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).
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).
- Direct light - Direct light is light that goes straight from the light source (like the sun) to your eyes. Too much direct light can wash out the details of your surroundings and make it almost painful to try to focus your vision on anything.
- Reflected light - Reflected light, usually in the form of glare (see explanation above), is light that has bounced off a reflective object to enter your eyes. Just like direct light, strong reflected light can make it difficult to perceive the details or directly view an object. Snow, water, glass and white sand are all good reflectors.
- Ambient light - Ambient light is light that has bounced and scattered in many directions so that it is does not seem to have a specific source. A good example of ambient light is the glow in the sky around a major city. It would be very hard to identify a single source of light for that glow. Ambient light is how you are able to see when there is no direct source of light.
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.