Ultraviolet absorbing photochromic glass of low silver content

There is disclosed a method of producing a photochromic glass having a reduced transmittance for ultraviolet radiation while having substantially unimpaired photochromic properties. The method comprises providing a combination of 0.1-1.0% CeO.sub.2 and 0.1-1.5% Sb.sub.2 O.sub.3 and/or As.sub.2 O.sub.3 as part of the glass composition. In an embodiment utilizing minimal silver content, the glass article in thicknesses of 1.3 to 2.0 mm. can be chemically strengthened to surpass the impact specification imposed by the Federal Food and Drug Administration for eyewear, will transmit less than 0.2% of radiation having wavelengths between 290-315 nm, and will demonstrate a darkened luminous transmittance at 20.degree.-25.degree. C. below 35% and a fading rate such that after five minutes the luminous transmittance will be at least 1.75 times that of the darkened transmittance. These glasses consist essentially of: ______________________________________ SiO.sub.2 55-61 Ag >0.1-<0.2 B.sub.2 O.sub.3 18-21 Cl 0.2-0.6 Al.sub.2 O.sub.3 5-11 Br 0.05-0.2 Li.sub.2 O 1.5-3 CuO 0.003-0.015 Na.sub.2 O 2-5 CeO.sub.2 0.1-1.0 K.sub.2 O 4.5-8 Sb.sub.2 O.sub.3 0.1-1.5 ______________________________________

BACKGROUND OF THE INVENTION 
Photochromic or phototropic glasses had their genesis in U.S. Pat. No. 
3,208,860. Such glasses become darker, i.e., they change color, when 
subjected to actinic radiation, customarily ultraviolet radiation, and 
return to their original state when withdrawn from exposure to that 
radiation. That patent was addressed generally to silicate-based glasses, 
most preferably alkali metal aluminoborosilicate-based glasses, containing 
crystallites of at least one silver halide from the group of silver 
chloride, silver bromide, and silver iodide. The mechanism underlying the 
reversible darkening capability of those silver halide glasses was 
discussed in some detail in that patent and that explanation is 
incorporated herein by reference. 
The most widespread commercial application for photochromic glasses has 
been as spectacle lenses, i.e., as prescription lenses and as 
non-prescription sunglasses. One of the first commercial photochromic 
ophthalmic lenses was marketed by Corning Glass Works, Corning, N.Y., 
under the trademark PHOTOGRAY. 
Recently, some manufacturers of ophthalmic products have questioned whether 
the eye might be damaged by long term exposure to ultraviolet radiation. 
Particular attention has been directed to limiting transmission in the 
UV-B, or erythemal, zone. Such a limitation, if imposed, would present a 
potential problem to a photochromic lens manufacturer who considers using 
thinner glass with lower silver content in lens manufacture. 
The thinner the lens the more lightweight it will be. Further, since silver 
is the single most expensive component of the glass, a reduction in the 
content thereof self-evidently decreases the cost of the glass. 
Unfortunately, however, those actions reduce the capability of the glass 
to absorb ultraviolet radiations in the erythemal zone. There is, of 
course, lack of agreement on the need to limit ultraviolet transmission. 
Nevertheless, to the extent that a glass lens of reduced transmission is 
deemed necessary or desirable, the present invention makes provision 
therefor in a thin, low-silver-content blank. 
U.S. Pat. No. 4,407,966, issued Oct. 4, 1983 in the names of David J. Kerko 
and David L. Morse, discloses the utility of CeO.sub.2 to reduce the 
transmittance at wavelengths in the near ultraviolet region of the 
radiation spectrum. However, it was also observed that the addition of any 
significant level of CeO.sub.2 substantially degraded photochromic 
performance. 
OBJECTIVES OF THE INVENTION 
A basic objective is to provide a means of utilizing the ultravoilet 
absorbing properties of CeO.sub.2 in a photochromic glass without 
degrading the photochromic properties of the glass. 
Another objective is to provide photochromic glasses having a high degree 
of darkening capability coupled with a fast fading rate and low silver 
content. 
A specific objective of the present invention is to provide 
copper-sensitized, silver halide-containing, photochromic glasses having a 
silver content less than 0.2% by weight which, when present in thickness 
dimensions of less than 2 mm, will transmit less than 0.2% of radiations 
having wavelengths between 290-315 nm. 
Another specific objective is to provide photochromic glasses for sunglass 
use wherein the silver content is in the range of 0.10% to 0.20% and 
wherein the thickness of the glass is about 1.5 mm. 
SUMMARY OF THE INVENTION 
In one aspect then, our invention is a method of producing a photochromic 
glass having reduced transmittance for ultraviolet radiation while having 
substantially unimpaired photochromic properties, which comprises 
providing a combination of 0.1 to 1% CeO.sub.2 and 0.1 to 1.5% Sb.sub.2 
O.sub.3 and/or As.sub.2 O.sub.3 as part of the glass composition. 
Application of our method permits producing a photochromic glass which 
contains a silver halide as a photochromic agent, which is copper 
sensitized and contains &lt;0.2% Ag but &gt;0.1% Ag, that, in cross section of 
&lt;2 mm but &gt;1.3 mm, can be chemically strengthened to pass the strength 
standards mandated by the FDA, and that, at 20.degree.-25.degree. C., 
exhibits a darkened luminous transmittance below 35% and a fading rate 
such that after five minutes the luminous transmittance will be at least 
1.75 times that of the darkened transmittance. 
