an inorganic glass capable of reversibly changing its light transmission in the visible region of the spectrum upon exposure to ultraviolet or short-wavelength visible radiation.

The photosensitivity of photochromic glass is caused by photochemical processes, which may be associated either with electron transfer between elements of variable valence—for example, Eu^II and Ce^III—or with the photolysis of halides of heavy metals. In the case of photolysis, the halides are uniformly distributed as microcrystallites throughout the glass.

Glasses containing silver halides are the most widely used photochromic glasses because of their desirable photochromic characteristics—such as achieving a sufficient optical density upon darkening and rapid rates of darkening and fading—and technological properties. Photochromic glasses containing copper halides or thallium chloride are also used.

Photochromic glass may be a soda-lime-silica, borate, borosilicate, germanate, or phosphate glass. Photochromic glasses and the glasses used in technology are manufactured under the same conditions.

Photochromic glasses may be used, for example, in instrumentation as variable-transmission light filters, in construction to control lighting and heating in buildings, in holography as a medium for recording information, in medicine for special-purpose eyeglasses, and in aircraft and rocket manufacture for cockpit, cabin, or compartment windows.