| The latest fashion trend, the glasses that that | | | | directions, the entire process happens very |
| automatically become sunglasses when we move | | | | rapidly. |
| out in the sun. cool! Sunglasses or prescription | | | | In the original Photo Brown and Photo Grey |
| eyeglasses that darken when exposed to the sun | | | | products made by transitions, the lenses are |
| were first popularized by Transitions in the 1990s. | | | | made of glass, and the molecules are distributed |
| In fact, because of the extreme popularity of the | | | | evenly throughout the entire lens. The problem |
| Transitions brand, these lenses are usually | | | | with this method was apparent in prescription |
| referred to as transition lenses. The correct term | | | | glasses where different parts of the lens were of |
| for these glasses is photo chromic or photo | | | | varying thickness. The thicker parts would appear |
| chromatic, which refers to a specific chemical | | | | darker than the thinner areas. But with the |
| reaction the lenses have to ultraviolet (UV) | | | | increasing popularity of plastic lenses, a new |
| radiation. | | | | method has been developed. By immersing the |
| | | | lenses in a chemical bath, the photo chromatic |
| Photo chromic lenses have millions of molecules of | | | | molecules are actually absorbed to a depth of |
| substances such as silver chloride or silver halide | | | | about 150 microns into the plastic. This is much |
| embedded in them. The molecules are transparent | | | | better than a simple coating, which would only be |
| to visible light in the absence of UV light, which is | | | | about 5 microns thick and would not provide |
| normal for artificial lighting. But when exposed to | | | | enough molecules to make the lenses sufficiently |
| UV rays, as in direct sunlight, the molecules | | | | dark. This plastic lens absorption process has been |
| undergo a chemical process that causes them to | | | | popularized by Transitions, the leading |
| change shape. The new molecular structure | | | | manufacturer of photo chromic lenses. |
| absorbs portions of the visible light, causing the | | | | An important note about photo chromic lenses: |
| lenses to darken. The number of molecules that | | | | because they react to UV light and not to visible |
| change shape varies with the intensity of the UV | | | | light, there are circumstances under which the |
| rays. | | | | darkening will not occur. A perfect example of this |
| When you go indoors and out of the UV light, a | | | | is in your car. Because the windshield blocks out |
| different chemical reaction takes place. The | | | | most UV light, photo chromic lenses will not |
| absence of the UV radiation causes the molecules | | | | darken. For this reason, most sunglasses with |
| to "snap back" to their original shape, resulting in | | | | photo chromic lenses also have a certain amount |
| the loss of their light absorbing properties. In both | | | | of tint already applied to them. |