GB2137189A

GB2137189A - Optical Glass

Info

Publication number: GB2137189A
Application number: GB08328871A
Authority: GB
Inventors: Hidema Tajima; Toshiharu Yamashita
Original assignee: Hoya Corp
Current assignee: Hoya Corp
Priority date: 1983-02-14
Filing date: 1983-10-28
Publication date: 1984-10-03
Also published as: JPS6335579B2; JPS59152240A; DE3340005A1; FR2540857B1; FR2540857A1; GB2137189B; GB8328871D0; DE3340005C2

Abstract

An optical glass having a high refractive index, a large Abbe number and a low specific gravity and which is suitable for use in spectacle lenses, comprises, in % by weight, 56 to 67% SiO2; 0 to 7% B2O3; 0 to 5% Al2O3; 0 to 11% Na2O; 0 to 15% K2O; 0 to 7% Li2O; with the proviso of 11 to 18% Na2O+K2O+Li2O; 12 to 19% TiO2; 0 to 4% MgO+CaO+SrO+ZnO; and 0 to 2% La2O3+ZrO2+Nb2O5. The glass may include small amounts of Co2O3, Nd2O3 and/or Pr6O11.

Description

SPECIFICATION Optical Glass The present invention relates to an optical glass. More particularly, it relates to an optical glass having a refractive index of 1.59 or more, an Abbe number of 36 or more and a specific gravity of 2.6 or less. Such a glass having a low specific gravity and a large Abbe number for a high refractive index is very suitable for use in a glass for use as a spectacle lens.

The correction of spectacle lenses depends on the refractive index and the curvatures of both surfaces of the lens used, but when the degree of myopia or hypermetropia advances, the thickness of the lens for correcting it increases at the outer edge portion and the center portion and hence the appearance is spoiled and the lens becomes heavy in weight to give an unpleasant feeling to the user.

Therefore, to overcome this problem, a glass for a spectacle lens having a light weight and a high refractive index has heretofore been proposed as disclosed in, for example, U.S. Patent 3,615,770 and Japanese Patent Publication 2645/1 969. According to the teachings of those publications, the weight of the spectacle lens can be reduced if it has a diopter value of -3 or less and +3 or more.

However, where a diopter value within the range of +3 is required, even if a glass having a specific gravity of about 3.0 is used, the weight of the eye glass lens cannot be reduced.

Further, glass having a low specific gravity and a relatively high refractive index necessarily has a small Abbe number and particularly when the glass is used as a spectacle lens and an object is seen through the outer edge portion of the lens, the image blurs at the periphery thereof due to chromatic aberration. In particular, blurring due to a yellow light is appreciable and in an extreme case, the ability of lens to correct the visual acuity is decreased 30%.

Accordingly, an object of the present invention is to provide a glass which can produce a thin and light spectacle lens having a low diopter value. Such a glass has a low specific gravity as in conventional spectacle lens glass but has a high refractive index.

Another object of the present invention is to provide a glass which can be used for a spectacle lens having a reduced chromatic aberration at the outer edge portion.

The optical glass according to the present invention comprises, in % by weight 56 to 67% SiO2; O to 7% B203; 0to5%Al2O3; 0to 1 1 % Na2O; 0to 15%K2O; Oto 7% Li2O, with the proviso of 11 to 18% Na2O+K2O+Li2O; 1 2 to 1 9% TiO2; O to 4% MgO+CaO+SrO+Zn0; and O to 2% La203+ZrO2+Nb2o5- In order to increase the Abbe number, the amount of TiO2 must be decreased. However, if the amount of TiO2 is too small, the desired specific gravity and refractive index cannot be obtained. In order to increase the Abbe number while maintaining the desired refractive index (nd) and specific gravity (Sg), the amount of K2O must be increased.The preferred amount of K2O is 1 0 to 1 5% by weight. Further, K2O is useful in having a good compatibility with the coating of the lens.

The use of Cho203 is effective to completely decolorize the coloration of glass by TiO2. The preferred amount of Co203 is 0.5 to 3.0 ppm per 100 parts by weight of the above basic glass composition.

