How Sunglasses Work

A pair of sunglasses seems so simple -- it's two pieces of tinted glass or plastic in some sort of plastic or metal frame. How much more straightforward can something get? It turns out that there are many different things you can do with two pieces of glass, and these things can have a big effect on you when you use the lenses. As you will see in this article, there really is a difference between the various sunglasses you'll find out there.

There are four things that a good pair of sunglasses should do for you:

When you buy a pair of cheap sunglasses, you often give up all of these benefits and can even make things worse. For example, if your sunglasses offer no UV protection, you increase your exposure to UV rays. The cheap sunglasses block some of the light, causing your iris to open to allow more light in. This lets in more of the UV light as well, increasing the damage UV light can cause to the retina.

So there is a difference. Buying the right pair of good sunglasses for the conditions in which you use them gives you maximum protection and performance.

The sidebar shows some of the top sunglass manufacturers. Manufacturers of other products sell sunglasses, too. From Nike and Timberland to Gucci and Kenneth Cole, many big brands include sunglasses among their product lines. Many sunglass manufacturers make huge claims about the features and special qualities of their products. Prices can range from less than $20 up to several hundred dollars depending on the features and the name.

Then there are the imposters. You go to a discount shop or a flea market and see vendors offering sunglasses that look exactly the same as the high-dollar brand names for a fraction of the cost. Are you really paying that much for a name or are there fundamental differences between the look-alike sunglasses and the brand-name ones?

The biggest problem with cheap sunglasses is in the way the lenses are made. Inexpensive sunglasses have lenses made of ordinary plastic with a thin tinted coating on them. While the tint color and a similar frame design may make them look like Oakley X-Metal Romeos or Ray-Ban Predators, the actual lenses are very different. You will learn exactly how different they are, and how important the differences are, in the following sections.

Keep reading for a complete description of specific technologies like polarization, photochromic lenses, anti-reflective coatings and so on.

Good sunglasses are extremely effective "light conditioners." They modify incoming light to match it to your eyes. In the next section we will discuss all of the different technologies used by sunglass manufacturers to modify light. In order to understand those technologies it is important to understand something about light.

A light wave consists of electromagnetic energy. The size of a wave is measured by its wavelength. The wavelengths of the light we can see range from 400 to 700 billionths of a meter (nanometers). The amount of energy in a light wave is proportionally related to its wavelength: shorter wavelengths have higher energy. Of visible light, violet has the most energy and red the least. Just above the visible light spectrum is ultraviolet (UV) light, and it turns out that natural sunlight is rich in UV light. Because of its high energy, UV light can damage both your cornea and your retina.

What is Glare?

The brightness or intensity of light is measured in lumens. For example, when you are indoors, most artificial light is around 400 to 600 lumens. If you go outside on a sunny day, the brightness ranges from about 1,000 lumens in the shade to more than 6,000 lumens on a large stretch of concrete, like a highway. Our eyes are comfortable until we get to around 3,500 lumens. When the brightness of the direct or reflected light gets to about 4,000 lumens, our eyes begin to have difficulty absorbing the light. What we see when we try to look at these brighter areas are flashes of white -- this is glare. To reduce the discomfort caused by the amount of light entering our eyes, we squint. Once you get to around 10,000 lumens, your eyes are so bombarded that they begin to completely block out the light. Prolonged exposure to light of such intensity can cause damage resulting in temporary or even permanent blindness. That's why unprotected viewing of a large snowfield, which on a bright day can reflect light at more than 12,000 lumens, can result in being "snowblind."

Visible light is light that can be perceived by the human eye. When you look at the visible light of the sun, it appears to be colorless, which we call white. It is made up of many color frequencies. The combination of every color in the visible spectrum produces a light that is colorless, or white (see How Light Works for details).

On the next page, we'll take a look at how we see in color. 

There are two basic ways by which we can see colors. Either an object can directly emit light waves in the frequency of the observed color (a neon light does this), or an object can absorb all other frequencies, reflecting back to your eye only the light wave, or combination of light waves, that appears as the observed color (any painted object does this). For example, to see a yellow object, either the object is directly emitting light waves in the yellow frequency, or it is absorbing the blue part of the spectrum and reflecting the red and green parts back to your eye, which perceives the combined frequencies as yellow.

When we talk about light in reference to sunglasses, three types of light are important.

Good sunglasses can eliminate the ultraviolet part of the spectrum, cut down on direct light to the point where it is comfortable and eliminate or decrease reflected light (depending on the reflecting surface).

One interesting property of light is polarization. When reflected off of certain surfaces -- such as water -- light gets polarized. Polarized sunglasses can eliminate reflected light off of water and similar surfaces because of its polarization. See the section on sunglass technologies for details.

See How Light Works to learn more about the properties of light. See What makes glass transparent? for details on transparency.

