Low refractive optical glass of a flint glass type

Abstract: An optical glass which is a SiO₂ - Nb₂O₅ - R'₂O (R' being an alkali metal element) - F system of a specific composition range exhibits optical constants of a refractive index (Nd) of about 1.60 or below and an Abbe number ( νd) of about 57 - 40, is free of PbO and has an improved light transmissivity.

Description: This invention relates to a low refractive flint glass type optical glass of a SiO 2 - Nb 2 O 5 - TiO 2 - R' 2 O - F (where R' is an alkali metal element) having optical constants of a refractive index (Nd) of about 1.60 or below and an Abbe number (νd) of about 57 - 40. Known in the art are a variety of glasses having these optical constants. These glasses are of a relatively low refractive and low dispersion property in the property region of flint glasses and have been produced mostly as a SiO 2 - PbO system glass. These glasses, however, have a serious drawback of containing a toxic PbO ingredient. For overcoming this drawback, the specification of USP 3,877,953 discloses a SiO 2 - B 2 O 3 - Nb 2 O 5 - K 2 O system glass and the specification of USP 4,719,186 disclose a SiO 2 - B 2 O 3 - TiO 2 - R' 2 O system glass. Journal of Non-Crystalline Solids, Vol. 129, No. 1 to 3, 1991, pages 54 to 63 describes an optical glass consisting of KF, Nb 2 O 5 and SiO 2 . EP 0 564 802 A2 discloses a glass being free of Pb and Ba having a high light transmission. Said publication published on 13 October 1993 discloses the presence of a minimum amount of CaO being 3 % by weight. JP-01133956-A discloses a glass composition for grated index type lens which is obtained by replacing a part of O 2- -ion in a glass composition by 1 to 20 mol.-% of F - -ion. The glass composition comprises by mol.-% 40 to 65 SiO 2 , 0 to 20 B 2 O 3 , 10 to 25 Al 2 O 3 , 0 to 25 Li 2 O, 0 to 20 Na 2 O, 0 to 20 K 2 O, 0 to 25 Cs 2 O, 0 to 25 TiO 2 , 8 to 35 Li 2 O + Na 2 O + K2O + Cs 2 O + TiO 2 , 0 to 25 TiO 2 , 0 to 10 ZrO 2 , 0 to 20 GeO 2 , 0 to 20 MgO, 0 to 10 CaO, 0 to 10 SrO, 0 to BaO, 0 to 10 SnO 2 , 0 to 15 PbO, 0 to 15 Ta 2 O5, 0 to 10 Nb 2 O5 0 to 2 As 2 O3 and 0 to 2 Sb 2 O 3 . JP-52-25812 discloses a low density optical flint glass. The glass contains SiO 2 (partially replaceable with B 2 O 3 ) 10 to 60 % by weight, Nb 2 O 5 (partially replaceable with Ta 2 O 5 ) 5 to 60 % by weight, TiO 2 0,5 to 50 % by weight, alkali metal oxides 5 to 50 %, ZrO 2 < 20 %, alkaline earth metal < 30 %, Al 2 O 3 < 10 % and Nb 2 O 5 + TiO 2 10 to 70 % by weight. The refractive index Nd is 1,61 to 1,80. The Abbé number νd is 38 to 25. JP-58-46459 discloses a low density of optical flint glass compositions containing oxides of silicone, niobium, barium, sodium, potassium and titanium. The glass comprises (in weight %) 10 to 60 SiO 2 , 5 to 60 Nb 2 O 5 , 5 to 40 R 2 O, 5 to 40 RO, 0 to 20 La 3 P 3 , 0 to 20 ZrO 2 , 0 to 20 TiO 2 , 0 to 10 Al 2 O 3 , where SiO 2 can be partially substituted with B 2 O 3 and Sn 2 O 5 can be partially substituted with Ta 2 O 5 . the low density optical glass has a high quality refractive index 1.61 to 1.8 nd and 43 to 28 νd. DE 973 350 discloses optical silicate glasses having a low refractive index. 