Sunglasses: Comfort and Protection
Besides looking good, sunglasses provide comfort, by toning down bright light, and protection from Ultra Violet light and impact injury. After we review the main features of sunglasses we will make a field trip to the beach and look through the different types of lenses to show their effects.
|The features that provide comfort are: darkness level, tint color, and polarization. Protection from Ultra Violet is a separate issue. These subjects are covered in more depth in the videos on YouTube. Links are provided at the end of this page.|
On the left is a light tint. This lens blocks 10% of the light, allowing 90% to come through the lens. Some people find this comforting in situations with bright indoor lighting.
In the middle is Level 2, medium tint, letting 50% of light through. This is not commonly used because it is too dark for indoors and not dark enough for outdoors.
On the right is dark tint, Level 3. It allows about 20% of light to come through; it blocks out 80%. This is a typical level for sunglasses. The idea is to reduce the amount of light coming through the lens to a comfortable level, but you don’t want a lens for everyday use that is too dark. It will make objects in shadow, like pedestrians, hard to see.
Tint color makes a big difference in what you experience with the glasses.Typical colors for sunglasses are gray, green and brown.
Green, has similar qualities, but seems less popular these days.
Brown, which is gaining in popularity, gives a sense of higher contrast and a brighter image, but adds some color changes.
“Photo-chromic” lenses are the ones that darken up when exposed to sunlight. They were first introduced by Dow Corning in the late 1960’s, and they seemed like a bit of magic when they came out. The lens contains a photosensitive chemical which is nearly clear indoors.
The lens in the upper picture I have just taken out of my pocket. It is mostly clear. When exposed to the Ultra-Violet part of sunlight, the lens darkens up fairly rapidly. Below is the same lens that has been exposed to the sun for about a minute. When you go back indoors, away from the sun’s UV rays, the lens gradually returns to its almost clear state.
The first Corning lenses were made out of glass, with names like photo-gray and photo-sun. More recently a lighter- weight plastic lens was developed under the name “Transitions,” which is a brand name that has taken over like the word “Kleenex.”
This graph shows how fast the darkening happens. It comes from Corning’s technical specs for their SunSensor lens. The left axis is percent of light transmission. The bottom axis shows minutes. Let’s follow the curve. At the start (1) the lens is not fully clear, with about 15% absorbance, like a light indoor tint. After exposure to sunlight for a couple of minutes (2), the lens darkens to 80% absorbance, a typical level for a sunglass lens. At the 15 minute mark (3) the lens is about as dark as it’s going to get. At this point the lens is removed from sunlight. You can see it takes quite a bit longer for the lens to lighten up (4). It never becomes totally clear.
Polarized lenses. When light reflects off certain surfaces, like water, it produces annoying glare. Certain glasses are able to filter that out.
Here is how that works. The sun gives off a wide spectrum of electro-magnetic waves. By that we mean there are individual light rays or waves, of many wavelengths. We perceive the mixed rays of the visible part of the spectrum as white light. The arrows on the lower right are meant to show the light waves are oriented in random directions.
For the purpose of this discussion I am only showing one vertical wave, in blue, and one horizontal wave, in orange. The color choice is only for illustration; the different colors could be oriented in any direction. When light bounces off certain objects, like water, the vertical waves, shown here in blue, are absorbed. The horizontal waves, colored orange, are reflected, which is what you perceive as “glare.” Note that the reflected waves are all oriented in the same direction, horizontally, which means they have been “polarized, by reflection.”
There is another way to polarize light. There exists lens material that, because if its internal structure, allows light waves to pass through, only if they are oriented in a particular direction. That is what is meant by a “polarizing” lens. In this illustration the polarized lens is oriented so that it blocks the horizontal rays and allows only the vertical rays to pass. Can you see what’s coming? Your sunglasses work the same way.
The first polarization comes about by reflection, usually from a horizontal surface, like water or a road surface, so the light has a horizontal orientation. The polarizing material in your sunglasses is oriented vertically, so it blocks the horizontal waves and reduces glare. If your sunglasses were oriented in the other direction, they would let those waves through. By blocking out glare the lens provides a nice level of comfort without having to be as dark.
