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.