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 % SiO 2 , 1 to 50 % Nb 2 O 5 , 17.5 to 30 % alkali metal oxides and 0 to 10% fluorides (s). Further, 0.5 to 15 % B 2 O 3 and 0.5 to 30 % P 2 O 5 are present. The proposed glasses having the above described optical constants, however, have the disadvantage that they are inferior in light transmissivity. It is therefore an object of the present invention to eliminate the above described disadvantages of the prior art glasses and provide an optical glass which has optical constants of a refractive index (Nd) of 1.60 or below and an Abbe number (νd) of 57 to 40 and has improved light transmissivity. Accumulated studies and experiments made by the inventor of the present invention for achieving the above described object of the invention have resulted in a finding, which has led to the present invention, that a stable glass can be obtained in a novel SiO 2 - Nb 2 O 5 - TiO 2 - R' 2 O - F system glass which is substantially free of B 2 O 3 and that a PbO-tree optical glass which has the above described desired optical constants and is improved in light transmissivity can thereby be obtained. The optical glass of a flint glass type achieving the above described object of the invention is characterized in that the glass being substantially free of PbO consists of in weight percent: SiO 2 45 - 70 % Al 2 O 3 0 - 20 % Nb 2 O 5 + TiO 2 0.5 - 17 % in Nb 2 O 5 0.5 - 2 % TiO 2 up to 15 % ZrO 2 0 - 10 % Li 2 O + Na 2 O + K 2 O 15 - 40 % in which Li 2 O 0 - 20 % Na 2 O 0 - 40 % K 2 O 0 - 40 % CaO less than 3 % ZnO + MgO + CaO + SrO + BaO 0 - 10 % in which BaO 0 - 5 % and a fluoride of a metal element or elements contained in the above metal oxides, a total amount of F contained in the fluoride or fluorides 0.1 - 8 % having a refractive index Nd of 1.60 or below and an Abbé number (νd) of 57 to 40 and having a wavelength of light producing light transmissivity of 5 % at a thickness of 10 mm being 330 nm or less and a wavelength of light producing light transmissivity of 80 % at a thickness of 10 mm being 360 nm or less. 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 SiO 2 which is a principal glass forming ingredient should be within the range of 45 - 70% for chemical durability and melting property of the glass. The Al 2 O 3 ingredient may be added for adjusting optical constants and improving stability of the glass. If the amount of this ingredient exceeds 20%, the melting property decreases with resulting difficulty in obtaining a homogeneous glass. The Nb 2 O 5 ingredient is the most important ingredient for imparting desired optical constants to the glass of the present invention and besides is effective for improving chemical durability of the glass. For obtaining the expected results, it is necessary to add 0.5% or more Nb 2 O 5 . The TiO 2 ingredient has the same effect as the Nb 2 O 5 ingredient, and is present in an amount up to 15% because, if the amount exceeds 15%, the glass tends to become tinted, The total maximum amount of the Nb 2 O 5 and TiO 2 ingredients, should be 17% for preventing the glass from being tinted. For improving the chemical durability of the glass, the ZrO 2 ingredient may be added up to 10% which is a limit at which significant devitrification can be prevented. The ingredients Li 2 O, Na 2 O and K 2 O are effective for increasing the melting property of the glass and the ingredients Na 2 O and K 2 O are effective for lowering the refractive index, so that one or more of these ingredients should be added to the glass. For maintaining the chemical durability, the addition of these ingredients should be respectively up to 20%, 40% and 40% and a total amount of one or more of these ingredients should be within the range of 15 - 40%. The ZnO, MgO, SrO and BaO ingredients are effective for improving the melting property of the glass and one or more of these ingredients may be added. A total amount of one or more of these ingredients should be up to 