Heat treatment method for making photochromic gradient lenses

Claim 1

In a method of making ophthalmic quality lenses, or lens blanks, having a reversible, progressive, local variation in phototropic, or photochromic, behavior, said lens or lens blank including a portion ultimately to serve as a distance portion and another portion ultimately to serve as a near portion, the distance portion and near portion being contiguous with the near portion being below the distance portion when the lens is glazed in a frame, the reversible, progressive, local variation in phototropic or photochromic behavior being substantially more resistant to light transmission near the top which defines the upper limit of the distance portion thereof and substantially less resistant to light transmission in the near portion when said lens is glazed in a frame, the transition in light transmission from the top of the lens to the bottom being substantially uniform laterally of said lens when glazed, comprised of an unnucleated oxide glass lens or lens blank, exhibiting no photochromic properties to the eye before nucleation, and being made of glass consisting of ophthalmic quality glass, said lens, or lens blank, containing through its volume at least 0.005 volume percent of at least one silver halide selected from the group consisting of silver chloride, silver bromide, silver iodide and mixtures thereof, after nucleation in at least that portion of the lens, or lens blank, showing phototropic or photochromic behavior, said halide being in the form of particles, there being a progressive variation in the average linear dimension of said particles substantially corresponding to the variation in phototropic or photochromic behavior of the lens, or lens blank, the average linear dimensions of said silver halide particles being smaller than about 5 nanometers in the near portion and said particles substantially, progressively, increasing in size to about 50 nanometers in the distance portion, the improvement comprising the steps of forming a lens, or lens blank, from a glass making batch, said lens, or lens blank, containing throughout its volume at least 0.005 vol. % of at least one silver halide selected from the group consisting of silver chloride, silver bromide, silver iodide, and mixtures thereof, subjecting said glass lens, or lens blank, to a heat treatment, said heat treatment consisting of subjecting the distance portion of such a lens, or lens blank, to a temperature above the strain point but below the softening point thereof for a time period sufficient to grow silver halide particles having an average linear dimension of about 50 nm, substantially progressively lessening the temperature of heat treatment across said lens from the top edge which defines the upper limit of the distance portion to that area spaced therefrom which constitutes the near portion in such a manner as to form silver halide particles of progressively smaller average linear dimension to thereby produce a lens characterized by a local variation in phototropic or photochromic behavior the greatest photochromic or phototropic behavior being exhibited in the distance portion.