The Importance of Visible Light Transmission

I live in Michigan, so because we have four seasons we have days with bright sunshine and other days with haze.

Today it's a beautiful, bright sunny day outside, the weekend is here and we decide to spend some time outdoors. Perhaps we will mow the grass, have a barbeque or take a drive to the lake shore and spend the day.

But before I head out the door for the day there are a few things I need to consider including wearing a pair of sunglasses to protect my eyes. Most of us are aware of the danger of uv radiation to our eyes, but do we know whether or not our sunglasses will adequately protect our eyes?

Many times the choice of sunglasses is based upon how they look and fit into our wardrobe, but the truth is the main purpose of any pair of sunglasses is to protect our eyes from damage, injury or disease. This is where understanding the importance of visible light transmission comes in.

Visible Light Transmission

Any time sunlight passes through a window, a lens or any other medium some of it is absorbed, some of it is reflected back and some of it passes right through. The amount of light that passes through any of these mediums can be measured, and that measurement is represented as a percentage of the total sunlight present. This number is known as the visible light transmission percentage or VLT. Some materials like glass naturally limit the VLT, but in most cases it is necessary to apply a film to the material in order to control the amount of light that is allowed to pass through the material.

But Why Is VLT Important?

The visible light wavelength is sandwiched between the ultraviolet wavelengths and the infrared wavelengths. In other words, it is the light we can see with our eyes. While visible light may not be harmful to your eyes the amount of light reaching your eyes can cause discomfort and even pain if too much of it reaches your eyes. Therefore it is useful to limit the amount of visible light that reaches your eyes. Remember, even glare can hurt your eyes and cause redness, soreness and serious discomfort. If it has nothing to do with UV radiation then why care about the percentage of visible light that reaches our eyes? The reason visible light transmission is important that the amount of light determines how comfortable your eyes are when looking through sunglass lenses but more important than comfort is the amount of glare that reaches your eyes. Glare is particularly problematic when on or near the water, sand or snow.

VLT And Car Windows

The amount of allowable visible light transmission has been considered by each state in the United States for either a legal or mandatory requirement for automobile windshields and car window tinting. Many states limit the amount of visible light transmission blocking can be done as a safety issue. Imagine for a moment that your windshield only allowed 8-12% of visible light transmission through the windshield. It would make seeing things at night extremely difficult... and dangerous. The same problem exists with sunglass lenses.

Typical Visible Light Transmission Guidelines

Visible light transmission is affected by both the tint, thickness and material of your sunglass lenses as well as the coatings that have been placed on the lenses. With this in mind we can make some recommendations when considering a pair of sunglasses:

Select sunglasses with glass lenses;
Make sure the VLT of the sunglass lenses is around 15% for best results;
Always check the UV rating for the lenses.

Measuring VLT

The better sunglass manufacturers like Randolph Engineering, Inc. will actually measure the visible light transmittance of their lenses and let the consumer know what that value is. For most Randolph sunglass lenses the VLT is 16%, which is quite good. This measurement can be made with a machine for great accuracy making it easy to tell consumers what the VLT for any particular pair of sunglasses or sunglass lens. You simply place the lens in the slot at the top of the machine and the readout will tell you what the VLT for that particular lens is.