10% for maintaining the target optical constants. For the same reason, the amount of the BaO ingredients should be up to 5%. The fluorine (F) ingredient is an important ingredient for lowering the refractive index and improving light transmissivity of the glass. For achieving these effects, it is necessary to add 0.1% or more fluorine. The addition of fluorine, however, should not exceed 8% because otherwise striae would be produced in the glass and the quality of the glass would thereby be deteriorated. As a defoaming agent, Sb 2 O 3 or other known defoaming agent in the amount not exceeding 1% may be added to the glass of the present invention. Table 1 shows examples of compositions (No. 1 - No. 9), a reference example and comparative examples (No. A and No. B) of the prior art optical glasses as well as the refractive index (Nd), Abbe number (νd) and wavelengths of light in the unit of 10 nm producing light transmissivity of 80% and 5% in a specimen with two polished surfaces having thickness of 10 mm, i.e., T 80 /T 5 (10nm) These glasses were obtained by preparing and mixing selected raw materials such as oxides, carbonates, nitrates and fluorides and melting the mixture of the raw materials in a platinum crucible, stirring the mixture to homogenize it, casting it in a mold and annealing it. In Table 1, the examples of the optical glasses according to the present invention all exhibit optical constants within the predetermined region and smaller values of T 80 and T 5 than the glasses of the comparative examples having equivalent optical constants, which indicates that the wavelength of light is Shifted substantially toward the short wavelength side, so that light transmissivity is remarkably improved. Moreover, the glasses of the examples of the present invention have good chemical durability. Furthermore, these glasses have excellent melting property and property against devitrification and have a low softening temperature and, accordingly, are easy to manufacture. Examples Comparative Examples No. 1 2 3 4 5 A B SlO 2 60.8 48.0 51.3 57.0 59.0 28.5 60.0 B 2 O 3 10.5 6.6 Al 2 O 3 5.0 1.7 Nb 2 O 5 2.0 1.0 2.0 1.0 2.0 10.0 TiO 2 9.5 5.0 12.2 5.0 14.0 9.9 ZrO2 0.5 ZnO 2.5 MgO 0.2 CaO SrO 5.0 BaO 2.5 Li 2 O 5.0 8.0 8.0 Na 2 O 7.5 36.0 10.0 5.0 5.6 K 2 O 7.5 17.0 36.0 3.0 21.0 7.0 AlF 3 5.0 Ti 2 F 6 30.0 NaF 0.5 KF 2.5 7.5 1.0 4.0 (F) (0.8) (3.4) (2.4) (0.3) (1.3) (15.5) (0.2) Sb 2 O 3 0.2 0.2 ηd 1.5714 1.5236 1.5534 1.5260 1.5970 1.5886 1.5789 νd 45.5 49.1 44.5 52.7 40.9 39.2 46.4 T 80 /T 5 (10nm) 35/33 35/33 35/33 34/32 36/33 37/34 38/35

Claims: A low refractive optical glass of a flint glass type being substantially free of PbO and consisting of in weight percent SiO 2 45 - 70 % Al 2 O 3 0 - 20 % Nb 2 O 5 + TiO 2 0.5 - 17 % in Nb 2 O 5 0.5 - 2 % TiO 2 up to 15 % ZrO 2 0 - 10 % Li 2 O + Na 2 O + K 2 O 15 - 40 % in which Li 2 O 0 - 20 % Na 2 O 0 - 40 % K 2 O 0 - 40 % CaO less than 3 % ZnO + MgO + CaO + SrO + BaO 0 - 10 % in which BaO 0 - 5 % and a fluoride of a metal element or elements contained in the above metal oxides, a total amount of F contained in the fluoride or fluorides 0.1 - 8 % having a refractive index Nd of 1.60 or below and an Abbé number (νd) of 57 to 40 and having a wavelength of light producing light transmissivity of 5 % at a thickness of 10 mm being 330 nm or less and a wavelength of light producing light transmissivity of 80 % at a thickness of 10 mm being 360 nm or less.

Inventors: ENOMOTO GORO, ENOMOTO, GORO
Applicants: OHARA KK, KABUSHIKI KAISHA OHARA
Application year: 1993
Main classification: C03C3
IPC codes: C03C3