Magic of Polarization
Here is an example of two polarizing lenses laying in the same direction (above left). The first lens is polarizing the light, letting through light waves of only one orientation. The second lens has the same orientation, so it will also let those waves pass. Where they overlap things are a little darker, because of the double tinted lenses. But, if the second lens is oriented at 90 degrees (above right), it will block the waves let through by the first lens. Now the area where they overlap is a lot darker. The magic of polarization.
One unexpected problem with polarized lenses may be difficulty reading some LCD displays, like instruments for a pilot, or on some gas pumps. Keep in mind that polarization does not block UV, so it is for comfort, not protection.
Lastly we come to Ultra-Violet light. Remember our illustration of sunlight, showing that it contains many wavelengths or a “spectrum” of electro-magnetic waves (radiation). Here is the part of the spectrum we are interested in.
On the left is Infra-Red, which has a long wavelength and is responsible for the warmth you feel from sunlight. In the middle is visible light that colors our world. Ultra-Violet light is past the blue end of the visible spectrum, thus the name “ultra-violet.” Because UV waves contain more energy, they have the potential to cause more damage.
The UV spectrum is divided into 3 regions. The most energetic UV-C, is blocked by the atmosphere, so that is not a problem. UV-B is your friend in small doses, but not your friend, in large doses. You need a certain amount of UV-B for your own production of Vitamin D, which starts in your skin. But too much UV-B is potentially a direct cause of skin damage. While less than 5% of UV-B reaches the earth’s surface, over 95% of UV-A gets through, and it is also potentially damaging.
It has been clearly shown that cumulative exposure to Ultra-Violet radiation causes damage to exposed skin in the form of tanning, sunburn, accelerated skin aging, and skin cancers. And surprisingly immuno-suppression.
Regarding eye structures: The skin of the eyelids develops cancer like any other area of the skin. The exposed conjunctiva can form a scar-like pterygium, shown at the right, outlined by the white dashed line. The cornea can be affected. In the lens, U-V accelerates clouding, called cataract. There are experimental reasons to suspect possible retinal damage, but that link is hard to prove.
You want transparent materials, like glass, to allow the visible spectrum through, but block out as much of the UV as possible. Regular window glass transmits Infra-Red, visible light and most of the UV. (UV-A, but it absorbs UV-B. [Balk]) In your car, for safety reasons, the windshield is made of laminated glass, which blocks almost all of the U-V. That is why your photo-chromic lenses do not darken in the car. The tempered side windows are not as protective, but the sun roof is pretty good.
Common materials for eye-glasses include glass, standard CR-39 plastic, and newer plastics, Polycarbonate and Trivex. This diagram shows how the different lens materials perform. The vertical scale shows the percent of transmittance, with the different wavelengths on the horizontal axis. As we saw above, untreated Glass allows just about all the UV-A and some B through. CR-39 Plastic is better, absorbing about half the UV-A. It can be treated to absorb over 90% of the UV. The newer materials, Polycarbonate and Trivex, absorb over 90% of the UV by themselves. So if your glasses are made out of one of these 2 materials you don’t need additional UV coating. If the lenses are made out of glass or CR-39 plastic, you should consider UV coating if you spend a significant amount of time outdoors.
Here are general measures for UV protection. Wearing a hat will significantly decrease UV exposure to the face and eyes. Protect your skin with sunscreen and clothing. Decide if a tanning booth is worth the risk.
Now, even though we know the risks of UV exposure, it doesn’t seem likely that people will stop going to the beach. This view is through a polarized, gray tinted lens. (My sunglasses actually.) You can compare the glare off the water, outside the lens, and through the lens.
On the left is the same view through a polarized brown lens of the same tint level. The feel is quite different; the image seems brighter. On the right the lens is rotated to demonstrate the effect of lens orientation we talked about. In this case letting more glare come through.
On the left is a Transitions lens, gray tint, just out of my pocket. In the time it takes to set up the shot it is already starting to darken. On the right it has had a couple of minutes to fully darken up.
So sunglasses provide comfort by dimming bright light and cutting reflected glare. They also protect against UV radiation and impact injury. This subject is covered in greater detail in the following videos on YouTube.