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I Can
See Clearly Now: Anti-Reflective Coatings |
If we knew of a way to optimize our
patients' vision, wouldn't it only make sense to do so? The
best vision possible is a combination of an accurate
prescription, appropriate lens material, and lens options.
One to consider for every patient is anti-reflective
coating.
What is it?
Anti-reflective coating is just that, a thin film on the
surface of the lens that diminishes reflections. It is an
incorrect assumption that anti-reflective coating is
anti-'glare' coating. Glare is generally defined as
difficulty seeing in the presence of bright light, or
scatter from bright light that decreases vision. Reflection
is the image of something reflected by a reflective
material, such as a mirror, chrome, water, or a lens.
Anti-reflective coating is not meant to
decrease brightness in a way that a tinted lens would.
Instead, it increases the transmission of light through the
lens into the eye to 99%, which is 8% more than with
non-coated lenses. The increased light does not translate
into increased brightness, but rather increased contrast and
clarity.
Who benefits?
Any patient who faces reflective surfaces can benefit.
Driving at night, using a computer, watching television,
working under fluorescent lights, and other lighting
situations can be easier to navigate when the vision is
clearer, the contrast sharper, and the eye less fatigued.
How it works:
The reason light reflects off a surface, in this case a
lens, is because the refractive index of air and the
material is different, resulting in the reflection that we
see. Think of light as a wave. When it strikes the surface,
some of it will bounce back in the form of a reflection.
Both the reflected wave and the incident ray (the original
wave of light striking the surface.) have the same
characteristics, although they move in opposite directions.
This wave would be considered a constructive wavelength, in
that it builds onto itself, therefore making the reflection
larger.
When a reflective coating is applied to
the lens, it is done so with slightly different placement.
More precisely, the second reflective surface (the coating)
is to be ¼ wavelength from the first surface (the lens). ¼
wavelength for the incident wave and ¼ wavelength for the
reflected wave place this wave half a wavelength from the
original reflection. The lowest point of this wave would be
aligned with the original wave's highest point, canceling it
out. This second wave is considered the destructive
wavelength, in that it is the opposite phase, and will
decrease and eliminate the reflection problems.
Magnesium Fluoride has a refractive index
of 1.37. This makes it a practical choice for the index
matching aspect of anti-reflective coating.
Types:
Anti-reflective coating can be placed in two categories:
single coatings and multiple layer coatings. The process of
multiple coatings builds off of the single coating mentioned
above. As the name indicates, there are multiple layers at
work. Each layer of coating is specifically applied with
great consideration to the index of refraction of the
surface, eliminating all reflections.
Single layer anti-reflective coatings
perform well, but are not as advanced as multiple layer
coatings. Where single layer coatings will eliminate most
reflections, multiple layer coatings will eliminate all
reflections. The effectiveness of the coating somewhat
depends on how many layers there are.
The first anti-reflective coating was
invented in 1935 for the Carl Zeiss Company as a top secret
tool of the German army in World War II.
Older types of anti-reflective coatings
were problematic. Patients would be unsatisfied with how
easily the coating seemed to scratch or craze, even if the
lenses were cared for properly. Having a negative impression
of the product caused patients to avoid it. All
anti-reflective coatings have a hard coat/scratch coat
applied to the lens before the multiple layer
anti-reflective 'stack' would be applied.
Well received premium options such as
Essilor's Crizal family and Sola's Teflon Clear are multiple
layer coatings which have oleophobic and hydrophobic
elements. This means that they repel oils and water, making
them easier to care for, and producing clear, crisp vision.
Another premium anti-reflective coating
is Hoya's Super Hi-Vision, which stresses a 'substrate
matched' hard dip coat that creates an extremely clear lens
with a slippery surface which makes it is easier to care
for.
Anti-reflective coating and sun wear:
Anti-reflective coating is also an important step in
creating sun wear. For many opticians, it is standard
practice to apply it to the back side of the lens. This
brings more light into the eye, increasing the contrast of
the sun wear as well as making the most of the tint.
Applying traditional anti-reflective coating to the front of
a tinted lens can alter the intensity and hue of the tint.
In January 2007, Essilor has debuted
Crizal Sun, the first anti-reflective coating specifically
for sun wear. It is integrated into the lens instead of
being applied to the surface only. The anti-reflective
portion is on the back side only, reducing glare and
reflection. A double sided hydrophobic hard coat on the
front protects the lens from scratches and smudges.
There are many types of anti-reflective
coating on the market, with more products being offered all
the time. The skill lies in knowing both our products and
our patients' needs.
Anti-reflective coatings can be a benefit
to most patients, but it is important to consider the life
style and occupations of our patients.
Carpenters, stone masons, construction
workers, wood workers, and others who work in or spend their
leisure time in dusty environments or are faced with debris
that could harm the lens may not be the best fit for even
the most advanced anti-reflective coating. While they would
no doubt benefit from the enhanced vision anti-reflective
coating affords, the increased care or frequent replacement
of the lenses may irritate some patients. These same
patients may find that anti-reflective coating would be a
great addition on a second pair.
Care for anti-reflective lenses does not
differ entirely from that of non-coated lenses. The patient
should rinse their eyewear under tap water before cleaning
with eyeglass cleaner, and then dry with a cotton cloth. The
cloth should also be laundered regularly without using
fabric softener. The step of rinsing the lenses is often
over looked by both patients and even eye care
professionals, but it does make a difference in keeping the
lenses clean all day long.
As with anything, education is very
important when discussing anti-reflective coatings with
patients. They should be informed of what the benefits and
limitations are, and how the coatings can help them. |
