Tints, Filters and Sunglasses
Tinted lenses are worn for the sake of appearance, comfort (discomfort glare), vision (disability glare) and protection against UV exposure. Sunglasses typically reduce transmission to 15%-25% and can be even darker for very bright conditions such as snow. At least 8% transmission is required to drive a car safely. Wearing sunglasses enables one to avoid the long delay of dark adaptation that otherwise occurs naturally in 2 to 3 hours of exposure to bright sunlight. Neutral gray and green gives the truest color perception. Brown lenses block blue light, enhancing contrast against a blue sky. Some skiers, boaters and pilots favor amber to enhance contrasts. Polarized lenses filter out the horizontally polarized light that is reflected from surfaces such as water, roads and snow. Polarized lenses are particularly useful for driving, in that they block the horizontally polarized image of the automobile dashboard reflected by the windshield. They are also helpful to the angler who wishes to see the fish, rather than the reflections coming from the surface of the water.
Lenses allowing more than 20% or 25% transmission are traditionally called comfort tints rather than sunglasses. In some situations other than bright light, people are said to benefit from the filtering of various colors. For example, red lenses may be retinitis pigmentosa, orange for macular degeneration, green for some cases of deuteranopia.(as long as they do not have problems seeing traffic lights). Brown for protanopia, neutral gray for other color deficiencies, flesh-tones or pink for people working in fluorescent light, and yellow for target shooters. Gradient tints, fading to clear from the top to bottom of the lens help some patients read through the bifocal better as well as serve as a fashionable look.
Photochromic lenses are activated by UV light. They may not serve the purpose as sunglasses in all conditions but can be worn in any situation and allow you not to be switching glasses. When activated a transitions CR-39 lens blocks 85% in cool weather, but only 72% (and 65% in poly) on a hot day.
UV protection is the most important eye health consideration for sunglasses. If CR-39 material is used a UV coating should be applied to the sunglass lens. If poly is used the material itself has UV blocking properties and does not need the additional coating.
Backside antireflective coating can be applied to sunglasses to reduce reflections.