overcome and is no longer a problem. It is this material which is the basic
ingredient for the product we call AOLITE.
With this brief history of plastic lenses for ophthalmic use, we will
now turn to AOLITE and discuss its properties, the manufacturing process
involved and the product advantages and limitations.
As previously stated, AOLITE material uses Allyl Di-Glycol Carbonate,
or CR-39, as it is more commonly called, as a basis. Other ingredients
have been added to improve its properties for ophthalmic application, but
being basically a thermosetting material, it has extremely good thermal
stability. Lenses made from this material will not soften and distort at
high temperatures. In fact, the lenses may be subjected to temperature
of 200° or 300° for short periods of time without any detrimental
effect. At 400° or 500°, the lens will become somewhat yellow and
brittle, but even then it does not soften significantly. If elevated to
a high enough temperature, the material will burn, but not so as to be
a hazard in any respect. In fact, the material will just barely support
combustion, and can be likened to safety film in this respect.
The chemical durability of AOLITE is very good indeed, and, in fact,
is better than glass in some respects. It is a common fallacy to believe
that glass is impervious to all kinds of chemical attack; this simply is
not so. True, glass is an extremely inert material, but it is, to some
extent, attacked by a variety of chemicals, including plain water. In fact,
we check all of our ophthalmic glasses at regular intervals to be sure
their water solubility, as well as acid solubility, is not beyond established
AOLITE is not completely impervious to chemical attack either, but like
glass, it is very inert. One of the chemicals to which it is most vulnerable
is chloroform, but even here it is not readily soluble in the sense that
we commonly think of a material being soluble. One has to work on it even
in order to cause this material to attack AOLITE.
The dispersion, that property which determines how constant is the index
refraction for various colors of light, is also very similar to glass.
AOLITE has a reciprocal relative dispersion of 57, compared with about
58 for glass. This low dispersion, or high nu value, means that lenses
made from AOLITE will have the same freedom from chromatic aberration,
as will lenses made from ophthalmic Crown glass.
With regard to light transmission, AOLITE is again very similar to glass.
The internal quality of AOLITE results in a clarity almost exactly the
same as glass. Because of the slightly lower index of refraction, there
is slightly less surface reflection from AOLITE, and this results in ½
to 1 percent gain in visual transmittance, as compared to ophthalmic Crown
glass. Whereas, glass has a visual transmittance of about 91.5%, AOLITE
has a transmittance of about 92.0%.
At this point, it would be well to call attention to some misleading
advertising, which has been prevalent for some time. Claims are being made
that certain plastic lenses have 6% to 8% more light transmission than
glass, and this claim is simply not so. It can be seen from the foregoing
figures that in order for this to be true, the plastic material would have
to have essentially 100% visual transmittance, and the laws of physics
dictate that any transparent material having an index of refraction suitable
for ophthalmic use will have about an 8% loss of light due to surface reflections.
It is likely that the erroneous figure of 6% to 8% more light transmittance
for plastic came about through a misunderstanding of the facts, or a deliberate
distortion of them. Because plastic materials have a visual transmittance
of 92.0% (or 8.0% absorption) and glass has a visual transmittance of 91.5%
(or 8.5% absorption), it is true that plastic materials have about 8% less
loss than does glass. This, however, is nothing more than double talk,
and has absolutely no significance to the wearer.
The facts are, then, that AOLITE has, for all practical purposes, the
same visual transmittance properties as does glass. It has, however, one
added advantage over regular ophthalmic Crown glass, in that it absorbs
ultraviolet radiation to about the same extent as does Cruxite. AOLITE
has a transmittance of 5% at 350 millimicrons, which classifies it as an
ultraviolet absorbing material.
Now, let us turn to a brief description of the manufacturing process
involved in the production of AOLITE lenses. An exclusive patented process
is utilized at American Optical Company, involving a set of glass molds.
The mold used to form the convex lens surface serves as a dish into which
the liquid resin is poured. This mold is held in a specially designed retaining
ring onto which is placed the other mold which will form the concave lens
surface. The entire unit is then placed in an oven and baked for a period
of about 16 hours. During the 16-hour cycle, the temperature varies from
100 degrees to 180 degrees F. at a predetermined cycle. At the end of the
16-hour period, the mold unit is removed and the molds separated. During
this period of time, polymerization has taken place, which is a chemical
reaction converting the liquid resin into a solid lens. As previously stated,
this solid material will never again soften or revert to the liquid state.
The lens is now put into another oven and baked for an additional three-hour
period. This is called a post-cure, and the purpose is to relieve any stresses
which have set up in the lens and to increase its surface hardness.
Thus, it can be seen that the manufacturing cycle for AOLITE lenses
is nearly 24 hours long, and this means that a given set of molds can be
used to produce only one lens per day.
Now let us take a look at some of the product advantages - in other
words, why should people want to wear AOLITE lenses? In the first place,
there is a weight advantage since AOLITE lenses weigh only about 50% as
much as glass. This is a particularly important advantage in the stronger
prescriptions, where the weight of the heavy glass lenses is a serious
The second important reason for wearing AOLITE lenses is that of safety.
