text: This invention relates to a light sensitive plastic element whose absorbance varies directly with the intensity of ambient activating radiation. More particularly, it relates to transparent polymeric materials having photosensitive silver halide absorbed into the structure of the surface layer thereof, which silver halide can undergo a reversible, light induced chemical change wherein light absorbing species and nonabsorbing species are alternately produced depending on the intensity of ambient electromagnetic radiation.
  Silver halides have several advantages over organic photochromic materials as an operative photochromic element. First, when properly treated, they are inherently stable and hence fatigue free, i.e. do not lose their photochromic properties in repeated exposures to activating radiation. Second, on activation, the light transmittance of silver halide based photochromic articles decreases over the entire visible spectrum resulting in activated gray or gray-brown shades.
  Attempts to impart silver halide based photochromic properties to windows, lenses, and other articles made from transparent polymeric materials have not, in general, been overly successful. While it is well known that silver halides undergo a chemical change which results in a light absorbing species, it is not altogether clear what features of the environment of such photosensitive silver halides must be controlled in order to assure that the activated species, in the absence of activating radiation, will return to its light transmissive state in a reasonable period of time.
  One condition that seems to be important is that the silver halide crystals must be shielded from the effects of the chemistry present in polymerizing materials, i.e., catalysts and initiators, which in general have a deactivating effect on the photosensitive crystals. For this reason, in contrast to the conditions present in silver halide based photochromic glass, it is not possible simply to form silver halide crystals directly in prepolymers which thereafter may be polymerized to form optical devices.
  Another aspect of the silver halide crystal environment which seems to be important is that the material surrounding the photosensitive crystals must define a barrier which can prevent any substantial diffusion of halogen, since recombination of halogen and the activated silver species which is believed to occur during recovery is obviously not possible if the halogen is lost.
  Still another important parameter is the size of the silver halide crystal which is incorporated into the host material. In this regard, it appears that crystals over 1 micron in diameter either do not undergo reversible, light-activated reactions, or do so to a degree insufficient to impart significant photochromism to host materials.