Preferred embodiments of our invention are selected from an alkali 
metal-aluminoborosilicate sub-family of compositions. This sub-family, in 
addition to cerium plus antimony and/or arsenic oxides, is composed 
essentially, expressed in terms of weight percent on the oxide basis, of: 
______________________________________ 
SiO.sub.2 
55-61 K.sub.2 O 
4.5-8 
B.sub.2 O.sub.3 
18-21 Ag &gt;0.1-&lt;0.2 
Al.sub.2 O.sub.3 
5-11 Cl 0.2-0.6 
Li.sub.2 O 
1.5-3 Br 0.05-0.2 
Na.sub.2 O 
2-5 CuO 0.003-0.015 
______________________________________ 
PRIOR LITERATURE 
PHOTOGRAY lenses, marketed by Corning Glass Works, are produced from a 
glass having the following approximate analysis in weight percent. 
______________________________________ 
SiO.sub.2 
55.6 PbO 5.0 
B.sub.2 O.sub.3 
16.4 ZrO.sub.2 
2.2 
Al.sub.2 O.sub.3 
8.9 Ag 0.16 
Li.sub.2 O 
2.65 CuO 0.035 
Na.sub.2 O 
1.85 Cl 0.24 
K.sub.2 O 
0.01 Br 0.145 
BaO 6.7 F 0.19 
CaO 0.2 
______________________________________ 
As can be understood, and PHOTOGRAY lenses are no exception, a commercial 
glass reflects comprises drawn between various process and product 
desiderata. For example, PHOTOGRAY lenses constitute the result of 
tradeoffs made between such parameters as photochromic behavior, 
ophthalmic properties, chemical durability, chemical strengthening 
capability, along with melting and forming characteristics. Stated in 
another way, to be suitable for commercial ophthalmic lenses a 
photochromic glass must possess the base physical properties and 
processing parameters of a standard ophthalmic glass. 
Ophthalmic photochromic lenses, more recently marketed by Corning Glass 
Works under the trademark PHOTOGRAY EXTRA, are encompassed within the 
disclosure of U.S. Pat. No. 4,190,451. The glass employed, in the standard 
2 mm thickness of ophthalmic lenses, displays the following photochromic 
properties: 
(1) at about 40.degree. C. will darken to a luminous transmittance below 
55% in the presence of actinic radiation; will fade at least 25 percentage 
units of transmittance after five minutes' removal from the actinic 
radiation; and will fade to a luminous transmittance in excess of 80% in 
no more than two hours after being removed from the actinic radiation; 
(2) at about 20.degree. C. will darken to a luminous transmittance below 
40% in the presence of actinic radiation; will fade at least 30 percentage 
units of transmittance after five minutes' removal from the actinic 
radiation; and will fade to a luminous transmittance in excess of 80% in 
no more than two hours after being removed from the actinic radiation; and 
(3) at about --18.degree. C. will not darken to a luminous transmittance 
below 15% in the presence of actinic radiation. 
As defined in that patent, the luminous transmittance of a glass is 
represented by the value Y delineated in terms of the 1931 C.I.E. 
trichromatic colorimetric system utilizing the light source Illuminant C. 
This colorimetric system and light source are described by A. C. Hardy in 
Handbook of Colorimetry, Technology Press, M.I.T., Cambridge, Mass. 
(1936). 
The glass compositions encompassed within that patent consisted 
essentially, expressed in terms of weight percent on the oxide basis, of: 
______________________________________ 
Li.sub.2 O 0-2.5 
Na.sub.2 O 0-9 
K.sub.2 O 0-17 
Cs.sub.2 O 0-6 
Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 
8-20 
B.sub.2 O.sub.3 14-23 
Al.sub.2 O.sub.3 5-25 
P.sub.2 O.sub.5 0-25 
SiO.sub.2 20-65 
CuO 0.004-0.02 
Ag 0.15-0.3 
Cl 0.1-0.25 
Br 0.1-0.2 
Ag: (Cl + Br) 0.65-0.95 
molar ratio of alkali metal oxide: B.sub.2 O.sub.3 
0.55-0.85 
______________________________________ 
PHOTOGRAY EXTRA lenses have the approximate composition, expressed in terms 
of weight percent on the oxide basis, of: 
______________________________________ 
SiO.sub.2 
55.8 ZrO.sub.2 
4.89 
Al.sub.2 O.sub.3 
6.48 TiO.sub.2 
2.17 
B.sub.2 O.sub.3 
18.0 CuO 0.011 
Li.sub.2 O 
1.88 Ag 0.24 
Na.sub.2 O 
4.04 Cl 0.20 
K.sub.2 O 
5.76 Br 0.13 
______________________________________ 
PHOTOGRAY and PHOTOGRAY EXTRA lenses have been formed in accordance with 
the standard techniques employed in the fabrication of conventional, i.e., 
non-photochromic, ophthalmic lenses. Hence, a glass blank is pressed and 
that blank ground and polished to a predetermined prescription. The glass 
blank is heat treated in accordance with a predefined schedule to develop 
silver halide crystallites therein which, in turn, impart photochromic 
properties thereto. 