With regard to the spectrum at the periphery of image due to the chromatic aberration, use of Nd203 is effective from the necessity of eliminating a yellow light having a high visual sensity. This effect can be achieved by adding Nd203 in an amount of 0.1 to 1.0 part by weight per 100 parts by weight of the above basic glass composition.

Further, a glass having a preferred blue tone can be obtained by adding Per501, in an amount up to 0.3 part by weight per 100 parts by weight of the above basic glass composition.

In the above basic glass composition according to the present invention, SiO2 is a glass-forming oxide. If the amount of SiO2 is more than 67% by weight, the glass is difficult to melt and also becomes unstable and hence the tendency of the glass to devitrify increases. If the amount of SiO2 is less than 56% by weight, the specific gravity of the glass increases.

B203 is effective to decrease the specific gravity and increase the Abbe number and also makes the glass to melt easily. If the amount of B20 is more than 7% by weight, a tendency of phase separation of the glass increases.

Al203 is necessary to stabilize the glass. If the amount of Al203 is more than 5% by weight, a tendency of phase separation of the glass increases.

The alkali metal oxides of Na2O, K20 and Li2O are the components necessary to make it easy to melt the glass and also prevent the tendency of phase separation of the glass, thereby stabilizing the glass. If the total amount of the alkali metal oxides exceeds 1 8% by weight, the specific gravity increases and the chemical durability deteriorates and as a result, the glass cannot be used for a spectacle lens. If the total amount thereof is less than 1 1%, a tendency of phase separation of the glass increases. Of those three components, if the amount of Na2O is more than 11% by weight and the amount of K20 is more than 1 5% by weight, the chemical durability of the glass deteriorates.

K20 is a component effective to prevent the coloration and increase the Abbe number. K20 is preferably used in an amount of 1 0 to 1 5% by weight to achieve these effects.

Li20 is a component effective to increase the refractive index and decrease the specific gravity. If the amount of Li2O is more than 7% by weight, the glass tends to crystallize and the cost of the raw materials increases. Therefore, Li2O should not be used in an amount of more than 7% by weight.

TiO2 is an indispensable component to achieve the highest refractive index and the lowest specific gravity. If the amount of TiO2 is more than 1 9% by weight, the Abbe number decreases and the desired specific gravity cannot be obtained. If the amount of TiO2 is less than 1 2% by weight, the desired refractive index cannot be obtained.

The divalent oxides of MgO, CaO, SrO and ZnO are used to adjust the optical constants of the glass. The divalent oxides other than MgO increase the specific gravity as compared to the alkali metal oxides. Therefore, if the total amount of the divalent oxide is more than 4% by weight the desired specific gravity cannot be obtained. Further, use of MgO in an amount of more than 4% by weight causes a strong phase separation tendency.

A part of TiO2 can be replaced with a small amount of at least one of La203, ZrO2 and Nub205 to adjust the Abbe number. If the replacement is 2% by weight or more, the desired gravity cannot be obtained.

Ta2O5, Y203 and Gd2O3 can be added to the glass composition in the small amounts to adjust the optical constants. However, those components are expensive and are not preferably used.

Co2O3 is a component extremely effective to decolorize the coloration by TiO2. The preferred amount of Co2O3 is 0.5 to 3.0 ppm per 1 00 parts by weight of the basic glass composition. If the amount of Co203 is outside the above range, a proper effect for preventing the coloration cannot be obtained.

Nd2O3 is a component effective to eliminate the yellow light due to the chromatic aberration and is used in an amount of 0.1 to 1.0 part by weight per 100 parts by weight of the basic glass composition to maintain the appropriate transmittance over the visual light region.

As2O3 and/or Sb2O3 are used to purify the glass in an amount of 0.5% by weight or less based on the basic glass composition.

Examples of the glass according to the present invention and their properties are shown in Table 1 beiow. The composition of each glass is expressed in % by weight.

TABLE 1 Example No.