Sunglasses use a variety of technologies to eliminate the problems with light that were discussed in the previous section. The following sections discuss all of the different technologies currently in use:

Tinting The color of the tint determines the parts of the light spectrum that are absorbed by the lenses. Manufacturers use different colors to produce specific results.

Many manufacturers employ a process called constant density to tint the lenses. It is the oldest method of creating sunglasses and involves a glass or polycarbonate mixture with a uniform color throughout the material. The tint is built right into the lenses when they are created.

Tinting can also be accomplished by applying a coat of light-absorbing molecules to the surface of clear polycarbonate. The most common method for tinting polycarbonate lenses is to immerse the lenses in a special liquid containing the tinting material. The tint is slowly absorbed into the plastic. To make a darker tint, the lenses are simply left in the liquid longer.

In the next few sections, we'll take a look at other sunglass technologies.

Light waves from the sun, or even from an artificial light source such as a light bulb, vibrate and radiate outward in all directions. Whether the light is transmitted, reflected, scattered or refracted, when its vibrations are aligned into one or more planes of direction, the light is said to be polarized. Polarization can occur either naturally or artificially. You can see an example of natural polarization every time you look at a lake. The reflected glare off the surface is the light that does not make it through the "filter" of the water, and is the reason why you often cannot see anything below the surface, even when the water is very clear.

A polarized filter passes only the light that does not match its orientation.

Only the part of the light wave that is not aligned with the slots in the filter can pass through. Everything else is absorbed. The light coming through the filter is considered polarized.

Polarized filters are most commonly made of a chemical film applied to a transparent plastic or glass surface. The chemical compound used will typically be composed of molecules that naturally align in parallel relation to one another. When applied uniformly to the lens, the molecules create a microscopic filter that absorbs any light matching their alignment.

Most of the glare that causes you to wear sunglasses comes from horizontal surfaces, such as water or a highway. When light strikes a surface, the reflected waves are polarized to match the angle of that surface. So, a highly reflective horizontal surface, such as a lake, will produce a lot of horizontally polarized light. Therefore, the polarized lenses in sunglasses are fixed at an angle that only allows vertically polarized light to enter. You can see this for yourself by putting on a pair of polarized sunglasses and looking at a horizontal reflective surface, like the hood of a car. Slowly tilt your head to the right or left. You will notice that the glare off the surface brightens as you adjust the angle of your view.

A lot of sunglasses advertised as polarizing actually are not. There's a simple test you can perform before you buy them to make sure. Find a reflective surface, and hold the glasses so that you are viewing the surface through one of the lenses. Now slowly rotate the glasses to a 90-degree angle, and see if the reflective glare diminishes or increases. If the sunglasses are polarized, you will see a significant diminishing of the glare.

On the next page, we'll take a look at photochromatic sunglasses.­

Sunglasses or prescription eyeglasses that darken when exposed to the sun are called photochromic, or sometimes photochromatic. Developed by Corning in the late 1960s and popularized by Transitions in the 1990s, photochromic lenses rely on a specific chemical reaction to UV radiation.

Photochromic lenses have millions of molecules of substances, such as silver chloride or silver halide, embedded in them. The molecules are transparent to visible light in the absence of UV light, which is the normal makeup of artificial lighting. But when exposed to UV rays in sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken. The number of the molecules that change shape varies with the intensity of the UV rays.

When you go indoors and out of the UV light, the reverse chemical reaction takes place. The sudden absence of UV radiation causes the molecules to "snap back" to their original shape, resulting in the loss of their light absorbing properties. In either direction, the entire process occurs very rapidly.

In the PhotoBrown and PhotoGrey products made by Corning in the '60s, the lenses were made of glass, and the molecules are distributed evenly throughout each entire lens. The problem with this method became apparent when it was applied to prescription glasses, in which different parts of the lens can vary in thickness. The thicker parts would appear darker than the thinner areas. But with the increasing popularity of plastic lenses, a new method has been developed. By immersing plastic lenses in a chemical bath, the photochromic molecules are actually absorbed to a depth of about 150 microns into the plastic. This proved to be much better than a simple coating, which would only be about 5 microns thick and would not provide enough molecules to make the lenses sufficiently dark. This plastic lens absorption process has been popularized by Transitions, the leading manufacturer of photochromic lenses.

On the next page, we'll look at mirroring and scratch-resistant coatings. 

Reflective sunglasses often have a mirrored look. The lenses in these sunglasses have a reflective coating applied in a very thin, sparse layer -- so thin that it's called a half-silvered surface.

The name "half-silvered" comes from the fact that the reflective molecules coat the glass so sparsely that only about half the molecules needed to make the glass an opaque mirror are applied. At the molecular level, there are reflective molecules speckled all over the glass in an even film but only half of the glass is covered. The half-silvered surface will reflect about half the light that strikes its surface, while letting the other half go straight through.

