The Principle Of Photochromic Lenses

The principle of photochromic lenses mainly relies on a special chemical reaction of ultraviolet rays.

The photochromic lenses of sunglasses contain millions of molecules of certain substances, such as silver chloride or silver halide. Visible light, a common component of artificial light sources, penetrates these molecules when there is no UV light. But when exposed to sunlight's ultraviolet rays, the molecules undergo a chemical process that changes their shape. The new molecular structure absorbs some of the visible light, darkening the lens. The number of molecules whose shape changes changed with the intensity of the UV light.

When you come indoors and leave the UV rays, the corresponding chemical inverse changes will occur. When suddenly removed from ultraviolet light, these molecules will quickly revert to their original structure, thereby losing their light-absorbing properties. Whether it is a positive change or a negative change, the whole process happens very quickly.

In Corning's PhotoBrown and PhotoGrey lines from the 1960s, the lenses were made of glass with molecules evenly distributed throughout the lens. The problems it poses become apparent when using this method in prescription glasses, as the thickness of different parts of the prescription lens can vary, with slightly thicker areas appearing darker. But with the increasing popularity of plastic lenses, a new method has been developed. By soaking the plastic lens in a chemical bath, the color-changing molecules will be absorbed into the plastic lens to a depth of about 150 microns. The new method is significantly better than simple coating processes in which the color-changing molecules are only 5 microns thick, which doesn't provide enough molecules to darken the lenses. Transitions, a leader in photochromic lens manufacturing, has popularized this plastic lens adsorption process.