Short flint special glasses

Details Short flint special glasses Short flint special glasses

2000-05-26

2002-04-30

Group, Karl (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S078000, C501S079000, C501S903000, C065S061000, C065S122000, C065S134100

Reexamination Certificate

active

06380112

ABSTRACT:

The invention relates to optical glasses which have refractive indexes n
d
of from 1.52 to 1.58, Abbe numbers v
d
of from 50 to 57 and negative anomalous partial dispersions &Dgr;P
g,F
of <−0.0090. These glasses are of the short flint special glass (SFS) type of optical glass.
Since in recent years the glass components PbO and As
2
O
3
have been debated in public as environmental pollutants, the manufacturers of optical equipment also have a need for PbO-free and preferably also As
2
O
3
-free glasses having the respective optical properties.
Simple replacement of lead oxide by one or more constituents generally does not succeed in reproducing the desired optical and glass-engineering properties affected by PbO. Instead, new developments or extensive changes in the glass composition are necessary.
Parameters which are decisive for the use of an optical glass are the refractive index, for example n
d
, and the change in refractive index with wavelength, known as the dispersion. A term for the change in refractive index with wavelength is the Abbe number, for example
v
d
=
n
d
-
1
n
F
-
n
C
The difference n
F
−n
C
is known as the principal dispersion. Other differences are partial dispersions. Relative partial dispersions are the ratio of a partial dispersion to the principal dispersion, for example
P
g
,
F
=
n
g
-
n
F
n
F
-
n
C
Like the Abbe number, the relative partial dispersion is an important material constant for an optical glass. The majority of glasses satisfy an approximately linear relationship between P
x,y
and v, according to which P
x,y
=a
x,y
+b
x,y
·v (standard straight line).
Glasses which do not satisfy this equation are referred to as glasses having anomalous partial dispersion. The equation must then be expanded by an additional correction term &Dgr;P
x,y
:
P
x,y
=a
x,y
+b
x,y
v+&Dgr;P
x,y
Depending on whether &Dgr;P
x,y
is greater than or less than “0”, the glasses are then referred to as glasses having positive or negative anomalous partial dispersion. Through a suitable combination of optical glasses having different Abbe numbers, the imaging flaw, chromatic aberration, can be eliminated or at least improved in lens systems, for example for 2 colours. The residual chromatic aberration (chromatism) which remains for the uncorrected colours is referred to as the secondary spectrum. This effect is particularly disadvantageous for high-performance optics, since it impairs the imaging sharpness and resolving power of the optic. However, use of glasses having anomalous partial dispersion in optical lens systems would succeed in reducing the secondary spectrum and thus giving corrected lens systems having excellent imaging sharpness and high resolving power.
Particularly desirable is correction in the blue region of the visible spectrum, for which the relative partial dispersion P
g,F
mentioned above by way of example is characteristic. In its standard straight line, a
g,F
=0.6438 and b
g,F
=−0.001682.
The patent literature has already revealed some specifications which describe glasses having optical values n
d
and v
d
from these or adjacent regions, only some of the glasses having high negative anomalous partial dispersion &Dgr;P
g,F
. The glasses described have a wide variety of disadvantages:
Patent Specification DD 1603 07 relates to optical crown glasses having refractive indexes n
e
=1.500-1.555 and Abbe numbers ve=57-62 with negative anomalous partial dispersion which have a very high B
2
O
3
content, namely 73-87% by weight, and thus do not have adequate chemical resistance for practical purposes.
By contrast, DE-B 13 03 171 relates to a batch for the production of optical glasses having anomalous partial dispersion and an Abbe number v
e
=40-60 and refractive indexes n
e
=1.52-1.64, where the batch only contains up to 34% by weight of B
2
O
3
, but up to 40% by weight of SiO
2
. These glasses do not achieve the desired magnitude of the negative anomalous partial dispersion.
Like the glasses of said DD 1603 07, the glasses of DE-B 1 022 764 having negative anomalous partial dispersions and Abbe numbers of from 64 to 35 and refractive indexes of from 1.53 to 1.73 do not contain SiO
2
. Consequently, these glasses likewise do not have adequate chemical resistance.
JP 60-469 46 A discloses UV-transparent glasses of the borosilicate glass type which are predominantly in the quaternary system CaO·Al
2
O
3
·B
2
O
3
·SiO
2
(CaO+Al
2
O
3
+B
2
O
3
+SiO
2
=9-100% by weight). However, with other components <10% by weight, highly negative partial dispersion cannot be achieved.
German Patent Specification DE 42 18 377 C1 describes optical crown glasses having negative anomalous partial dispersion &Dgr;P
g,F
, a refractive index n
d
>1.52 and an Abbe number v
d
>57. In these glasses, in which, inter alia, SiO
2
at up to 15% by weight and Ta
2
O
5
at up to 10% by weight are only optional components, a defined water content of 0.1-0.5% by weight is necessary in order to ensure sufficiently high anomaly of the partial dispersion. Such setting of a defined water content makes glassmaking undesirably complex.
JP 63-222 040 A claims glasses from a very broad composition range which varies greatly with respect to the possible components, but lists only a few specific glasses having only a few particular constituents. This specification, which relates to glasses for ultrasound retardation lines, gives, due to the completely different objective, absolutely no indication of how a lead-free glass having a refractive index n
d
of from 1.52 to 1.58 and an Abbe number v
d
of from 50 to 57 and having a highly negative anomalous partial dispersion, namely &Dgr;P
g,F
<−0.0090, can be produced.
This, however, is the object of the present invention. It is achieved by the glasses described herein.
The glasses contain predominant proportions of the glass formers Al
2
O
3
, B
2
O
3
and SiO
2
: they contain 8-13% by weight of Al
2
O
3
, which has a positive effect on the chemical resistance. At higher contents, however, the melting properties would be impaired and the tendency toward crystallization increased.
The glasses contain 45-55% by weight of B
2
O
3
. B
2
O
3
dissolves components which are difficult to melt, and B
2
O
3
lowers the negative partial dispersion &Dgr;P
g,F
(i.e. increases its value) of the glasses. At higher contents, the chemical resistance would be impaired and the Abbe number would be too high.
The glasses contain SiO
2
in contents of greater than 15% by weight and at most 25% by weight. At lower contents, the good chemical resistance drops, while at higher contents, the good melting properties are impaired. The SiO
2
content helps to achieve an Abbe number in the low region desired.
CaO, present to the extent of 2-8% by weight, and optionally also ZnO (0-5% by weight), increase the refractive index and contribute to the lowering of &Dgr;P
g,F
. At excessively high contents, the Abbe number would be too high and the chemical resistance would be impaired, and therefore the sum of CaO and ZnO should remain limited to 10% by weight.
The glasses contain from 3 to 8% by weight of alkali metal oxides, which improve the melting properties of the glasses and stabilize them against separation. The said total content can be achieved by means of 0-4% by weight of Na
2
O, 0-4% by weight of K
2
O and 0-4% by weight of Li
2
O. Preferably, however, at least two alkali metal oxides are present, in particular at least Na
2
O and K
2
O. It is particularly preferred for all three components to be present.
The glasses may furthermore contain up to 14% by weight of La
2
O
3
. La
2
O
3
supports a high Abbe number at the same time as a high refractive index.
The glasses contain Ta
2
O
5
in an amount of at least >10% by weight in order to achieve the desired high negative anomalous partial dispersion, and at most 17% by weight, since otherwise separation would occur. In addition, Ta
2
O
5
improves the chemical resistance and increa

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