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How are Reflection
Free lenses made?
Anti-reflection coatings
are made up from extremely thin layers of different
dielectric materials that are applied in a high
vacuum onto both surface of the lenses. The quality
of the AR depends upon the number of layers applied
to the lens. The early coatings had only a single
layer of magnesium fluoride or perhaps two but
nowadays most coatings have at least six layers
and are known as broadband coatings.
The number of layers affects
the amount of light transmitted over the visible
spectrum. The three lines shown on this diagram
represent the advances made in vacuum coating
technology over the years.
| Red
line |
early single layer
magnesium fluoride coating for glass lenses
only
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| Orange
line |
early
coatings for plastic (organic) lenses.
|
| Green
line |
modern broadband
coating where the transmission is more than
99% over the visible spectrum.
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One of the characteristics
of an AR coating is the colour of the residual
1% reflection on the lens - sometimes called the
bloom. On modern broadband Reflection Free lenses
it can be tuned to be either a soft green or blue
without compromising the quality of the anti-reflection
properties. The AR bloom should not be confused
with a permanent lens tint. An AR bloom is almost
imperceptible and can only be seen when holding
the lens up to sunlight or artificial light.
For a more technical description
of how anti-reflection coatings work click
here
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