The first record
of A/R research was done by an Englishman, Taylor, in the
late 1600's. He noticed that very old lenses allowed more
light to pass through them than new lenses. We now know
that this phenomenon was caused by oxidation of the surface
of the glass lens over many years. This oxidation resulted
in a layer, which acted as a simple A/R coating.
Unfortunately for Mr. Taylor, he never discovered the
reason for his observation nor was he able to duplicate it
in the lab.
The first man-made A/R coating was developed by the Carl
Zeiss company in Germany. Its original use was coated
optics for artillery, and to this day many of the
improvements in A/R technology can be attributed to
military research.
Uncoated Lenses / Coated
Lenses
Differences in
index of refraction of transparent materials cause part of
the spectrum of light rays to bounce back instead of
passing through. This is a reflection. For example, CR-39,
which has an index of refraction of 1.498, will allow 92%
of the visible spectrum to pass through. If there are two
layers of transparent material, then a portion of the
reflected rays from the bottom material are reflected back
and forth until virtually all pass through the material.
The few that do not pass through result in a residual
color, usually in the blue, green, or gold visible range.
The perfect A/R coating would allow 100% of light through,
and would render the material invisible. Theoretically this
is possible, but the process parameters are so unforgiving
that it probably is not feasible for ophthalmic purposes.
But they are quite close. While a raw CR-39 lens allows 92%
transmission, this lens with an A/R coating achieves 99.3%.