A FLUOROPHOSPHATE OPTICAL GLASS

Abstract: A fluorophosphate optical glass having optical constants, i.e. refractive index (nd) of 1.54 to 1.60 and Abbe number (νd) of 68 to 75 and having abnormal partial dispersion represented by a relative partial dispersion of at least 0.537 is provided, which has a chemical composition (% by weight) comprising: Al(PO₃)₃ 15 - 32% Ba(PO₃)₂ 0 - 10% Sr(PO₃)₂ 0 - 10% Ca(PO₃)₂ 0 - 10% Mg(PO₃)₂ 0 - 10% sum of metaphosphates 20 - 32% BaF₂ 20 - 70% SrF₂ 5 - 40% CaF₂ 0 - 15% MgF₂ 0 - 10% AlF₃ 0 - 5% GdF₃ 0 - 5% sum of fluorides 55 - 75% Gd₂O₃ 5 - 22% La₂O₃ 0 - 7% Y₂O₃ 0 - 10% Yb₂O₃ 0 - 10% sum of rare earth oxides 5 - 22%

Description: 1. Field of the Invention This invention relates to a fluorophosphate optical glass having optical constants, i.e. refractive index (nd) of 1.54 to 1.60 and Abbe number (νd) of 68 to 75 and having abnormal partial dispersion represented by a relative partial dispersion of at least 0.537. 2. Description of the Prior Art An optical glass having abnormal partial dispersion, capable of effecting correction of the secondary spectrum in designing a lens, is very important for designing the same and is of high utility value. As to fluorophosphate glasses having abnormal partial dispersion and a refractive index (nd) of at most 1.50, a number of reports have been made and it is known that the fluorophosphate glass disclosed in Japanese Patent Laid-Open Publication No. 144141/1988 has been produced on a commercial scale and used for various optical instruments. On the other hand, optical designers have eagerly had requirements for an optical glass having a higher refractive index, lower dispersion and abnormal partial dispersion. The optical glasses having optical constants satisfying such requirements are known as disclosed in Japanese Patent Publication Nos. 28169/1978, 4145/1985 and 14500/1987. The known optical glasses contain large amounts of Ba(PO₃)₂ and BaO as essential components as in a glass composition, for example, comprising 2.0 to 13.0% of MgF₂, 0 to 20.0% of SrF₂, 1.0 to 33.0% of BaF₂, 0 to 8.0% of AlF₃, 0 to 8.0% of YF₃, 13.0 to 39.0% of Al(PO₃)₃, 0 to 24.0% of Mg(PO₃)₂, 1.0 to 20.0% of Ca(PO₃)₂, 5.0 to 19.0% of Ba(PO₃)₂, 10.0 to 36.0% of BaO, 1.5 to 12.0% of Y₂O₃ and/or Yb₂O₃, 0 to 6.0% of ZnO, 0 to 29.0% of PbO and 0 to 22.0% of Nb₂O₅, % being by weight, as described in Japanese Patent Publication No. 4145/1985. However, the large contents of Ba(PO₃)₂ and BaO result in not only increase of the dispersion but also lowering of the abnormal partial dispersion. These known optical glasses having a relatively high liquidus temperature are so unstable that crystals tend to be precipitated and comparable properties to those of the optical glass according to the present invention cannot be given in stable manner. SUMMARY OF THE INVENTION It is an object of the present invention to provide a fluorophosphate optical glass having a refractive index (nd) of 1.54 to 1.60, an Abbe number (νd) of 68 to 75 and a relative partial dispersion of at least 0.537. It is another object of the present invention to provide a stable fluorophosphate optical glass having an excellent chemical durability, which can be produced on a commercial scale. These objects can be attained by a chemical composition (% by weight) comprising 15 to 32% of Al(PO₃)₃, 0 to 10% of Ba(PO₃)₂, 0 to 10% of Sr(PO₃)₂, 0 to 10% of Ca(PO₃)₂ and 0 to 10% of Mg(PO₃)₂, the sum of the metaphosphates being 20 to 32%, 20 to 70% of BaF₂, 5 to 40% of SrF₂, 0 to 15% of CaF₂, 0 to 10% of MgF₂, 0 to 5% of AlF₃ and 0 to 5% of GdF₃, the sum of the fluorides being 55 to 75%, and 5 to 22% of Gd₂O₃, 0 to 7% of La₂O₃, 0 to 10% of Y₂O₃ and 0 to 10% of Yb₂O₃, the sum of the rare earth element oxides being 5 to 22%. DETAILED DESCRIPTION OF THE INVENTION The inventors have made various efforts to overcome the disadvantages of the prior art fluorophosphate optical glasses, as described above, and consequently, have found that an optical glass having a glass composition comprising Al(PO₃)₃, BaF₂, SrF₂ and Gd₂O₃ as essential components has optical constants at which the inventors have aimed. The present invention is based on this finding. Accordingly, the present invention provides a novel fluorophosphate optical glass having a refractive index of 1.54 to 1.60, an Abbe