U.S. Pat. Nos. 4,018,965 and 4,130,437 observed that non-prescription 
photochromic lenses, for example, sunglass lenses, could be produced 
without resort to the above-described techniques of grinding and 
polishing. Both patents described potentially photochromic glass 
compositions having the capability of being drawn into glass sheet; the 
expression potentially photochromic designating glass which, as drawn into 
sheet, is essentially non-photochromic but which, after being exposed to 
an appropriate heat treatment, will manifest photochromic character. 
The glasses of each patent possess the capability of being chemically 
strengthened, when employed as sheet in thicknesses over the interval of 
about 1.3-1.7 mm, to comply with the strength standards for eyeglass lens 
safety mandated by the Federal Food and Drug Administration (FDA). The 
preferred glass compositions of U.S. Pat. No. 4,130,437 can be 
simultaneously heat treated to develop photochromic behavior therein and 
to sag the glass sheet into molds to form eyeglass lens blanks of the 
desired curvature in the manner disclosed in U.S. Pat. No. 4,088,470. That 
faculty, plus the capability of being readily formed into sheet, enables 
the relatively inexpensive and rapid production of sunglass lenses. 
The operable ranges of glass compositions disclosed in those patents are 
set out below in terms of weight percent on the oxide basis: 
______________________________________ 
U.S. Pat. No. 4,018,965 
U.S. Pat. No. 4,130,437 
______________________________________ 
SiO.sub.2 54-66 54-66 
Al.sub.2 O.sub.3 
7-15 7-15 
B.sub.2 O.sub.3 
10-25 10-25 
Li.sub.2 O 0.5-4 0.5-4 
Na.sub.2 O 3.5-15 3.5-15 
K.sub.2 O 0-10 0-10 
Li.sub.2 O + Na.sub.2 O + K.sub.2 O 
6-16 6-16 
PbO 0-3 0-1.25 
Ag 0.1-1 0.1-0.3 
Cl 0.1-1 0.2-1 
Br 0-3 0-0.3 
CuO 0.008-0.16 0.002-0.02 
F 0-2.5 0-2.5 
______________________________________ 
U.S. Pat. No. 4,168,339 describes the fabrication of photochromic glass as 
microsheet, i.e., sheet glass with thickness dimensions between about 
0.25-0.5 mm. Glasses suitable for forming such sheet have compositions, 
expressed in terms of weight percent on the oxide basis, of: 
______________________________________ 
SiO.sub.2 
54-66 PbO 0.4-1.5 
Al.sub.2 O.sub.3 
7-16 Br 0.2-0.5 
B.sub.2 O.sub.3 
10-30 Cl 0.5-1.2 
Na.sub.2 O 
3-15 F 0.2-0.5 
Li.sub.2 O 
0-4 CuO 0.008-0.03 
K.sub.2 O 
0-10 Ag &gt;0.03-1 
______________________________________ 
U.S. Pat. No. 4,358,542 is also directed to the production of photochromic 
glasses in sheet form which, where desired, can be simultaneously heat 
treated to induce photochromic behavior therein and to sag the glass into 
conformance with molds to form spectacle lens blanks of a suitable 
curvature. Those glasses are stated to consist essentially, expressed in 
terms of weight percent on the oxide basis, of. 
______________________________________ 
SiO.sub.2 
58.5-60 PbO 0.1-0.25 
Al.sub.2 O.sub.3 
9-10 Ag 0.1-0.15 
B.sub.2 O.sub.3 
19-20.5 Cl 0.3-0.5 
Li.sub.2 O 
2-2.5 Br 0.05-0.15 
Na.sub.2 O 
2-3 CuO 0.0065-0.01 
K.sub.2 O 
6-7 
______________________________________ 
U.S. Pat. No. 4,407,966, issued Oct. 4, 1983 in the names of David J. Kerko 
and David L. Morse, is directed to the production of photochromic glasses 
displaying very rapid fade rates. The glasses disclosed consist 
essentially, expressed in terms of weight percent on the oxide basis, of: 
______________________________________ 
SiO.sub.2 
56-60 TiO.sub.2 
0-3 
B.sub.2 O.sub.3 
18-21 PbO 0.1-0.15 
Al.sub.2 O.sub.3 
6-9 Ag &gt;0.15-0.25 
Li.sub.2 O 
&gt;2.5-3.5 Cl 0.2-0.35 
Na.sub.2 O 
0.5-2.5 Br 0.075-0.15 
K.sub.2 O 
5-7 CuO 0.004-0.02 
ZrO.sub.2 
3.75-5 
______________________________________ 
The optional addition of CeO.sub.2 is disclosed as being effective to 
reduce the transmission in the near ultraviolet portion of the radiation 
spectrum. 
