High dispersion optical glass

Abstract: An optical glass which is a SiO₂ - Nb₂O₅ - R'₂O (where R' is an alkali metal element) - F system of a specific composition range exhibits optical constants of a refractive index (Nd) of about 1.52 - 1.67 and an Abbe number (ν d) of about 32 - 50, is free of PbO and has an improved light transmissivity.

Description: This invention relates to a high dispersion optical glass of a SiO₂ - Nb₂O₅ - TiO₂ - R'₂0 (where R' is an alkali metal element) - F system having optical constants of a refractive index (Nd) of about 1.52 - 1.67 and an Abbe number (ν d) of about 32 - 50 which is excellent in light transmissivity and contains a relatively large content of Nb₂O₅ ingredient. Known in the art as optical glasses having the above described optical constants are glasses of a SiO₂ - PbO - R'₂O system, a SiO₂ - TiO₂ - R'₂O system and a SiO₂ - TiO₂ - R'₂O - F system disclosed in the specification of USP 2,481,701. The first mentioned prior art optical glass, however, contains toxic PbO and the latter two optical glasses are disadvantageous in that they do not have excellent light transmissivity. Journal of Non-Crystalline Solids, Vol. 129, No. 1 to 3, 1991, pages 54 to 63 describes an optical glass consisting of KF, Nb₂O₅ and SiO₂. JP-A-63 265 840 (as interpreted by the Derwent Abstract, No. 88-356301) describes an optical glass having a refractive index of 1.59 to 1.82, Abbe's No. of 20 to 40 and exhibiting excellent light transparency with the composition, comprising 5 to 55 % SiO₂, 1 to 50 % Nb₂O₅, 17.5 to 30 % alkali metal oxides and 0 to 10 % fluorides (s). Further, 0.5 to 15 % B₂O₃ and 0.5 to 30 % P₂O₅ are present. Japanese Patent Application Laid-open No. Sho 52-25812 discloses a glass of a SiO₂ - TiO₂ - Nb₂O₅ - R'₂O system and Japanese Patent Application Laid-open No. Sho 52-45612 discloses a glass of a SiO₂ - Nb₂O₅ - R''₂O (where R'' is a divalent metal element) - R'₂O system. These glasses, however, do not sufficiently satisfy both a high dispersion optical characteristic and an excellent optical transmissivity which are the target of the present invention. It is, therefore, an object of the invention to eliminate the disadvantages of the prior art optical glasses comprehensively by providing a novel optical glass which is free from the toxic PbO ingredient, has optical constants of a refractive index (Nd) of about 1.52 - 1.67 and an Abbe number (ν d) of about 32 - 50, and has an excellent light transmissivity. Accumulated studies and experiments made by the inventors of the present invention for achieving the above described object have resulted in a finding, which has led to the present invention, that an optical glass having the desired optical constants and an improved light transmissivity can be obtained in a SiO₂ - Nb₂O₅ - TiO₂ R'₂O - F system glass within a certain composition range. The optical glass being substantially free of PbO achieving the above described object of the invention is characterized in that the glass consists of in weight percent: SiO₂ 47 - 70% Al₂O₃ 0 - 15% Nb₂O₅ 15.5 - 35% TiO₂ upto 15% ZrO₂ 0 - 10% Li₂O + Na₂O + K₂O 10 - 35% in which Li₂O 0 - 10% Na₂O 0 - 20% K₂O 0 - 20% ZnO + MgO + CaO + SrO + BaO 0 - 5% and a fluoride or fluorides of a metal element or elements contained in one or more of the above metal oxide or oxides, said fluoride or fluorides substituting for said metal oxide or oxides partially or entirely, and a total amount of F contained in the fluoride or fluorides 0.1 - 10% In a preferred embodiment of the invention, the high dispersion optical glass as defined above has the total amount of F which is in the range 0.1 - 7%. In the optical glass made according to the invention, the above described content ranges of the respective ingredients have been selected for the reasons stated below. In the following description, the content range of the respective ingredients are expressed in weight percent. The amount of SiO₂ which is an indispensable ingredient for forming a glass should be within the range of 47 - 70%. If the amount is less than 47%, chemical durability of the glass will deteriorate whereas if the amount exceeds 70%, melting property of the glass will deteriorate. The Al₂O₃ ingredient may be added as desired for adjusting optical constants and improving stability of the glass. If the amount exceeds 15%, melting property of the glass will deteriorate with resulting difficulty in obtaining a homogeneous glass. The Nb₂O₅ ingredient is an important ingredient in the glass of the present invention because it contributes to improving of high dispersion characteristic and light transmissivity. If, however, the amount is less than 15.5 %, the target optical constants cannot be obtained whereas, if the amount exceeds 35%, devitrification tendency of the glass will increase. The TiO₂ ingredient is added for improving the refractive index and decreasing the Abbe number of the glass. If the amount exceeds 15%, the glass tends to become tinted. The ZrO₂ ingredient may be added as desired for improving chemical durability of the glass. If the amount exceeds 10%, devitrification tendency of the glass will increase. The Li₂O, Na₂O and K₂O ingredients may be added as desired because these ingredients are effective for facilitating melting of the glass. If, however, the amounts of these ingredients exceed 10%, 20% and 20% respectively, the refractive index of the glass will decrease. On the other hand, if the total amount of one or more of these ingredients is less than 10%, the above described effect cannot be obtained sufficiently whereas, if the total amount exceeds 35%, chemical durability of the glass will deteriorate. The ZnO, MgO, CaO, SrO and BaO ingredients may be added as desired because these ingredients are effective for improving melting property of the glass. If, however, the total amount of one or more of these ingredients exceeds 5%, it will become difficult to maintain the target optical constants. The fluorine (F) ingredient is an important ingredient in the glass of the present invention in that it increases the high dispersion characteristic to the refractive index and increases light transmissivity. If the amount of F is less than 0.1%, these effect will not be obtained sufficiently whereas the amount should not exceed 10%, and preferably should be 7% or below, because if the amount exceeds 10%, cord occurs in the glass during melting with resulting difficulty in obtaining a homogeneous glass. In addition to the above described ingredients, a known defoaming agent such as Sb₂O₃ may be added up to about 0.5%. Table 1 shows examples of the high dispersion optical glass according to the invention (No. 1 to No. 9) a reference example and comparatives examples of the prior art optical glasses (No. A and No. B) as well as refractive index (Nd), Abbe number (ν d) and wavelengths of light in the unit of 10 nm producing light transmissivity of 80% (T80) in a specimen with two polished opposite surfaces having thickness of 10 mm. As shown in Table 1, the glasses of No. 8 and No. 9 of the examples of the present invention have wavelengths of light shifted largely to the short wavelength side compared with the glasses of No. A and No.B of the prior art glasses which have about the same optical constants and this clearly indicates improvement in light transmissivity in the glasses of the present invention. The glasses of No. 1 to No. 7 of the examples of the present invention also exhibit an excellent light transmissivity. The glasses of the examples of the present invention can be obtained by weighing and mixing normal optical glass raw materials such as oxides, carbonates, nitrates and fluorides, melting and defoaming the raw materials in a platinum crucible at 1200 - 1400° C for about 2 to 5 hours, stirring the molten glass to homogenize it and thereafter casting it in a mold and annealing it.