number of 68 to 75 and a relative partial dispersion of at least 0.537, which has a chemical composition (% by weight) comprising: The grounds for limiting the proportion ranges of the components to the foregoing are as follows: Al(PO₃)₃ is a main component essential for forming the network structure of glass, contributing to stability and increasing the chemical durability and is in a proportion of 15 to 32%, since if less than 15%, not only the chemical durability is diminished but also the glass becomes unstable, while if more than 32%,the crystallization tendency is too large to obtain the glass. Ba(PO₃)₂, Sr(PO₃)₂, Ca(PO₃)₂ and Mg(PO₃)₂ are optional components for raising the refractive index, increasing the chemical durability and improving the workability and are respectively in a proportion of 0 to 10%, since if more than 10%, the abnormal partial dispersion is lowered. This should be avoided. If the sum of the amounts of these metaphosphates is less than 20% or more than 32%, the glass tends to be too crystallized and a satisfactory glass is not produced. BaF₂ is an essential component for lowering the dispersion and raising the abnormal partial dispersion and is present in a proportion of 20 to 70%, since if less than 20%, the glass tends to be crystallized, while if more than 70%, the chemical durability is diminished and the glass is too unstable. SrF₂ is an essential component functioning similary to BaF₂ and serving to stabilize the glass jointly with BaF₂ and is in a proportion of 5 to 40%, since if less than 5%, the stabilizing effect on the glass is inadequate, while if more than 40%, the chemical durability is diminished and the crystallization tendency is too great to maintain the glass. CaF₂ is an optional component, although having the similar effect to BaF₂ and SrF₂, and is in a proportion of 0 to 15%, since if more than 15%, the glass tends to be crystallized and becomes unstable. MgF₂ is an optional component, although having the similar effect to BaF₂ and SrF₂, and is in a proportion of 0 to 10%, since if more than 10%, the glass tends to be crystallized and becomes unstable. AlF₃ is an optional component having an effect of lowering the dispersion and is in a proportion of 0 to 5%, since if more than 5%, the glass tends to be to crystallized to maintain it. GdF₃ is an optional component having an effect of raising the refractive index without raising the dispersion and an effect of increasing the hardness of the glass and is in a proportion of 0 to 5%, since if more than 5%, the glass tends to be crystallized too maintain it. If the sum of the amounts of these fluorides is less than 55%, the glass becomes unstable, while if more than 75%, not only the chemical durability is diminished, and also the glass becomes unstable. Gd₂O₃ is an essential component capable of raising the refractive index without raising the dispersion as well as imparting a high refractive index and abnormal partial dispersion without substantial introduction of barium oxide component, and further capable of raising the hardness of the glass and serving to improve the mechanical strength and workability, and is in a proportion of 5 to 22%, since if less than 5%, the intended refractive index cannot be obtained and the mechanical strength and workability are deteriorated, while if more than 22%, dissolving of it is difficult and the insoluble material is precipitated, resulting in the crystallization tendency. La₂O₃, Y₂O₃ and Yb₂O₃ are optional components, although having the similar effect to Gd₂O₃, and are respectively in a proportion of 0 to 7%, 0 to 10% and 0 to 10%, since if the amounts of La₂O₃, Y₂O₃ and Yb₂O₃ are respectively more than 7%, 10% and 10%, the glass tends to be too crystallized to maintain it. If the sum of the amounts of these rare earth oxides is less than 5%, the intended refractive index cannot be obtained, while if more than 22%, dissolving of them is difficult and insoluble ones are precipitated, so that the glass tends to be too crystallized to maintain it. Production of the fluorophosphate optical glass of the present invention can generally be carried out by mixing raw materials in a predetermined proportion to give the above described composition, melting the mixture at a temperature of 1000°C to 1250°C, and then subjecting the melt