Most likely, every eye care professional has heard of
Anti-reflective coating and knows fundamentally what the
coating is intended to accomplish.
The basic definition of
AR coating is that it is a coating that is applied to the
surface of a lens to help eliminate reflections off the
surface of the lens. It is a great benefit to the patient
because it increases contrast, eliminates a majority of
reflections, improves peripheral vision, improves night
vision and eases eye fatigue. What the ECP may not know is
exactly the how and why of AR coating.
How Anti-Reflective Coatings Work
AR coatings work by utilizing the phenomenon of wave
interference. Wave interference occurs when two light waves
meet while traveling along the same medium. There are two
types of interference: constructive interference and
destructive interference.
Constructive interference occurs at the location where
the two light waves have a displacement in the same
direction. The two waves are either peaking or falling at
the same place causing the wave pulse to be greater than
each wave pulse on its own. The result is greater reflection
or illumination on the lens medium.
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| Before
Interference |
During
Interference |
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Destructive interference is the opposite. It occurs when
the two waves of light displace in opposite directions. By
achieving complete amplitude while at opposites, the two
light pulses completely destroy each other.
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| Before Interference |
During Interference |
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The amplitudes do not have to be equal for destructive
interference to occur. If one wave with an amplitude of 1 up
meets a wave with an amplitude of 2 down, the destruction
would be 1.
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| Before Interference |
During Interference |
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Anti-Reflective coatings work by utilizing the theories
of destructive interference.
Types of Anti-Reflective Coatings
Index Matching
Index matching is the earliest form of AR coating and was
discovered by accident. In 1886, English Physicist Lord
Rayleigh discovered this coating when he was inspecting a
few tarnished pieces of glass. Optical glass at the time
tarnished due to chemical reactions between the glass and
the environment, and when Lord Rayleigh was analyzing the
glass he discovered that the lenses transmitted more light
than the new, untarnished lens. Intrigued, Rayleigh began
experimenting on the lenses and discovered that the tarnish
creates two new lens interfaces: a tarnish to glass
interface and a tarnish to air interface. Since the index of
refraction of the tarnish falls in between that of the air
and the glass, less reflection occurred at these interfaces
than the air to glass interface.
Today, most traditional Index Matching (IM) AR coatings
consist of a thin layer of quartz laid on top of an idium-tin
oxide (ITO) layer. ITO is a colorless and transparent
compound when applied in thin layers and it is normally
deposited on the lens surface by a vacuum process. The
oxygen in the atmosphere then produces stress in the low
emissive coating and hardens it. Magnesium-fluoride may then
be applied to improve scratch resistance. Traditional IM ITO
coatings are not commonly used in optics today due to the
high cost of idium-tin oxide. Other chemical compounds, such
as aluminum zinc oxide, have a similar transparency rate and
are much cheaper so are more commonly found used in the
optical industry. Of course, different companies utilize
different materials and like to keep that information to
themselves for proprietary reasons.
Single-Layer Interference
Originally developed by Carl Zeiss in 1935, single layer
interference is a single quarter-wave layer of a transparent
chemical compound whose refractive index is the square root
of the substrate’s refractive index. For example, crown
glass has an optical index of refraction of 1.523. The
square root of 1.523 is 1.234, so the optimum material for
the single layer antireflective coating would have an index
of 1.234. Unfortunately, no such optical quality material
exists. The closest transparent material that is readily
available for use would be magnesium fluoride (n=1.38).
Magnesium fluoride is an excellent material for single layer
interference AR coatings because it:
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Works well on a wide range of indices, especially
higher index materials because it is closer to the square
root of the material.
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Is cheap.
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Gives good AR qualities across most of the visible
band of light
Other excellent chemical compounds are germanium,
silicon, sapphire, zinc selenide and gallium arsenide.
However, the effectiveness of the single layer interference
AR coat is limited due to the availability of the suitable
lens materials with the proper index. This can be rectified
by utilizing a multi-layer interference AR coating which
also increases the percentage of light transmitting through
the lens.
Multi-Layer Interference
Common multi-layer AR coatings are composed of very thin
layers, typically one-quarter to one-half the wavelength of
light, or about 10 to 20 millionths of an inch. They are
designed to be broad band, or work over the entire visible
light spectrum by utilizing pairs of wavelengths to
establish the destructive interference. A multi-layer
interference coating can outperform a single-layer
interference coating by a factor of ten. Using magnesium
fluoride as the example again, a single-layer AR coat may
reflect about 2% at the light wave spectrum of 550nm. A
multi-layer AR coat at the same central wavelength of 550nm
may only reflect approximately 0.2%.
Improving the AR Coating
Anti-Scratch
A lot of patients had complaints about the strength and
endurance of the earlier forms of AR coating. Previous forms
of AR had several complications such as scratching, crazing,
and peeling. To prevent this, many of the premium AR coats
have a scratch coating applied to the top of the multi-layer
AR coat. The advance coats are a nano-composite varnish.
Typically composed of silica along with other proprietary
substances, nano-composite varnishes are hard enough to
provide scratch coating while maintaining enough flexibility
not to craze or peel when strained.
Anti-Smudge
Low end and older AR coatings have a tendency to smudge.
These smudges break up the surface of the AR coating making
the wave interference method less effective. To prevent
this, oleophobic and hydrophobic top coats are applied to
the AR coat. Oleophobic simply means that the layer repels
oil. As a result finger prints do not adhere to the lens
surface as readily. Hydrophobic top coats allow water to
bead up on the lens surface instead of spotting on the lens.
The result is lenses that stay smudge free longer and are
easier to clean because the surface has been sealed. The
concept here is no different than that of waxing your car
– the surface is sealed and water and dirt are repelled as
a result.
Anti-Dust
Dust scratches. The best way to prevent scratches is to
prevent the particles that cause scratches to adhere to the
lens. This can be achieved by utilizing an anti-static
coating on the lens. Not only does the anti-static coat
repeal dust, it also repels other airborne particles such as
pollen and dander, thereby keeping the lens surface cleaner.
In effect, creating a clearer visual surface and making
cleanings required less often.
The Benefits to AR Coating
Improved Night Driving
AR improves the flow of light through a lens, thereby
increasing visual acuity. How this especially helps with
night driving is through the:
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Elimination of ghost images.
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Reduction of the “rearview mirror effect”. This
occurs when the light from behind the driver reflects off
the patients lens back into the eye of the driver.
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Improved visual acuity because more light is entering
the eye
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Increased peripheral vision
Enhanced Contrast
When light doesn’t transmit through the lens and
through the eye clearly, the quality of the image that
reaches the retina is not as sharp as one would like. The
lack of contrast can cause blurring, double vision and eye
fatigue. Because of this, an AR coating is especially
important for patients who:
Wear “flat surface” lenses such as higher powers
in aspheric and/or high index lens designs.
Cosmetic
Lenses have a windowpane reflective effect when not
coated with AR. Although it does not disturb the vision of
the wearer, it does cause others to not be able to see the
wearer’s eye. This can be a hindrance to individuals who
work in sales, present themselves to the public, or are in
an occupation in which they have to do a lot of public
speaking. It can also be a problem for those who just want
to look their best. AR coating helps with all these issues.
Conclusion
Every patient appreciates an ECP who is well informed and
knowledgeable about their products and is able to present
lens information to them in an easy to understand manor.
Patients are technologically savvy and want their glasses to
represent their savviest choice. Another way for an ECP to
meet or exceed the patients needs is with AR coating in
their arsenal of recommendations.
