Lightly tinted glasses of variable transmission

Claim 1

A lightly tinted, variable transmission glass having a base composition consisting essentially, reported in terms of weight percent on the oxide basis, of ______________________________________ SiO.sub.2 46-60 MgO 0-3.5 B.sub.2 O.sub.3 16-28 CaO 0-6 Al.sub.2 O.sub.3 4-11 SrO 0-6 ZrO.sub.2 2-6 BaO 0-6 Li.sub.2 O 1.5-5 P.sub.2 O.sub.5 0-5 Na.sub.2 O 0-6 TiO.sub.2 0-3 K.sub.2 O 2.5-9 ______________________________________ and also containing small amounts of the photochromic elements Ag, Cl, Br, and CuO, characterized in that (A) the said photochromic elements are present in the following proportions, reported in terms of weight percent as analyzed in the glass ______________________________________ Ag 0.08-0.18 Br 0.08-0.18 Cl 0.06-0.26 CuO 0.0001-0.0020 ______________________________________ with the conditions that (i) when the amount of Ag is greater or equal to 0.12%, the ratio Br:(Cl+Br) should be greater or equal to 0.25 and (ii) when the amount of Ag is less than 0.12% the ratio Br:(Cl+Br) should be greater or equal to 0.30; and (B) the glass, when it is in the form of a lens of 2 mm thickness, exhibits the following optical properties: (a) a luminous transmission in the clear state (T o ) greater than about 86%; (b) a luminous transmission in the darkened state after 15 minutes' exposure to actinic radiation (T D15 ) over the full extent of temperatures of 0°-25° C. between about 60-80%; (c) a fading rate at ambient temperature (20°-25° C.) such that, after five minutes' withdrawal from actinic radiation, the glass exhibits a luminous transmission (T F5 ) of at least 78%; (d) a difference of luminous transmission in the darkened state over the temperature interval of 0°-25° C. of less than five points of transmission percentage; and (e) a difference of luminous transmission in the darkened state over the temperature interval of 25°-40° C. of less than six points of transmission percentage.