TECH TIPS – FEBRUARY 2012

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EACH ISSUE, TECH TIPS WILL EXPLORE SOME INTERESTING ASPECT OF OPTICAL TECHNOLOGY. THIS MONTH LOOKS AT AR TREATMENTS VS. LENS INDEX OF REFRACTION.
When is an anti-reflective (AR) treatment most useful in terms of light transmission? Are some ophthalmic lens materials better suited for AR treatments than others? These are good questions and worth investigating.

THE EQUATION
An AR treatment is designed to eliminate reflections off a lens surface. Here is the formula that expresses the percentage of reflection that occurs off the
surface of an optical material:

Admittedly, this is a simplified equation that has been factored down to make it easy to utilize for ophthalmic purposes, but even so, it tells us a great deal about AR treatments. Notice there are only three variables in the equation. Since the intensity of the light falling onto the front surface is something that will occur outdoors or indoors and is beyond your control (and the amount of reflected light is what we want to measure), it’s the refractive index of the lens material that’s the only factor to reflectance you can control clinically in this formula.

REFLECTANCE AMOUNTS
The table below shows the reflectance amounts for various ophthalmic lens materials. Notice that as the index of refraction increases, so does the amount of reflection from the surfaces of the lens. This is important to know because it will have an impact on your patients. For example, you attempt to provide a light and thin lens option for a patient who has a -6.00D Rx in each eye and because of this you supply a 1.70 lens material. That’s a good choice for thinness and for making the lenses lighter, but it increases the surface reflections to 13%. That’s substantial. Think of it this way, that 13% reflection is like providing 13% gray tinted lenses to the patient. Why? Because of the dimmer amount of light that will come through that lens due to surface reflections.

To say this another way, the higher the index of refraction of the lens material you choose for a patient, the higher the surface reflections the lens will produce, which provides less light for the patient through the lens. How can you avoid this problem? Simple—supply an AR treatment. Lens manufacturers and lens coaters have become adept at producing high quality AR treatments, some of which claim reflection elimination over 99%. Reducing reflections enables more light to pass through a lens, and if your supplying clear lenses for a patient, that seems like a logical thing to do.

OTHER FACTORS
Many sun lenses only have the backside of the lens coated with an AR treatment. Why? If you’re trying to provide a reduction in light with sun lenses, why would you put a treatment on the front surface that increases the light level? The AR treatment on the back surface eliminates what most patients would tell you is the most annoying reflection, the one of seeing their own eye’s reflection.

If you’ve been told to always suggest an AR treatment with high-index lens materials, then you’ve been told something valuable. While an AR treatment is more important with higher index lens materials, it should be considered an essential feature of every pair of lenses, even low-index lens materials that lose nearly 8% of their light to reflections.

In summary, AR-treated lenses transmit nearly 100% of the light that falls upon it—an advantage your patients will appreciate.


Ed De Gennaro is Director, Professional Content of First Vision Media Group.

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