Science

• According to KnowYourEyes.com, the change in ultraviolet, or UV, radiation from indoors to outdoors triggers the change in tint in the glasses. Glass photochromic lenses used silver halide crystals embedded in the glass. When exposed to the UV rays, electrons from the glass combined with the crystals to form visible silver, which darkened the lenses. When removed from the UV light, the process reverses itself. For plastic lenses, organic dyes are now used that break down chemically when exposed to the UV light. That process causes the lenses to darken.
  
  

Benefits

• In addition to health benefits, photochromic lenses provide the convenience of one pair of glasses for indoor and outdoor use. For your health, excess exposure to UV light causes permanent damage to the cornea of the eyes. By having glasses that automatically darken, the user is protected without having to worry about changing glasses or forgetting to carry that second pair of glasses.
  
  

Problems

• According to KnowYourEyes.com, one problem with photochromic lenses occurs when wearing the glasses inside an automobile that has windows with UV protection. Because the windows block the UV rays, the glasses will not darken. The extent of the problem depends upon the type of windows and how much of the UV radiation is being blocked. Another problem is the amount of time the glasses take to darken. Improvements in the organic dyes used in plastic lenses have lowered the time, but the user can still encounter problems when leaving a building and going out into the bright sunlight, waiting for the lenses to darken.
  
  

Military Uses

• According to ShadowSpear.com, the transition problem for military users has been solved. Navy Special Warfare Command personnel are set to receive fast-tint protective eyewear. The Office of Naval Research reports that the transition time will be 0.5 seconds. This will allow military personnel to enter a dark building from the bright sunlight without having to switch lenses in their goggles. When engaged in combat activities, that time difference can be crucial. The lenses contain liquid crystal solutions with customized dyes in addition to meeting ballistic impact safety requirements.