SunGlass Information
Archive for April, 2009
Light reflected by shiny transparent materials is partly or fully polarized, except when the light is normal to the surface. It was through this effect that polarization was first discovered in 1808 by the mathematician Etienne Louis Malus. A polarizing filter, such as a pair of polarized eyewear, can be used to observe this effect by rotating the filter while looking through it at the reflection off of a distant horizontal surface. At certain rotation angles, the reflected light will be reduced or eliminated. Polarizing filters remove light polarized at 90° to the filter’s polarization axis. If two polarizers are placed atop one another at 90° angles to one another, there is minimal light transmission.
Polarization by scattering is observed as light passes through the atmosphere. The scattered light produces the brightness and color in clear skies. This partial polarization of scattered light can be used to darken the sky in photographs, increasing the contrast. This effect is easiest to observe at sunset, on the horizon at a 90° angle from the setting sun. Another easily observed effect is the drastic reduction in brightness of images of the sky and clouds reflected from horizontal surfaces, which is the main reason polarizing filters are often used in sunglasses. Also frequently visible through polarizing eyewear is rainbow-like patterns caused by color-dependent birefringent effects, for example in toughened glass (e.g., car windows) or items made from transparent plastics. The role played by polarization in the operation of liquid crystal displays (LCDs) is also frequently apparent to the wearer of polarizing sunglasses, which may reduce the contrast or even make the display unreadable.
In the early 1900s, the use of sunglasses started to become more widespread, especially among the pioneering stars of silent movies. It is commonly believed that this was to avoid recognition by fans, but the real reason was they often had perennially red eyes from the powerful arc lamps that were needed due to the extremely slow speed film stocks used. The stereotype persisted long after improvements in film quality and the introduction of ultraviolet filters had eliminated this problem. Inexpensive mass-produced sunglasses were introduced to America by Sam Foster in 1929. Foster found a ready market on the beaches of Atlantic City, New Jersey, where he began selling sunglasses under the name Foster Grant from a Woolworth on the Boardwalk.
Sunglasses first became polarized in 1936, when Edwin H. Land began experimenting with making lenses with his patented Polaroid filter. Since then, polarized lenses have evolved, whereby new substrates and lens technology processes offer consumers better clarity, protection and comfort.
If you work outdoors, you probably get a lot of sun. That means that you need to keep your eyes and skin protected to prevent damage. One way to protect your eyes is to wear polarized sunglasses. But don’t fall into the trap of buying fake polarized sunglasses.
In order to make sure your sunglasses are polarized look through the glasses at the reflection of any object on a window panel. Then, turn the glasses around as if they were the hands of a clock facing you. If the intensity of the reflection doesn’t change with respect to what you see through the window, they are not polarized.