Often, the mirror coating is applied as a gradient that gradually changes shades from top to bottom. This provides additional protection from light coming from above while allowing more light to come in from below or straight ahead. What that means is that if you are driving, the sun's rays are blocked but you can see the dashboard. Sometimes the coating is bi-gradient, shading from mirrored at top and bottom to clear in the middle.

The key problem with reflective sunglasses is that the coating is easily scratched. Apparently, sunglass manufacturers have not been able to successfully apply a scratch-resistant layer on top of the reflective coating. Therefore, the scratch-resistant coating is applied first to protect the lenses and the reflective coating is applied over it.

Scratch-resistant Coating

While glass is naturally scratch resistant, most plastics are not. To compensate, manufacturers have developed a variety of ways to apply optically clear hard films to the lens. Films are made of materials such as diamond-like carbon (DLC) and polycrystalline diamond. Through a process of ionization, a thin but extremely durable film is created on the surface of the lens. See Patent 5,268,217 for details.

We'll take a look at anti-reflective coatings on the next page. 

A common problem with sunglasses is called back-glare. This is light that hits the back of the lenses and bounces into the eyes. The purpose of an anti-reflective (AR) coating is to reduce these reflections off the lenses.

Similar to a scratch-resistant coating, AR is made of a very hard, thin film that is layered on the lens. It is made of material that has an index of refraction that is somewhere between air and glass. This causes the intensity of the light reflected from the inner surface and the light reflected from the outer surface of the film to be nearly equal. When applied in a thickness of about a quarter of light's wavelength, the two reflections from each side of the film basically cancel each other out through destructive interference, minimizing the glare you see. AR coatings are also applied to the front of prescription eyewear and some sunglasses to eliminate the "hot spot" glare that reflects off the lens.

Ultraviolet Coating

Several of the most serious eye problems can be linked to one cause: UV light. UV is often separated into two categories based on the frequency and wavelength of the light: UV-A and UV-B.

As a natural protection mechanism, the cornea of your eye absorbs all of the UV-B and most of the UV-A light. But some of the UV-A light reaches the lens of the eye, and over time this absorption can lead to cataracts. The small amount of UV-A that gets past your cornea and reaches the retina can eventually lead to macular degeneration, the leading cause of blindness in people older than age 65. Intense and prolonged exposure to UV radiation can cause either cancer of the eye or photokeratitis, which is basically a sunburn on your retina. Because it occurs most often when a person is outside on bright winter day, with sunlight glaring off the snow, this condition is commonly known as snow blindness.

A good UV coating on your sunglasses can eliminate UV radiation, and you should check to make sure that your sunglasses filter out 100 percent of both types of UV rays. There should be a statement on the label telling you how much UV protection the sunglasses have. You want 100-percent protection.

The previous section on UV coatings discusses the problem of UV radiation. While the effects of UV radiation are a general concern, other health considerations are more specific. For example, if you have very sensitive eyes, you may wish to use sunglasses that filter a larger portion of the visible light spectrum. Some sunglasses offer protection from infrared radiation as well as UV.

Sunglasses can also be tremendously useful as safety devices. By reducing glare, they allow you to be more visually aware of your surroundings. You may have noticed that most highway patrol officers wear sunglasses. Their sunglasses increase their ability to focus on objects, such as your car speeding by at 75 mph (121 kph), without having to squint.

Performance and Perception

Some sunglasses are equipped with specific filters that reduce glare, increase contrast, or even increase your depth perception. This makes them ideal for certain sports, like beach volleyball, or certain professions, like the highway patrol. Also, by filtering out harmful radiation, sunglasses allow you to spend more time engaged in outdoor activities without suffering some of the drawbacks. Serious fishermen love polarized sunglasses because polarized lenses get rid of the glare coming off the water. This makes it much easier to see deeper into the water and spot that elusive school of fish.

Choosing the Perfect Features for You

The key to finding the perfect pair of sunglasses is to pick the right features for your situation. Here are some of the most important features to compare when you buy a pair of sunglasses:

Fashion and image do play a role in sunglass selection. Ever since the 1960s, when FosterGrant ran the "Who's That Behind Those FosterGrants?" ad campaign, sunglasses have continued to increase in popularity. From Jack Nicholson in "Easy Rider" and John Belushi and Dan Akroyd in "The Blues Brothers" to Tom Cruise in, well, anything ("Risky Business," "Top Gun," "Jerry Maguire," "Mission Impossible II"), sunglasses have become a fashion icon. Styles range from conservative to outrageous, and come in every conceivable color.

There is a certain mystique when someone wears sunglasses. It is often said that the eyes are the windows to the soul, so it is no wonder that a device that hides them makes the wearer seem more provocative and mysterious.

For more information on sunglasses and related topics, check out the links on the next page.