Thermally darkenable photochromic glasses

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Abstract
Thermally darkenable photochromic (TDPC) glasses have been produced by precipitating copper−doped silver halide crystallites in certain lanthanum borate base glasses. A TDPC glass is characterized by a darkened state which is stable at room temperature but which can be bleached by visible light. The glass will return to the darkened state with the passage of time. The rate of darkening and the color of the darkened state are affected by temperature. These TDPC glasses are further characterized by the vanishing of the darkened color above a certain high temperature and redarkening of the glass below a certain somewhat lower temperature. These temperatures are believed associated with the melting and crystallization of the silver halide particles.