Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-05-31
2004-09-28
Group, Karl (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C051S302000, C051S302000, C051S302000
Reexamination Certificate
active
06797659
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an optical glass and, more particularly, to an optical glass which has optical constants of a refractive index (n
d
) within a range from 1.75 to 1.85 and an Abbe number (&ngr;
d
) within a range from 35 to 45 and is suitable for precision mold press forming.
In recent remarkable development of more compact and light-weight optical instruments, more aspherical lenses tend to be used for the purpose of reducing the number of lenses constituting an optical system of an optical instrument. For manufacturing an aspherical lens, the main current of the industry is to heat and thereby soften a preform obtained from a glass gob or glass block and press the softened preform with a mold having a high precision surface for transferring its mold surface to the preform. An aspherical lens obtained by this method can be processed to a product of a desired shape at a high productivity without grinding and polishing, or with minimum grinding and polishing. In the field of precision mold press forming of glass, various glass products including optical elements such as spherical and aspherical lenses and diffraction grating are now manufactured by the precision mold press forming.
There are generally two methods for manufacturing the preform. One is a dropping method according to which, as described, e.g., in Japanese Patent Application Laid-open Publication No. Hei 6-122526, melted glass is dropped from a tip of a flow tube, received and molded by a mold and then is cooled to provide a glass preform. According to this method, a preform is directly obtained while the glass is hot and, therefore, a preform can be manufactured at a high productivity in a large scale production. Besides, since a glass preform thus obtained has a lens shape which is either spherical or convex in both surfaces, an amount of change in the shape during the precision mold press forming can be held at the minimum.
The other method is to obtain a preform by cutting a glass block. This method has the problem that it requires increased steps of processing from cutting of a glass block to a step of processing to a shape which is close to a final lens shape but, since processing to a lens shape dose to a final lens shape is conducted, an amount of change in the shape can be reduced in forming various shapes of lenses besides a lens shape which is convex in both surfaces and, as a result, pressure of a press required for mold pressing can be reduced.
In producing a glass product by precision mold pressing, it is necessary to heat and press a glass preform under a high temperature for transferring a high precision mold surface to the glass product. For this reason, a mold used for this purpose is subjected to high temperature and pressure and, as a result, the surface of the mold tends to be oxidized and corroded in softening the preform by heating. This makes it difficult to maintain a high precision surface of the mold and, therefore, the number of times of replacing the mold increases and, therefore, a large scale production of the glass product at a low cost becomes difficult. For this reason, in an optical glass which constitutes a glass preform, for the purpose of preventing damage to a mold and also to a mold releasing film provided on the inner surface of the mold which takes place due to the high temperature environment in mold pressing, it is desired to reduce transition point (Tg) and yield point (At) at the lowest possible levels.
Glasses having various optical constants are sought as glass used for aspherical lenses. Among them, there is a strong demand for glass having a high refractive index (n
d
) within a range from 1.75 to 1.85 and an Abbe number (&ngr;
d
) within a range from 35 to 45. A typical glass having such optical constants is a lanthanum borate optical glass. For example, Japanese Patent Application Laid-open Publication No. Sho 60-221338 discloses a lanthanum borate optical glass having a low transition point. Such glass has a low transition point (Tg) and a low yield point (At) but, since difference (At−Tg) between the transition point and the yield point is small, difficulty arises in mold pressing for a large scale production of glass products in a stable manner. It is necessary to use glass having a wider range of At−Tg as a preform. Further, in a high refractive index region of n has decreased resistance to devitrification and, for this reason, a preform cannot be produced from this glass by the dropping method.
Japanese Patent Application Laid-open Publication No. Hei 6-305769 discloses a lanthanum borate optical glass maintaining a low transition point and an excellent chemical durability. This glass, however, is also insufficient for manufacturing a preform by the dropping method in respect of resistance to devitrification.
Various glasses containing PbO have been proposed as glasses having a low transition point (Tg) and a low yield point (At). Since, however, the glasses containing PbO tend to be fused to the mold during mold pressing, it is difficult to use the mold repeatedly and hence they are not suitable as an optical glass for mold pressing. In glass containing F
2
, the F
2
ingredient evaporates from the surface of glass melt and thereby causes cloudiness on the surface of a preform when the preform is produced from the glass melt, or, when the preform is molded by mold pressing, the F
2
ingredient evaporates and is deposited on the surface of the mold to cause cloudiness on the surface of the mold. For these reasons, the glass containing F
2
is not suitable as an optical glass for mold pressing.
It is, therefore, an object of the present invention to provide an optical glass which has eliminated the above described disadvantages of the prior art optical glasses, has optical constants of a refractive index (n
d
) within a range from 1.75 to 1.85 and an Abbe number (&ngr;
d
) within a range from 35 to 45, has a low transition point (Tg) and a low yield point (At), has a wide range of At−Tg, has excellent resistance to devitrification, and is suitable for precision mold pressing.
It is another object of the invention to provide an optical glass which has excellent resistance to devitrification and therefore is suitable for manufacturing a preform for precision mold pressing by the dropping method.
SUMMARY OF THE INVENTION
As a result of laborious studies and experiments, the inventor of the present invention has found, which has led to the present invention, that, in glass having optical constants of a refractive index (n
d
) within a range from 1.75 to 1.85 and an Abbe number (&ngr;
d
) within a range from 35 to 45, a lanthanum borate glass which can prevent oxidation of the surface of a mold having a high precision surface for mold pressing can be obtained and that only within a composition of extremely limited range can be found an optical glass which has the above described desired optical constants and suitable Tg, At and At−Tg, has excellent resistance to devitrification, is free from substance which is undesirable for protection of the environment and has excellent precision mold pressing characteristics.
For achieving the above described objects of the invention, there is provided an optical glass having optical constants of a refractive index (n
d
) within a range from 1.75 to 1.85 and an Abbe number (&ngr;
d
) within a range from 35 to 45, comprising in mass % on oxide basis:
SiO
2
+ B
2
O
3
16.5-less than 30%
in which SiO
2
1-7.5%
B
2
O
3
15.5-25%
La
2
O
3
25-40%
ZrO
2
1.5-10%
Nb
2
O
5
1-15%
Ta
2
O
5
1-10%
WO
3
1-10%
ZnO
15.5-30%
Li
2
O
0.6-5%.
having a transition point (Tg) within a range from 500° C. to 590° C. and a yield point (At) within a range from 530° C. to 630° C., and being free from devitrification in a devitrification test conducted under a condition of 950° C./2 hours.
In one aspect of the invention, there is provided an optical glass having optical constants of a refractive index (n
d
) with
Bolden Elizabeth A.
Group Karl
Hedman & Costigan ,P.C.
Kabushiki Kaisha Ohara
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