to casting in conventional manner. The fluorophosphate optical glass of the present invention has optical constants, i.e. a refractive index (nd) of 1.54 to 1.60, an Abbe number (νd) of 68 to 75 and abnormal partial dispersion represented by a relative partial dispersion of at least 0.537, is excellent in chemical durability as well as stability and can readily be produced on a commercial scale, as described above. The following examples are given in order to illustrate the present invention in greater detail without limiting the same. Example 1 to Example 20 Examples of the glass composition of the present invention are shown in Table 1 and the refractive indexes (nd), Abbe numbers (νd) and relative partial dispersion (ng-nF/nF-nC) are shown in Table 2. The fluorophosphate optical glass of the present invention was prepared by weighing the corresponding metaphosphates, oxides and fluorides, as raw materials, in predetermined proportions, mixing adequately these materials to prepare a starting material for the glass preparation, charging the starting material in a platinum crucible, melting in an electric furnace at a temperature of 1000 to 1250°C, stirring by a platinum stirrer to render the material clear and homogeneous, casting in a suitably preheated metal mold and then cooling gradually. In Example 1, for example, the raw materials corresponding to the composition 2 of Example 1 were weighed and mixed adequately according the above described procedure and then melted in an electric furnace previously heated at 1100°C for 3 hours, thus obtaining a fluorophosphate optical glass with a refractive index (nd) of 1.58222, an Abbe number (νd) of 69.2 and a relative partial dispersion (ng-nF/nF-nC) of 0.545. In these examples, the metaphosphates were used, but the present invention should not be construed to be limited thereto and of course, other raw materials capable of in situ formation of the metaphosphates can be used, for example, oxides and phosphoric acid in combination, or pyrophosphates to give the specified quantity according to the present invention. The above described fluorides can partly be replaced by several percents of LiF, NaF and/or KF as far as the object of the present invention is not lost. The relative partial dispersion (ng-nF/nF-nC) in Table 2 means the degree of abnormal partial dispersion that a glass has positioned apart from the straight line connecting two points of relative partial dispersion when taking the relative partial dispersion on the ordinate and the Abbe number (νd) on the abscissa choosing normal glasses 511605-K7 and 620363-F2 as a standard. Since in the scope of the present invention as claimed, the standard relative partial dispersion of the normal glasses are 0.532 and 0.521 respectively at an Abbe number (νd) of 68 and 75, a glass has abnormal partial dispersion if the relative partial dispersion of this glass is apart from the straight line connecting two points of the standard relative partial dispersion. As apparent from the results of Examples, for example, the glass of Example 1 has abnormal partial dispersion in view of its large relative partial dispersion (ng-nF/nF-nC), i.e. 0.545. In comparison with the prior art optical glasses having an Abbe number (νd) of 66 to 70 shown in Table 3, the glasses of Examples 1 to 20 according to the present invention show higher refractive indexes (nd) and larger relative partial dispersion (ng-nF/nF-nC).

Claims: A fluorophosphate optical glass having a refractive index (nd) of 1.54 to 1.60, an Abbe number (nd) of 68 to 75 and a relative partial dispersion of at least 0.537, which has a chemical composition (% by weight) comprising: A glass as claimed in claim 1 wherein the content of Al(PO₃)₃ is 20 to 27%. A glass as claimed in claim 1 or claim 2 wherein the content of BaF₂ is 30 to 45%. A glass as claimed in any one of the preceding claims wherein the content of SrF₂ is 15 to 30%. A glass as claimed in any one of the preceding claims wherein the content of Gd₂O₃ is 10 to 20%. A glass as claimed in any one of the preceding claims moulded into an optical part such as a lens.

Inventors: KOMIYA, MASAYUKI, NAGAHAMA, SHINOBU, OTSUKA, MASAAKI, SAWANOBORI, NARUHITO
Applicants: SUMITA OPTICAL GLASS, INC.
Application year: 1989
Main classification: C03C3
IPC codes: C03C4, C03C3