Component 1 2 3 4 5 6 7 8 9 SiO2 65.0 62.5 66.5 62.8 59.5 61.3 62.0 61.0 61.5 B203 4.5 4.5 3.5 4.5 4.5 4.5 4.5 6.5 Al2O3 2.5 2.5 2.5 2.5 2.5 2.5 2.5 Na2O 9.0 9.8 2.0 5.0 2.0 5.0 K20 3.8 6.8 13.7 3.8 10.8 6.8 8.8 3.8 Li20 4.0 4.0 3.0 4.0 7.0 4.0 4.0 4.0 TiO2 17.2 16.2 16.7 14.5 16.2 13.0 16.7 16.7 15.2 MgO 4.0 CaO 2.0 2.0 SrO 1.0 ZnO 1.0 La2O3 0.9 ZrO2 1.0 Nb205 Co2O3 Nd2O3 Pr6O11 nd 1.598 1.602 1.599 1.593 1.606 1.594 1;602 1.607 1.599 vd 37.3 39.9 39.4 40.0 39.4 43.0 39.4 38.6 39.9 Sg 2.57 2.60 2.59 2.58 2.59 2.59 2.59 2.59 2.59 TABLE 1 (Cont.) Example No.

Component 10 11 12 13 14 15 16 17 SiO2 61.5 56.0 64.2 64.2 64.2 59.5 62.5 62.5 B203 4.5 4.5 2.5 2.5 2.5 4.5 4.5 4.5 Al2O3 2.5 5.0 1.0 1.0 1.0 2.5 2.5 Na2O 9.0 10.8 10.8 10.8 3.0 3.0 9.8 K20 9.8 7.8 1.0 1.0 1.0 11.0 10.8 Li2O 4.0 5.0 5.0 5.0 2.4 2.6 4.0 TiO2 16.2 17.2 15.0 15.0 15.0 17.1 16.6 16.2 MgO CaO SrO ZnO La2O3 ZrO2 Nub205 1.0 Co2O3 0.00005 0.00025 0.00013 0.00010 Nd2O3 0.5 0.4 0.2 Pr6O11 0.1 nd 1.599 1.5960 1.599 1.599 1.599 1.600 1.597 1.602 vd 39.1 38.2 41.5 41.5 41.5 38.7 39.4 39.9 Sg 2.60 2.60 2.60 2.60 2.60 2.59 2.59 2.59 The optical glasses of the present invention including the glasses shown in the Examples can be obtained by melting and stirring the mixture of starting material powders such as siliceous stone powder, aluminum hydroxide, titanium oxide, borax, potassium carbonate and magnesium carbonate in a tank furnace or platinum crucible at 1,300 to 1,400 C to homogenize the mixture and remove bubbles, casting the molten mixture into a metallic mold pre-heated at a suitable temperature or directly press molding it and annealing the product.

Claims

1. An optical glass comprising, in % by weight, 56 to 67% of SiO2; O to 7% B203; O to 5% Al2O3; Oto 11%Na2O; O to 15%K2O; O to 7% Li20; with the proviso that the total of Na2O+K20+Li20 is 11 to 18%; 12 to 19%TiO2; O to 4% MgO+CaO+SrO+ZnO; and O to 2% La203+ZrO2+Nb2O5.

2. An optical glass as claimed in Claim 1, wherein the amount of K20 is 10 to 1 5% by weight.

3. An optical glass as claimed in Claim 1 or 2, which also contains Co203 in an amount of 0.5 to 3.0 ppm per 1 00 parts by weight of the glass composition.

4. An optical glass as claimed in Claim 1,2 or 3, which also contains Nd203 in an amount of 0.1 to 1.0 part by weight.

5. An optical glass as claimed in any preceding claim, which also includes Pr8O11 in an amount of O to 0.3 part by weight per 100 parts by weight of the glass composition.

6. An optical glass as claimed in any preceding claim, which also contains Co2O3 in an amount of 0.5 to 3.0 ppm.

7. An optical glass having a composition as shown in any of Examples 1 to 1 7 hereinbefore.

8. A lens made from an optical glass as claimed in any preceding claim.