The photochromic characteristics of the several above-disclosed glasses are 
stated to be as follows: 
The glasses of U.S. Pat. No. 4,018,965 are asserted to exhibit at ambient 
temperatures, viz., 20.degree.-25.degree. C., a clear luminous 
transmittance of at least 60%, a darkened luminous transmittance not 
exceeding 25%, and a rate of fading such that the glass evidences a faded 
luminous transmittance after a five-minute fading period from the darkened 
state of at least 1.5 times that of the darkened transmittance; 
The glasses of U.S. Pat. No. 4,130,437 are noted as demonstrating at 
temperatures of 20.degree.-25.degree. C. clear luminous transmittances of 
at least 60%, darkened luminous transmittances below 30%, fading rates 
such that the glasses exhibit faded luminous transmittances after a 
five-minute interval from the darkened state of at least 1.75 times those 
of the darkened transmittances, and, after one-hour fading period, the 
glasses manifest luminous transmittances greater than 80% of their 
original luminous transmittances; 
The glasses of U.S. Pat. No. 4,168,339 are stated to evidence darkened 
luminous transmittances at room temperature below 50% and fading rates 
such that, after a five-minute period of fading from the darkened state, 
the transmittances will have increased by at least 20 percentage units 
and, after a one-hour fading interval, the glasses will exhibit luminous 
transmittances in excess of 80%; 
The glasses of U.S. Pat. No. 4,358,542 are observed to demonstrate a clear 
luminous transmittance greater than 65%, darkened luminous transmittances 
at 20.degree. C. below 25%, fading rates at 20.degree. C. such that the 
glasses manifest faded luminous transmittances of at least twice those of 
the darkened transmittances after a five-minute fading period, darkened 
transmittances at 40.degree. C. below 45%, fading rates at 40.degree. C. 
such that the glasses display faded luminous transmittances of at least 
1.75 times those of the darkened transmittances after a five-minute fading 
period. 
The glasses of U.S. Pat. No. 4,407,966 are averred to exhibit darkened 
luminous transmittances at 40.degree. C. below 35%, five-minute fading 
rates at 40.degree. C. of at least 40 units of transmittance, darkened 
luminous transmittances at 25.degree. C. below 25%, and five-minute fading 
rates at 25.degree. C. of at least 35 units of transmittance. 
Finally, U.S. application Ser. No. 353,199, filed Mar. 1, 1982 in the name 
of David W. Morgan, now U.S. Pat. No. 4,390,135, describes the production 
of silver halide-containing, alkali metal aluminoborosilicate photochromic 
glasses sensitized by copper wherein the majority of the copper is present 
in the Cu.sup.+ state rather than the Cu.sup.+2 state. That phenomenon was 
accomplished via the inclusion of at least 0.025% As.sub.2 O.sub.3 and/or 
Sb.sub.2 O.sub.3 in the glass composition. 
Operable glass compositions are asserted to consist essentially, expressed 
in terms of weight percent on the oxide basis, of about 4-26% Al.sub.2 
O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, at least one alkali 
metal oxide in the indicated proportion of 2-8% Li.sub.2 O, 4-15% Na.sub.2 
O, 4-15% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, at least one 
halogen in the minimum effective porportion of 0.2% chlorine, 0.1% 
bromine, and 0.08% iodine, a minimum of silver in the indicated proportion 
of 0.2% in a glass wherein the effective halogen is chlorine, 0.05% in a 
glass containing at least 0.1% bromine, but less than 0.08% iodine, and 
0.03% in a glass containing 0.08% iodine, the sum of Al.sub.2 O.sub.3, 
B.sub.2 O.sub.3, SiO.sub.2, alkali metal oxide, silver, and halogen being 
at least 85% of the total composition, 0.004-0.03% CuO, and at least 
0.025% As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3, but less than that amount 
which will reduce a substantial portion of the silver ions to metallic 
silver. The content of As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3 generally 
ranges about 0.025--0.25%. 
U.S. Pat. No. 1,545,509 (Montgomery and Scott) discloses glasses containing 
up to 4.74% CeO.sub.2 to decrease the transmission of ultraviolet light, 
and the addition of up to 16% Sb.sub.2 O.sub.3 to supplement the 
absorption characteristics of the CeO.sub.2 and to reduce discoloration 
caused by it. The use of cerium oxide (CeO.sub.2) to reduce discoloration 
of glass in cathode ray tubes exposed to electron bombardment and 
X-radiation is disclosed in U.S. Pat. No. 2,477,329 (de Gier). Use of up 
to 0.05% CeO.sub.2 with up to 0.15 Sb.sub.2 O.sub.3 is shown in U.S. Pat. 
No. 2,515,275 (Stookey). The CeO.sub.2 enhances photosensitivity while the 
Sb.sub.2 O.sub.3 acts as a fining agent. U.S. Pat. No. 4,257,811 (Jahn) 
adds CeO.sub.2 to an optical glass to resist solarization and avoid 
discoloration. The glass is free of alkaline earth metal oxides and 
contains small amounts of ZnO and SrO. 
GENERAL DESCRIPTION 
Our invention is predicated in large measure on our discovery that the 
photochromic behavior in a thin, low silver content, photochromic glass 
that contains cerium oxide can be essentially restored by also including 
arsenic and/or antimony oxide (Sb.sub.2 O.sub.3) in the composition. 
Laboratory experience has demonstrated that about 0.1% CeO.sub.2 is 
demanded to insure that the glass will transmit less than 0.2%, and that 
about 0.3% will effect virtually total absorption, of ultraviolet 
radiation over the wavelength range of 290-315 nm. The actual amount 
required is, of course, dependent upon the thickness of the glass lens and 
the level of Ag present in the composition. 
Up to 1% CeO.sub.2, and even greater quantities, can be operable. However, 
because of the inherent high cost of CeO.sub.2 and the fact that no 
advantageous absorption flows therefrom, 1% has been deemed a practical 
maximum. Moreover, because the presence of CeO.sub.2 produces a highly 
oxidizing environment in the glass, values between about 0.1-0.4% have 
been deemed preferred. 
