Optical light weight glass with a refractive index of >1.70, an Abbe index of >22 and a density of <3.5 G/CM.sup.3

An optical glass with the familiar optical position of n.sub.d >1.70 and v.sub.d >22, which differs from the familiar optical glasses of the optical position due to its lower density and good chemical stability, and is composed of (in percent by weight): SiO.sub.2 25-42, TiO.sub.2 15-35, B.sub.2 O.sub.3 0.5-7, BaO 3-13, CaO 1-3, Nb.sub.2 O.sub.5 1-20, GeO.sub.2 0-2, Al.sub.2 O.sub.3 0-4, Na.sub.2 O 7.5-15, Li.sub.2 O 0-9, K.sub.2 O 2-10, SrO 0-6, ZnO 0-5, ZrO.sub.2 0-3, and WO.sub.3 0-3.

DESCRIPTION OF THE INVENTION 
1. Technical Field of the Invention 
This invention relates to optical glass compositions in the familiar range 
of refractive indices of 1.7-1.85 and having an Abbe number of &gt;22, which 
share such optical properties with conventional glasses but which differ 
from them because of greatly reduced density and improved chemical 
stability. 
2. Background Art 
The state of the art known to the present inventors which most closely 
approximates the present invention has been described in JP-OS No. 79 88 
917 and in JP-OS No. 80 126 549. According to these teachings, 
light-weight glasses of the above-mentioned optical range can be 
manufactured; however, these glasses exhibit some considerable 
disadvantages. The compositions described in JP-PS No. 79 88 917 do 
without the utilization of any Nb.sub.2 O.sub.5 whatsoever, and those in 
JP-OS No. 80 126 549 permit a maximum use of Nb.sub.2 O.sub.5 of 12 
percent by weight and a WO.sub.3 component of not more than 0.5 percent by 
weight. Therefore, in order to produce the high refractive value, very 
high contents of TiO.sub.2 and/or ZrO.sub.2 have to be used, which results 
in higher crystallization and which provides useable glass in the desired 
dimensions only after considerably greater technical effort and at lower 
yields. 
The stability of the glasses described in the above-quoted documents can be 
described by the ratio of the SiO.sub.2 to the TiO.sub.2 content. When 
this ratio is &lt;1.25, glass stability decreases greatly. 
On the other hand, the examples given in JP-PS No. 77 25 812 result in 
completely crystallized, unusable solids if the traditional processing 
methods are used, so that it is obviously difficult to obtain any usable 
optical glass at all for the range of composition of the patent document. 
DISCLOSURE OF THE INVENTION 
Accordingly, it is a general object of this invention to provide an optical 
quality glass combining the properties of a high refractive index, a 
moderate Abbe number, a low density, good stability towards melting 
procedures, and good chemical resistance. 
Another object of this invention is to produce such glass compositions 
which can be produced in large melting containers to form large 
dimensional products. 
A further object of this invention is to provide glass compositions having 
optical transmission characteristics considerably greater than 
conventional heavy flint glasses. 
An additional object of this invention is to provide glass compositions 
having a refractive index of 1.74-1.79, an Abbe number of at least 25-29, 
and a density of 3.1-3.3 g/cm.sup.3. 
A more particular object of this invention is to provide lens blanks and 
polished camera lenses made from the novel glass compositions herein. 
Upon study of the specification and appended claims, further objects, 
features and advantages of this invention will become more fully apparent 
to those skilled in the art to which this invention pertains. 
BEST MODE FOR CARRYING OUT THE INVENTION 
Briefly, the above and other objects, features and advantages of this 
invention are attained in one aspect thereof by providing a glass 
composition which meets all of the above-mentioned requirements and is 
characterized by containing (in percent by weight) the following 
components: 
(a) glass forming oxides: SiO.sub.2 25-42, GeO.sub.2 0-2, B.sub.2 O.sub.3 
0.5-7, Al.sub.2 O.sub.3 0-4, but preferably by: SiO.sub.2 30-35, GeO.sub.2 
0-1, B.sub.2 O.sub.3 0.5-3, Al.sub.2 O.sub.3 0-2; 
(b) alkali metal oxides: Li.sub.2 O 0-9, Na.sub.2 O 7.5-15, K.sub.2 O 2-10, 
but preferably: Li.sub.2 O 0-3, Na.sub.2 O 7.5-13, K.sub.2 O 4-6, with the 
sum of alkali metal oxides being 12-18; 
(c) MgO 0-5, CaO 1-3, SrO 0-6, BaO 3-13, ZnO 0-5, but preferably: MgO 0-2, 
CaO 1-3, SrO 0-2, BaO 7-13, ZnO 0-2, and the sum of these alkaline earth 
metal oxides being 8-15; 
(d) additional components: 
______________________________________ 
TiO.sub.2 
15-35 
ZrO.sub.2 
0-3 
Nb.sub.2 O.sub.5 
1-20 
WO.sub.3 
0-3, 
______________________________________ 
but preferably: 
______________________________________ 
TiO.sub.2 
20-30 
ZrO.sub.2 
0-3 
Nb.sub.2 O.sub.5 
7-19 
WO.sub.3 
0.2-2; 
______________________________________ 
wherein the ratio of SiO.sub.2 to TiO.sub.2 is in the range of about 1 to 
1.75; and 
(e) optionally one or more of the following additional oxides: 
______________________________________ 
PbO 0-5 
La.sub.2 O.sub.3 
0-3 
Y.sub.2 O.sub.3 
0-5 
Bi.sub.2 O.sub.3 
0-2 
Gd.sub.2 O.sub.3 
0-4 
Ta.sub.2 O.sub.5 
0-2. 
