Sunglasses or prescription eyeglasses that darken when exposed to the sun are called photochromic, or sometimes photochromatic. Developed by Corning in the late 1960s and popularized by Transitions in the 1990s, photochromic lenses rely on a specific chemical reaction to UV radiation.
Photochromic lenses have millions of molecules of substances, such as silver chloride or silver halide, embedded in them. The molecules are transparent to visible light in the absence of UV light, which is the normal makeup of artificial lighting. But when exposed to UV rays in sunlight, the molecules undergo a chemical process that causes them to change shape. The new molecular structure absorbs portions of the visible light, causing the lenses to darken. The number of the molecules that change shape varies with the intensity of the UV rays.
When you go indoors and out of the UV light, the reverse chemical reaction takes place. The sudden absence of UV radiation causes the molecules to "snap back" to their original shape, resulting in the loss of their light absorbing properties. In either direction, the entire process occurs very rapidly.
In the PhotoBrown and PhotoGrey products made by Corning in the '60s, the lenses were made of glass, and the molecules are distributed evenly throughout each entire lens. The problem with this method became apparent when it was applied to prescription glasses, in which different parts of the lens can vary in thickness. The thicker parts would appear darker than the thinner areas. But with the increasing popularity of plastic lenses, a new method has been developed. By immersing plastic lenses in a chemical bath, the photochromic molecules are actually absorbed to a depth of about 150 microns into the plastic. This proved to be much better than a simple coating, which would only be about 5 microns thick and would not provide enough molecules to make the lenses sufficiently dark. This plastic lens absorption process has been popularized by Transitions, the leading manufacturer of photochromic lenses.
On the next page, we'll look at mirroring and scratch-resistant coatings.