In general, an amount of Sb.sub.2 O.sub.3 and/or As.sub.2 O.sub.3 at least 
75% of that utilized of CeO.sub.2 will be incorporated into the 
composition. Because of its function in reducing Cu.sup.+2 ions to 
Cu.sup.+ ions and thereby improving the photochromic character of the 
glass, as much as 50% more Sb.sub.2 O.sub.3 and/or As.sub.2 O.sub.3 than 
CeO.sub.2 may advantageously be included. Such extra addition permits the 
reduction of extraneous oxidizing agents, e.g., Cr.sub.2 O.sub.3, present 
in the glass. However, care must be exercised to avoid reducing the silver 
ions (or other readily reducible metal ions such as lead ions) to 
colloidal metal particles. 
In summary, about 0.1-1% CeO.sub.2 and 0.1-1.5% As.sub.2 O.sub.3 and/or 
Sb.sub.2 O.sub.3 will be made part of the glass composition with the 
preferred amounts varying between about 0.1-0.4% CeO.sub.2 and 0.15-0.5% 
Sb.sub.2 O.sub.3 and/or As.sub.2 O.sub.3. 
The method for forming glass articles manifesting the desired photochromic 
properties comprises the three general steps customarily used in the art: 
(a) a glass-forming batch of the proper composition is melted; 
(b) the melt is simultaneously cooled and a glass article of a desired 
geometry is shaped therefrom; and thereafter 
(c) the glass article is subjected to a temperature between about 
650.degree.-675.degree. C. for a sufficient period of time to cause the 
growth of silver halide crystallites therein which produce the 
photochromic behavior in the glass. 
The inventive process appears to be applicable essentially irrespective of 
the base composition of the photochromic glass so long as AgCl, AgBr, 
and/or AgI crystals comprise the photochromic elements. For example, U.S. 
Pat. No. 3,548,060 describes glasses having base compositions within the 
Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --RO system, i.e., the glasses consist 
essentially, by weight, of 30-86% B.sub.2 O.sub.3, 2-35% Al.sub.2 O.sub.3, 
and 12-45% of an alkaline earth metal oxide. U.S. Pat. No. 3,703,388 
discusses glasses having base compositions within the La.sub.2 O.sub.3 
--B.sub.2 O.sub.3 field, i.e., the glasses consist essentially, by weight, 
of 15-75% La.sub.2 O.sub.3 and 13-65% B.sub.2 O.sub.3. U.S. Pat. No. 
3,834,912 discloses glasses having base compositions within the 
PbO--B.sub.2 O.sub.3 system, i.e., the glasses consist essentially, by 
weight, of 14.2-48% B.sub.2 O.sub.3, 29-73% PbO, 0-15% alkaline earth 
metal oxides, and 0-23% ZrO.sub.2, Al.sub.2 O.sub.3 and/or ZnO. U.S. Pat. 
No. 3,876,436 is directed to glasses having base compositions within the 
R.sub.2 O--Al.sub.2 O.sub.3 --P.sub.2 O.sub.5 field, i.e., the glasses 
consist essentially, by weight, of at least 17% P.sub.2 O.sub.5, 9-34% 
Al.sub.2 O.sub.3, not more than 40% SiO.sub.2, not more than 19% B.sub.2 
O.sub.3, and at least 10% alkali metal oxides. U.S. Pat. No. 3,957,498 is 
drawn to glasses having base compositions within the R.sub.2 O--Al.sub.2 
O.sub.3 SiO.sub.2 system, i.e., the glasses consist essentially, by 
weight, of 13-21% alkali metal oxides, 17-25% Al.sub.2 O.sub.3, and 45-56% 
SiO.sub.2. 
Finally, as was noted above in the discussion of U.S. Pat. No. 3,208,860, 
the currently commercially-marketed, photochromic glasses have base 
compositions within the alkali metal aluminoborosilicate system. The 
patent cites, as preferred compositions, glasses consisting essentially, 
by weight, of 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% 
SiO.sub.2, and at least one alkali metal oxide selected from the group of 
2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 
10-30% Cs.sub.2 O. Such glasses contain, by weight on the basis of 
chemical analysis, at least one halogen in the minimum effective 
proportion of 0.2% chlorine, 0.1% bromine, and 0.08% iodine, and a minimum 
of silver in the indicated proportion of 0.2% where the effective halogen 
consists of chlorine, 0.05% where the effective halogen is bromine, but 
the glass contains less than 0.08% iodine, and 0.03% where the glass 
contains at least 0.08 % iodine. Where a transparent article is desired, 
the total silver will not exceed 0.7% and the total of the three recited 
halogens will not exceed 0.6%. The sum of the recited base glass 
constituents, silver, and halogens will compose at least 85% of the 
composition. We prefer then to utilize a glass selected from this alkali 
metal aluminoborosilicate family in view of the available production 
experience. 
Within the broad alkali metal aluminoborosilicate family, there has been 
found a narrow sub-family that utilizes very low silver contents while 
providing superior photochromic properties. This sub-family includes 
glasses disclosed in U.S. Pat. No. 4,358,542 described earlier. 