Claims: A high dispersion optical glass being substantially free of PbO and consisting of in weight percent: SiO₂ 47 - 70% Al₂O₃ 0 - 15% Nb₂O₅ 15.5 - 35% TiO₂ upto 15% ZrO₂ 0 - 10% Li₂O + Na₂O + K₂O 10 - 35% in which Li₂O 0 - 10% Na₂O 0 - 20% K₂O 0 - 20% ZnO + MgO + CaO + SrO + BaO 0 - 5% and a fluoride or fluorides of a metal element or elements contained in one or more of the above metal oxide or oxides, said fluoride or fluorides substituting for said metal oxide or oxides partially or entirely, and a total amount of F contained in the fluoride or fluorides 0.1 - 10%. A high dispersion optical glass as defined in claim 1 wherein the total amount of F is 0.1 - 7%.

Inventors: ENOMOTO GORO, GOTO NAOYUKI, ISHIZAKI FUTOSHI, YAMAGUCHI KATSUHIKO, YASUI HIDEO, YATSUDA HISAO, ENOMOTO, GORO, GOTO, NAOYUKI, ISHIZAKI, FUTOSHI, YAMAGUCHI, KATSUHIKO, YASUI, HIDEO, YATSUDA, HISAO
Applicants: OHARA KK, KABUSHIKI KAISHA OHARA
Application year: 1993
Main classification: C03C3
IPC codes: C03C3