Photochromic Sunglasses

The lenses which darken on exposure to ultraviolet (UV) radiation are known as photochromic lenses. In the absence of UV rays, the darkened lenses return to their transparent or clear state. When such lenses are prescribed to a person for usage, they are called photochromic sunglasses. They are also referred to as transitional or photochromatic lenses. The photochromic sunglasses are generally made of glass, polycarbonate, or some other kind of plastic.

The glass version of photochromic lenses was developed by Corning in the 1960s. Plastic versions of this type of lenses is also available. They are commonly meant for commercial usage. Such sunglasses benefit regular users of eye lenses a lot. It prevents them from carrying two or three pairs of eyeglasses while going out. You might be wondering how photochromic sunglasses change their color on getting exposed to UV rays.

Photochromic Sunglasses: An Introduction
The reason behind the ability of the photochromic sunglasses to change their color from light to dark is its molecular structure and the effect of light on it. The polarized photochromic sunglasses contain millions of molecules of substances like silver chloride or silver halide. These molecules are transparent to visible light or in the absence of UV light. When they are exposed to UV rays, a chemical reaction take place which causes the molecules to change shape. The changed molecular structure absorbs portions of the visible light. This causes the lenses to get darkened. Therefore, polarized lenses are quite effective.

The number of the molecules which change shape differs from the intensity of the UV rays. When the photochromic sunglasses are no more exposed to UV rays, the reverse chemical reaction occurs. The sudden absence of UV light results in the snapping back of the molecules to their original shape. This, in turn, results in the loss of their light absorbing properties. In both the direction, the entire process of change in the molecular structure takes place quite quickly. The darkening of the glass is directly proportional to the intensity of the sunlight.

Photochromic Sunglasses: Pros and Cons
The photochromic lenses have both advantages and disadvantages. Their main advantage is that they get darkened like sunglasses on getting exposed to UV rays. They are particularly helpful to the people who ride motorcycle. Many motorcyclists are forced to carry 2-3 pairs of sunglasses or interchangeable lenses kits with clear, charcoal and amber color lenses. With photochromic lenses, they can buy and carry just one pair of sunglasses for all day and even for night as a replacement for night vision goggles. Therefore, these sunglasses eliminate the need to carry a separate pair of sunglasses for protecting the eyes against the harmful effects of UV rays of the sun. The disadvantage of photochromic sunglasses is that they do not adjust themselves to their darkened form immediately. They take about two minutes to completely change from clear to dark form or vice versa. There is one more disadvantage associated with these sunglasses. It is that they don't get darkened when worn inside a vehicle. This is because, the UV rays are not able to pass through the glass of the vehicle.

As has already been indicated in this present article, photochromic sunglasses are available only on prescription. Therefore, if you wish to have a pair of it for better eye care, fix an appointment with your ophthalmologist.