Glasses of this sub-family, as combined and modified in terms of the 
present invention, and as calculated in parts by weight on an oxide basis, 
come within these preferred ranges: 
______________________________________ 
SiO.sub.2 
55-61 La.sub.2 O.sub.3 
0-5 
B.sub.2 O.sub.3 
18-21 PbO 0-0.25 
Al.sub.2 O.sub.3 
5-11 CeO.sub.2 0.1-1.0 
Li.sub.2 O 
1.5-3 Sb.sub.2 O.sub.3 + As.sub.2 O.sub.3 
0.1-1.5 
Na.sub.2 O 
2-5 Ag 0.10-0.20 
K.sub.2 O 
4.5-8 CuO 0.004-0.02 
ZrO.sub.2 
0-5 Cl 0.2-0.5 
TiO.sub.2 
0-3 Br 0.05-0.15 
______________________________________ 
The initial clear luminous levels of transmittance displayed by the glasses 
are in the vicinity of 90% unless well-known tinting agents, such as the 
transition metal oxides CoO, NiO, and V.sub.2 O.sub.5 and/or the rare 
earth metal oxides Er.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Nd.sub.2 O.sub.3, 
and Pr.sub.2 O.sub.3, are added to reduce the initial transmittance 
values. However, because the quantity of colorant employed is very small 
(up to about 1% for the transition metal oxides and up to 5% of the rare 
earth oxides), the overall photochromic character of the glass is not 
substantially affected by such addition. 
The inclusion of up to 0.25% PbO may be helpful in improving the darkening 
capability of the glasses, especially where high Li.sub.2 O contents are 
utilized. Minor amounts of ingredients such as ZrO.sub.2, La.sub.2 
O.sub.3, and TiO.sub.2 may be added to adjust the refractive index of the 
glass. In general, individual amounts thereof will not exceed about 5% for 
ZrO.sub.2 and La.sub.2 O.sub.3 and about 3% for TiO.sub.2, with the total 
of the three being less than about 10%. TiO.sub.2 also has the effect of 
absorbing ultraviolet radiation in the near ultraviolet region of the 
radiation spectrum, i.e., over the range of about 315-380 nm. That feature 
can be useful in protecting the wearer of eyeglasses, but it also has an 
adverse effect upon the photochromic behavior of the glass. Accordingly, 
care must be exercised in its use.

DESCRIPTION OF PREFERRED EMBODIMENTS 
TABLE 1 reports several glass compositions, expressed in terms of parts by 
weight on the oxide basis, illustrating the compositional parameters of 
the inventive glasses. Because it is not known with which cation(s) the 
halides are combined and the fact that their quantities are so small, they 
are merely tabulated as chloride and bromide, in accordance with 
conventional glass analysis practice. Likewise, since the amounts of 
silver are extremely small, they are designated on an elemental basis. 
Finally, inasmuch as the sum of the individual components closely 
approximates 100, for all practical purposes the figures recorded may be 
considered to reflect weight percent. 
The actual batch ingredients may comprise any materials, either the oxide 
or other compound, which, when melted together, will be converted into the 
desired oxide in the proper proportions. The halides will normally be 
added as a metal salt. 
Batches for the exemplary compositions were compounded and melted in a 
laboratory scale, continuous glass melting unit operating at about 
1450.degree. C. After a period of three hours, the melt was drawn into 
glass sheet having a thickness dimension of about 1.5 mm and the sheet 
passed into an annealer operting at about 375.degree. C. Photochromic 
behavior was developed via heat treatment of about 15 minutes at a 
temperature of about 660.degree. C. This was followed by heat treatment at 
400.degree. C. for 16 hours to simulate the commercial chemical 
strengthening schedule. 
TABLE I 
______________________________________ 
1 2 3 4 
______________________________________ 
SiO.sub.2 57.4 59.2 59.1 59.3 
B.sub.2 O.sub.3 
19.7 19.6 19.6 19.7 
Al.sub.2 O.sub.3 
9.66 9.62 9.60 9.63 
Li.sub.2 O 
2.20 2.19 2.19 2.19 
Na.sub.2 O 
2.95 2.94 2.94 2.95 
K.sub.2 O 6.35 6.32 6.31 6.33 
Ag 0.175 0.174 0.174 0.175 
CuO 0.008 0.008 0.008 0.008 
PbO 0.13 0.13 0.13 0.13 
Sb.sub.2 O.sub.3 
-- 0.208 0.307 -- 
CeO.sub.2 -- 0.199 0.298 0.299 
Br 0.085 0.084 0.084 0.084 
Cl 0.441 0.439 0.438 0.439 
______________________________________ 
The inventive photochromic glasses are activated principally through 
radiation in the ultraviolet and short visible regions of the radiation 
spectrum. However, experience has indicated that the correlation between 
test data secured via exposure to an ultraviolet lamp and values measured 
with solar radiation outdoors was often imperfect. Consequently, to 
achieve better correlation with outdoor solar radiation, a "solar 
simulator" apparatus was devised and is described in U.S. Pat. No. 
4,125,775. 
The apparatus employs a 150 watt xenon arc source fitted with a filter to 
modify the spectral output such as to closely approximate the radiation 
spectrum of the sun, particularly in the ultraviolet, blue, and red 
portions. The infrared interval of the spectrum is attenuated with a film 
of water of sufficient thickness to provide irradiance equal to that of 
the sun, but with no special concern for the spectral distribution within 
that region. 
