Preparation of photochromic gradient lenses

Abstract From the Canadian patent

Glass lenses or lens blanks containing all the ingre-, dients necessary to produce phototropic or photochromic behavior are treated in a conventional production furnace to produce a locally variable heat treatment, wherein at least one portion thereof is raised to a temperature exceeding the glass strain point but not the softening point, and other portions are heated to variable temperature decreasing from the strain point. The treatment causes development of phototropic or photochromic behavior only in those portions of the lenses or lens blanks exposed to the temperature above the strain point.