Photochromic boro-silicate glass

DETAILED DESCRIPTION OF THE INVENTION Specific embodiments of the invention will now be described in more detail by way of example and with reference to the following table, which sets out examples of glass compositions in accordance with the invention, showing their compositions on the oxide basis and the photochromic effect achieved in terms of the induced optical density (ODd) and the time in seconds taken to fade from the darkened condition to a condition in which half of the lost light transmission has been restored, known as the half fading time (1/2 FT), measured with standard samples of glass 2 mm.
thick in standard simulated solar conditions at air mass 2 (see Parry Moon, J.
Franklin Inst, 230 (1940, pages 583-617).
The induced optical density is the difference between the optical density of the glass in the fully darkened state and the optical density of the glass in the fully faded state, the optical density being defined in the conventional manner as log.
sub.
10 (Ii/I.
sub.
t), where Ii is the intensity of the incident light and I.
sub.
t is the intensity of the transmitted light.
The induced optical density is thus a real measure of the photochromic effect and is in fact directly proportional to the number of photochromically activated silver atoms in a given volume of the glass.
The half fading time (1/2 FT) measures the speed of response of the glass to removal of actinic radiation.
The table also lists the temperature (HT.
degree.
C.
) and the time of the heat treatment applied to each of the glasses.
For glasses 3 to 27, a standard heat treatment time of one hour was used in each case, for comparative purposes only.
Finally the table lists the refractive index n.
sub.
D of each of the glasses.

______________________________________
Ag.
sub.
2 O 0.
08 to 0.
39%
CuO 0.
004 to 1%
Cl 0.
04 to 0.
5%
Br 0 to 1.
0%
F 0 to 0.
2%
______________________________________
The compositions listed in the table can be made up in the following manner