The intensity of the arc source was adjusted such that the measure of 
darkening was equivalent to that of several commercially available 
photochromic glasses, including PHOTOGRAY lens blanks, darkened outdoors 
at noon during a cloudless summer day in Corning, N.Y. (air mass value of 
about 1.06). Numerous experimental photochromic glasses of 
widely-disparate compositions have also been subjected to the outdoor 
sunlight and the solar simulator. A comparison of the data evidences 
excellent overall agreement. 
Continuous monitoring of the darkened transmittance manifested by the 
samples is obtained by interrogating each with a chopped beam of light 
from a tungsten-halogen lamp detected by a PIN silicon photodiode whose 
output was demodulated by a lock-in amplifier. A composite color filter 
was placed into the beam to approximate the response of the human eye 
under Illuminant C, as defined by C.I.E. 
The apparatus was interfaced to a PDP-11/04 computer (marketed by Digital 
Equipment Corporation, Maynard, Mass.) to permit automatic sample change, 
temperature selection, event sequencing, and data collection, storage, 
reduction, and retrieval with a minimum of operator's involvement. 
TABLE II reports (1) electron paramagnetic resonance (EPR) measurements 
conducted to determine the level of Cu.sup.+2 ions in the glasses and (2) 
photochromic properties demonstrated by the glasses at ambient 
temperatures, viz., 20.degree.-25.degree. C. With respect to the latter, 
T.sub.o signifies the initial transmittance of the samples prior to 
exposure to the solar simulator, T.sub.D10 designates the diminished 
transmittance of the samples after exposure to the solar simulator for 10 
minutes, T.sub.F5 indicates the units of transmittance observed for each 
sample five minutes after its removal from the solar simulator. 
.DELTA.T.sub.F5 indicates the fadeback, that is (T.sub.F5 -T.sub.D10) as a 
darkened sample is removed from activating radiation. Similarly, the rate 
of darkening is shown by subtracting T.sub.D10 from T.sub.o. 
TABLE II 
______________________________________ 
1 2 3 4 
______________________________________ 
EPR Data (%) 
Cu.sup.+2 
0.0027 0.0005 0.0005 0.0044 
*Cu.sup.+ 
0.0053 0.0075 0.0075 0.0036 
Photochromic Properties 
T.sub.o 90.6 90.3 90.0 90.3 
T.sub.D10 
32.7 33.3 32.4 34.8 
T.sub.F5 58.7 66.5 64.4 51.3 
.DELTA.T.sub.F5 
26.0 33.2 32.0 16.5 
______________________________________ 
*By difference between Cu.sup.+2 and batched level. 
An examination of TABLE II (Examples 1 and 4) illustrates the impairment 
which CeO.sub.2 exerts upon the photochromic performance of the glass. 
Hence, Example 4 does not darken to the extent of Example 1 and fades 
substantially slower. The reducing effect Sb.sub.2 O.sub.3 has upon the 
Cu.sup.+2 ions and the resulting improvement in photochromic behavior is 
apparent in a study of Examples 2 and 3. 
FIG. 1 exhibits transmittance curves for glass samples of Examples 1-4 over 
the range of wavelengths 290-400 nm. Radiation wavelength is plotted along 
the horizontal, while transmittance, as measured in standard manner, is 
plotted along the vertical. The numeral designations of the curves 
correspond to the example designations in TABLE I. To facilitate 
reference, the CeO.sub.2 and Sb.sub.2 O.sub.3 contents of the examples are 
tabulated on the drawing. 
In order to study effects of the minute magnitude here involved, it is 
necessary to substantially enlarge or magnify the graphical showing, or at 
least a small critical portion thereof. FIG. 2 is such an enlargement of 
that portion of FIG. 1 encompassing the so-called erythemal zone. The axes 
are plotted as in FIG. 1, but wavelength spans only 285-330 nms. and 
transmittance is plotted to 3.0%. A 0.2% limit line is shown dashed. 
The capability of CeO.sub.2 to absorb radiation in the 290-315 nm region is 
quite evident from these curves. It can be observed that Example 1 
transmits over 3% of the radiation at 315 nm, whereas Examples 2-4 are 
within the 0.2% limit up to that wavelength. The operation of Sb.sub.2 
O.sub.3 in reducing CeO.sub.2, thereby increasing transmission of 
ultraviolet radiation, is evident through a study of Examples 3 and 4. 
Thus, both Examples have the same level of CeO.sub.2, but Example 4, which 
is free from Sb.sub.2 O.sub.3, is more absorbing at 315 nm. 
When specimens of the glass sheet of the above compositions are subjected 
to the chemical strengthening practice customarily utilized with PHOTOGRAY 
EXTRA lenses, viz., immersion for 16 hours in a bath of molten salt 
consisting of 60% by weight KNO.sub.3 and 40% by weight NaNO.sub.3 
operating at 400.degree. C., followed by rinsing off any adhering salt 
with tap water, they readily pass the FDA strength standards for eyeglass 
safety. 
