Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Patent
1999-07-29
2000-12-19
Group, Karl
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
501 56, 501 63, 501 64, 501 67, C03C 406, C03C 402
Patent
active
061627496
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to photochromic glasses, for ophthalmic use, characterized by a high transmission and by the absence of coloration in the clear state, by the ability to darken in the presence of an actinic radiation source to a low level of transmission, by a low thermal dependence of the darkening level, and by a return to a high transmission when the light source is eliminated. These glasses are also characterized by a good stability of the photochromic properties with respect to the thermal treatment used in the antiglare treatments frequently applied to photochromic lenses.
BACKGROUND OF THE INVENTION
The photochromic glasses are now well known and are characterized by their ability to darken when exposed to actinic radiation, essentially ultraviolet radiation, and to brighten when this excitatory source disappears. Since the invention of these glasses (U.S. Pat. No. 3,208,860), now some 30 years ago, they have been applied with variations in a great number of versions, depending on whether one or the other of the attributes of photochromism was optimized for particular application. In general, the critical attributes of the photochromic glasses for ophthalmic applications are: their color and level of transmission in the clear state (in the absence of actinic radiation), their color (usually gray or brown) and transmission after darkening resulting from exposure to actinic radiation, the low amplitude of the variation in the level of transmission in the darkened state as a function of the temperature, normally between 0 and 40.degree. C., and their ability to brighten reversibly when the excitatory light source disappears.
For example, U.S. Pat. No. 4,746,633 describes glasses for ophthalmic use, having a transmission in the clear state of more than 88% and a transmission of 35-60% after 15 min of exposure to an actinic radiation source, and which then brighten to a minimum transmission level of 75% when this light source is eliminated. These glasses are also characterized by a transmission of less than 63% when the exposure to light occurs at a temperature of 40.degree. C. From numerous examples given in this invention, it appears that the thermal dependence of the transmission in the darkened state increases very significantly when the transmission in the darkened state at 25.degree. C. is decreased. Typically, a difference is much larger than 18.5 points of transmission exists between 25 and 40.degree. C., whereas the transmission in the darkened state at 25.degree. C. is less than 30%, measured at a thickness of 2 mm.
It would be useful to have available a photochromic glass for ophthalmic use, with a refractive index of approximately 1.5, presenting the following properties: 86%, preferably more than 88%, at a thickness of 2 mm, and essentially associated with the absence of coloration; min of exposure at 25.degree. C. to a sunlight-simulation device, of less than 35% at a thickness of 2 mm; min of exposure at 40.degree. C. to a sunlight-simulation device, of less than 45% at a thickness of 2 mm; absence of actinic radiation; and treatment used in the antiglare treatments, such that the difference between the values T.sub.d15(25.degree. C.) of the initial value and that of the glass after thermal treatment of 1 h at 280.degree. C. does not exceed 5 transmission points, in absolute value.
Such a photochromic glass does not exist today, to the knowledge of the applicant.
BRIEF SUMMARY OF THE INVENTION
The objective of the present invention, therefore, is to provide glasses having the above-mentioned characteristics. More specifically, the invention concerns glasses whose matrix has a composition, expressed in wt % based on the oxides, within the following ranges:
______________________________________ SiO.sub.2
51-66
B.sub.2 O.sub.3
15-23
Al.sub.2 O.sub.3
3-10
ZrO.sub.2
2.5-11
TiO.sub.2
0-1
Li.sub.2 O
3-7
Na.sub.2 O
0-8
K.sub.2 O
2-9
MgO 0-3
CaO 0-3
SrO 0-3
BaO 0-3
Nb.sub.2 O.sub.5
0-1
La.sub.2 O
REFERENCES:
patent: 3208860 (1965-09-01), Armistead et al.
patent: 4980318 (1990-12-01), Araujo
patent: 5244845 (1993-09-01), Kerko et al.
patent: 5426077 (1995-06-01), Brocheton et al.
patent: 5482901 (1996-01-01), Morgan et al.
Brocheton Yves Andre Henri
Prassas Michel
Corning S.A.
Group Karl
Peterson Milton M.
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