______________________________________ 
Additionally, the glass contains no more than 1.5 percent by weight of 
fluorine in the form of F- ions and/or 0.2-2 percent by weight of 
SnO.sub.2, and no more than about 2 percent by weight of other unmentioned 
glass-forming oxides. 
The glasses of the present invention meet both the requirement of low 
density (&lt;3.5 g/cm.sup.3 for refractive values of &gt;1.70 and Abbe indices 
of &gt;22) and also the requirement of a sufficient stability towards melting 
procedures customary in industry today. The relatively high 
crystallization stability is achieved by specific amounts of Nb.sub.2 
O.sub.5, WO.sub.3 and/or Al.sub.2 O.sub.3, CaO, MgO and by the ratio of 
SiO.sub.2 to TiO.sub.2, which has been greatly increased over those of the 
familiar composition ranges known in the prior art. The particularly good 
chemical stability of the glasses of the present invention is caused by 
small quantities of B.sub.2 O.sub.3 (up to 7 percent by weight), Al.sub.2 
O.sub.3 (up to 4 percent by weight) and WO.sub.3 (up to 3 percent by 
weight), and by the high content of TiO.sub.2 (15-35 percent by weight). 
Table 1 compares some examples of the present invention with conventional 
glasses (SF 4, SF 11 and SF 56, see SCHOTT Catalog No. 3111 D, 1980); acid 
resistance and alkali resistance were determined as described in the cited 
SCHOTT catalog, the contents of which are incorporated by reference 
herein. 
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Example Example Example 
SF 4 3 SF 11 7 SF 56 9 
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nd 1.7552 1.7552 1.7847 
1.7826 
1.7847 
1.7845 
vd 27.58 27.36 25.67 25.60 26.08 26.10 
density 
4.79 3.15 4.74 3.21 4.92 3.28 
SR* 5 1 2 1 3 1 
AR** 2.3 1.0 1.2 1.0 2.2 1.3 
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*SR = ac1d resistance (Saureresistenz): 
SR 1 SR 2 SR 3 SR 4 SR 5 
Time (h) 
&gt;100 100-10 10-1 1-0.1 
&lt;0.1 
Time for the removal of 0.1 .mu.m from a 
polished layer with 0.5 N HNO.sub.3, in hours. 
**AR = alkali resistance: 
AR 1 AR 2 
Time (h) &gt;120 120-30 
Time for the removal of 0.1 .mu.m from a 
polished layer with NaOH pH 10, in hours. 
Evaluation: 
0 - no visible change 
1 - scarred surface, no visible formation of layers 
2 - interference colors 
3 - white spot. 
In addition to the above stated main requirements, the glasses of the 
present invention also meet the requirement for sufficient crystallization 
stability so that they can be produced in large melting containers (e.g. 
100 liter crucibles) and in large dimensions (e.g. glass blocks of 
300.times.300.times.200 mm). Moreover, the glasses of the present 
invention are different from those known in the prior art because of their 
considerably greater transmission (Example: pure transmission at 400 nm 
and 25 mm thickness of layer SF 11=0.21; Ex. 7=0.55) than found in 
conventional heavy flint glasses. Thus, the glasses of the present 
invention permit a considerable improvement both with respect to weight 
and also the imagery quality of high quality optical equipment, e.g. 
portable television cameras. 
Compared with the heavy flint glasses cutomarily used for this optical 
range, the glasses of the present invention exhibit considerably less 
discoloration, which is expressed in increased pure transmission. By 
adding small quantities of F- (e.g., as NaF), this property can be further 
improved.