The efficacy of arsenic oxide (As.sub.2 O.sub.3) as an alternative to 
Sb.sub.2 O.sub.3 was demonstrated in a continuous melting program. In this 
program, a base glass was employed having the following composition 
expressed in terms of oxides on a cationic percentage base: 
______________________________________ 
Si 47.0 
B 26.9 
Al 9.0 
Li 7.0 
Na 3.7 
K 6.4 
______________________________________ 
Several additives were incorporated in constant amount in this base glass 
to impart photochromic potential. Finally, arsenic oxide was added in 
varying amounts to provide the following compositions as the melting 
campaign progressed. These compositions are set forth in weight percent on 
a calculated oxide basis in TABLE III hereafter: 
TABLE III 
______________________________________ 
5 6 7 8 
______________________________________ 
SiO.sub.2 59.2 58.9 59.1 59.2 
B.sub.2 O.sub.3 
19.7 19.6 19.6 19.7 
Al.sub.2 O.sub.3 
9.6 9.6 9.6 9.6 
Li.sub.2 O 
2.2 2.2 2.2 2.2 
Na.sub.2 O 
2.4 2.4 2.4 2.4 
K.sub.2 O 6.3 6.3 6.3 6.3 
Ag 0.178 0.178 0.178 0.178 
CuO 0.006 0.006 0.006 0.006 
PbO 0.130 0.129 0.129 0.130 
CeO.sub.2 0.299 0.298 0.299 0.299 
As.sub.2 O.sub.3 
-- 0.455 0.228 -- 
Cl 0.426 0.424 0.425 0.426 
Br 0.108 0.107 0.107 0.108 
______________________________________ 
The melting temperature was about 1400.degree. C., the batches were ball 
milled, and the glass melt stirred to promote homogeneity. The molten 
glass was formed as plano pressings which were ground and polished to 
provide a lens thickness of 1.5 mm. These were given a standard 
photochromic thermal development by heating for fifteen (15) minutes at 
660.degree. C. Darkening and fade rates were observed using a Huntermeter. 
The data is recorded in TABLE IV wherein T.sub.o, T.sub.D10 and 
.DELTA.T.sub.F5 have the meanings set forth earlier. 
TABLE IV 
______________________________________ 
T.sub.o T.sub.D10 
.DELTA.T.sub.F5 
______________________________________ 
90.3 25.4 16.3 
90.6 25.1 23.7 
90.6 24.9 23.3 
90.7 25.2 23.5 
90.8 24.2 22.2 
90.8 24.4 22.4 
91.0 24.6 22.8 
90.9 24.8 23.0 
90.9 24.2 22.3 
90.4 24.0 22.0 
90.7 25.0 22.5 
90.6 25.5 20.3 
______________________________________ 
It is apparent that the absence of arsenic oxide at the beginning and end 
of the melting run had a rather small, although noticeable, effect on 
extent of darkening in ten minutes. On the other hand, the effect on 
degree of fading, and hence on fade rate, is much more substantial. 
However, once an arsenic oxide presence was established, changes in amount 
appeared to have little effect. 
A further series of glass compositions was formulated to illustrate 
applicability of the present invention to photochromic glasses. This 
series was derived from the base glass composition employed above. This 
composition in terms of oxides on a cationic percent basis, is composed 
of: 
______________________________________ 
Si 47.0 
Al 9.0 
B 26.9 
Li 7.0 
K 6.4 
Na 3.7 
______________________________________ 
TABLE V reports the additives employed, in weight percent on an oxide 
basis, in formulating the illustrative series: 
TABLE V 
______________________________________ 
9 10 11 12 13 
______________________________________ 
Ag 0.189 0.188 0.187 0.188 
0.188 
CuO 0.008 0.008 0.008 0.008 
0.008 
PbO 0.120 0.119 0.119 0.120 
0.119 
Sb.sub.2 O.sub.3 
-- 0.306 0.599 0.296 
0.296 
CeO.sub.2 
-- 0.298 0.593 0.298 
0.297 
CoO 0.052 0.052 0.052 0.052 
0.052 
NiO 0.372 0.370 0.369 0.370 
0.370 
TiO.sub.2 
-- -- -- 0.295 
0.599 
Br 0.085 0.084 0.084 0.084 
0.084 
Cl 0.452 0.449 0.447 0.449 
0.448 
______________________________________ 
In each of these glasses, the variable additives are titanium, antimony and 
cerium oxides. Examples 12 and 13 have titania included to adjust index 
and to reduce transmission in the 315-380 nm region. Likewise, all 
examples contain additions of nickel and cobalt oxides which act as 
colorants and permanently reduce spectral transmission. 
The glasses were melted in a continuous melter for three (3) hours at 
1450.degree. C. The melted glass was drawn as 1.5 mm thick sheet glass 
suitable for producing sagged lenses, and was annealed at 375.degree. C. 
Transmittance values were measured for each glass at five nanometer 
intervals over a UV-B range of 290-315 nm and over a UV-A range of 315-380 
nm. The average over each range was determined and is shown in TABLE VI in 
conjunction with weight percent figures for the variable oxide additives. 
TABLE VI 
______________________________________ 
9 10 11 12 13 
______________________________________ 
Avg. (290-315 nm) 
0.80 0 0 0 0 
Avg. (315-380 nm) 
17.1 16.0 15.8 15.0 10.7 
Sb.sub.2 O.sub.3 
0 0.31 0.61 0.31 0.31 
CeO.sub.2 0 0.30 0.60 0.30 0.30 
TiO.sub.2 0 0 0 0.30 0.60 
______________________________________