Abstract Photochromic glass compositions for thin refractive-index-corrected lens elements consisting essentially of about: ______________________________________ SiO 2 54-58%
B 2 O 3 18-22%
Al 2 O 3 7-8%
Li 2 O
3.75-4.5%
Na 2 O
0-1%
K 2 O
5.5-7.5%
TiO 2 0-2%
ZrO 2 2-4.5%
Ag 0.20-0.33%
Cl 0.30-0.50%
Br 0.04-0.12%
CuO 0.007-0.012%
PbO 0-0.08%
Sb 2 O 3 0-0.20%
______________________________________ having a Li 2 O:Na 2 O molar ratio of at least 9:1 and exhibiting good darkening, fast fading, good chemical strengthenability and neutral dark-state coloration at thicknesses on the order of 1 mm, are described.
A composition for a photochromic glass consisting essentially, in weight percent on the oxide basis of about: ______________________________________
SiO.sub.2
54-58%
B.sub.2 O.sub.3
18-22%
Al.sub.2 O.sub.3
7-8%
Li.sub.2 O
3.75-4.5%
Na.sub.2 O
0-1%
K.sub.2 O
5.5-7.5%
TiO.sub.2
0-2%
ZrO.sub.2
2-4.5%
Ag 0.20-0.33%
Cl 0.30-0.50%
Br 0.04-0.12%
CuO 0.007-0.012%
PbO 0-0.08%
Sb.sub.2 O.sub.3
0-0.20%
______________________________________ wherein the mole ratio Li 2 O:Na 2 O equals or exceeds 9:1.
Claim 2
Dependent Claim
66 words
A composition for a photochromic glass consisting essentially, in weight percent on the oxide basis of about: ______________________________________
SiO.sub.2
54-58%
B.sub.2 O.sub.3
18-22%
Al.sub.2 O.sub.3
7-8%
Li.sub.2 O
3.75-4.5%
Na.sub.2 O
0-1%
K.sub.2 O
5.5-7.5%
TiO.sub.2
0-2%
ZrO.sub.2
2-4.5%
Ag 0.20-0.33%
Cl 0.30-0.50%
Br 0.04-0.12%
CuO 0.007-0.012%
PbO 0-0.08%
Sb.sub.2 O.sub.3
0-0.20%
______________________________________ wherein the mole ratio Li 2 O:Na 2 O equals or exceeds 9:1.
Claim 3
Dependent Claim
89 words
A photochromic glass having a composition according to claim 1 and exhibiting, when free of added tint at a thickness of about 1 mm, a clear luminous transmittance of approximately 88-92%, a fully darkened transmittance at 25° C. not exceeding about 35%, a fading rate at 25° C. from fully darkened state providing at least 35 percentage points of fading in 5 minutes, and a coloration in the fully darkened state corresponding to chromaticity coordinates in the range of: x=0.3070-0.3140 and y=0.3050-0.3200 in the 1931 C.I.E. trichromatic colorimetric system.
Claim 4
Dependent Claim
53 words
A photochromic lens element formed of a glass according to claim 2 having a thickness not exceeding about 1 mm, a refractive index n D =1.523, a chemically strengthened modulus of rupture strength of at least about 40,000 psi, and a surface compression layer after chemical strengthening of at least 0.003 inches thickness.
Claim 5
Dependent Claim
51 words
A photochromic lens element in accordance with claim 3 which has the following analyzed composition: ______________________________________
SiO.sub.2
55.2
B.sub.2 O.sub.3
20.6
Al.sub.2 O.sub.3
7.4
Li.sub.2 O
4.3
Na.sub.2 O
0.80
K.sub.2 O
6.1
ZrO.sub.2
4.0
TiO.sub.2
1.4
PbO 0.03
Ag 0.25
Cl 0.34
Br 0.095
CuO 0.009
Sb.sub.2 O.sub.3
0.03
______________________________________