Sunday, April 28, 2013

What is the best sunglasses lens in the world? Check the ESPF: eye-sun protection factor

What is ESPF?

ESPF is eye-sun protection factor - a way to measure the degree of UV protection provided by an eyeglass lens that was developed by the French optical company Essilor.

The higher the number, the higher the UV protection of a lens.  Most lenses that tout themselves as 100% UV protection, only block UV rays that hit the lens from the front - they block 100% the UV rays that hit the lens from the front only.  Those are ESPF 5 or 10 lenses.  Advanced UV technology lenses go up to ESPF 25 for a clear lens and ESPF 50 for a tinted sun lens.

With traditional UV lenses (the ones that only block rays form the front of the lens) you still get lots of  UV exposure from rays that reflect off the back of the lens and into your eyes, as shown in this diagram:

Some lenses, like Crizal Forte UV shown in the diagram, block UV rays that reflect of the back of the lens.  This gives you more than twice the UV protection of tranditional lenses.  Crizal Forte UV is an ESPF 25 lens.  It is the highest level of UV protection that you can get in a clear lens.

What is the best sunglasses lens?

To get the maximum protection, ESPF 50, you need to go with the Crizal Sun UV lens, which is the only (as far as we know) dark tinted lens that blocks UV rays from the front as well as reflected UV rays.  See the chart tat the end of this post for a list of the available ESPF 25 and ESPF 50 lenes.

Why worry about UV exposure?

UV exposure to the eyes increases the risk of diseases like cataracts and macular degeneration, which can cause permanent vision loss.  It also leads to skin cancer around the eyes, which is the most common kind of skin cancer.

Other UV related dangers include Pingueculae (small yellowish bumps on the white of the eye) and pterygium (opaque growth over the surface of the cornea). The more UV exposure you have during your life, the more likely you are to develop one of these damaging conditions.

Here is what vision is like with macular degeneration:

Here is what vision is like with cataracts:

ESPF 25 and ESPF 50 Lenses

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