While we must admit that not enough is known about the safety of plastic
materials as compared to glass, we do know that there is a very significant
difference between these two materials. Before one can say which is better
from a safety standpoint, it is necessary to define the kind of impact
from which we are trying to protect the eyes.
The standard test for determining the impact resistance of ophthalmic
lenses is to drop a 7/8" steel ball onto the lens from a height of 50".
There is a real question as to how valid this test is in the light of perils
which are apt to be encountered, for as someone once said - "When 7/8"
steel balls start flying around the room, I want to get out of there."
If, however, we are to use this test as a criterion for judging the safety
of plastic and glass lenses, then we would say that AOLITE lenses are about
on a par with regular glass lenses. They will not, however, withstand this
kind of impact as well as will a hardened, or heat-treated, industrial
thickness glass lens.
When we turn to other kinds of impacts, the story is a different one.
AOLITE lenses are much more resistant to impact from a high velocity small
particle than are either regular glass lenses or hardened industrial thickness
glass lenses. In this regard, a considerable amount of work has been done
by the Air Force at Randolph Field in Texas under the direction of Dr.
H. W. Rose. He has found that for small high velocity particles, plastic
materials are greatly superior to glass. To be more specific, he fired
steel spheres, 1 mm. in diameter, and weighing 3.7 milligrams at various
lens materials and determined the ballistic limit in feet per second at
which the lenses would break. He found that hardened glass lenses of industrial
thickness would break at a ballistic limit of 375 feet per second on the
average, that unhardened glass lenses of the same thickness would break
at 1,000 feet per second, and that plastic lenses of equal thickness would
not break at 1,600 feet per second, the fastest speed available to him.
Of further interest, however, was that the cornea of a rabbit's eye was
pierced at a ballistic limit of 475 feet per second, which is a greater
velocity than that which caused the hardened glass lens to shatter. Thus,
hardened industrial glass lenses are no protection at all to this kind
of impact, whereas plastic lenses offer very good protection, indeed.
The only real limitation of AOLITE is its scratch resistance. While
AOLITE's resistance to scratching is many times that of the earlier plastics
used for ophthalmic application, it still is not as scratch resistant as
glass. However, there is considerable reason to believe that glass is more
scratch resistant than is actually necessary. AOLITE lenses have been worn
by various individuals for two and three years without showing sufficient
scratches to cause the lenses to be replaced, and so it can be said that,
given adequate care, AOLITE lenses will last the useful life of the prescription.
There is, of course, hope that new and more scratch resistant plastic
materials will be developed in the future, and to this end, American Optical
Company is continually working in Research and Development to improve on
our material. The point is, however, that the material already available
in AOLITE will give a long life if given adequate care, and, at the same
time, the individual will benefit from its other advantages of lightweight
With regard to adequate care, it might be well to state that the proper
method for cleaning AOLITE lenses is to wet the lenses with water, or with
a special antistatic spray called "Antistat" just being introduced by American
Optical Company, and then to wipe them dry with a soft paper tissue such
as Kleenex, or with a clean soft cloth. If a cloth is used, the importance
of it being clean cannot be over-emphasized, for small particles of dirt
embedded in a cloth may damage the lenses. It should further be stated
that Silicone impregnated cloths or tissues should not be used on AOLITE
lenses, since these contain abrasive materials which will damage the lenses.
As to product availability, AOLITE Single Vision lenses are now available
in prescriptions having sphere powers from plus 10.00 to minus 10.00 diopters,
and cylinder powers to minus 4.00 diopters. This wide range represents
more than 98% of all Single Vision prescriptions.
With regard to bifocals, work is currently under way to make available
a One-Piece Bifocal, having a 22 mm. round segment on the convex surface.
It is anticipated that this product will be available early in 1959.
With regard to absorptive lenses, American Optical Company is currently
undertaking the development of the various ophthalmic colors. It is anticipated
that True-Color, Cosmetan and Calobar colors all will be available in AOLITE
in the not too distant future.
But what about the future of AOLITE lenses? Is there anything more that
we can expect from them? The answer to this is definitely yes, and it is
perhaps the most exciting aspect of the AOLITE story. American Optical
Company has just introduced a series of three AOLITE aspheric microscopic
lenses designed to aid the near blind. These magnifying lenses employ an
aspherical surface which results in many fold improvements in optical performance
over what is possible with the regular spherical or toric surfaces. Such
aspherical surfaces cannot economically be manufactured in glass, but since
the AOLITE process is one of casting from molds which can be utilized again
and again, aspheric surfaces have now become a practical reality.
American Optical Company expects to introduce, sometime this Fall, a
brand new series of AOLITE Aspheric Cataract lenses, both single vision
and bifocal, and these will exhibit optical properties greatly superior
to anything which is now available. For the first time, the Cataract patient
will have vision not only to the center of his lenses, but over the entire
lens area, just as does an individual wearing a normal Tillyer correction.
The added advantage of lightweight will make the new AOLITE Aspheric Cataract
lenses the most revolutionary ophthalmic development in many years, and
will mark the opening of a new era in eye care.
Return to AO History
Don Whitney and AO