A fluorophosphate optical glass

Abstract: A fluorophosphate optical glass with a low refrac­ tive index and low dispersion is provided in stable manner, which has a refractive index nd of 1.42 to 1.47 and an Abbe number νd of 90 to 97 and comprises 3 to 8 mole % of P₂O₅, 0.1 to 2 mole % of Al₂O₃, 1 to 7 mole % of BaO, 35.5 to 41 mole % of AlF₃, 8 to 13 mole % of MgF₂, 16 to 26 mole % of CaF₂, 15 to 21 mole % of SrF₂, 3.5 to 10 mole % of BaF₂ and 1 to 6 mole % of NaF. The invention also includes a process of making fluorophosphate optical glass and optical parts formed from the novel glass, and camera lenses and the like comprising such optical parts.

Description: This invention relates to a fluorophosphate optical glass having optical constants, i.e. a refractive index (nd) of 1.42 to 1.47 and Abbe number (νd) of 90 to 97. The low refractive index and low dispersion optical glass of the present invention has similar optical properties to those of crystalline calcium fluoride and is thus useful as a glass material serving for calcium fluoride as a lens or optical part in an optical instrument. Crystalline calcium fluoride, which has hitherto been used as a low refractive index and low dispersion optical material for a camera lens, meets with various problems because of being crystalline, for example, that the chemical resistance and workability are not good and the productivity is low to disadvantage mass production of it. In order to solve these problems, an optical glass having similar optical properties to crystalline calcium fluoride has thus been desired and some glasses have been proposed which consist of low refractive index and low dispersion fluorophosphates. For the purpose of realizing such optical properties, in particular, a low dispersion, fluoride ion must be introduced as an anion serving for oxide ion in a glass and when the Abbe number is above 90, the proportion of the fluoride ions is considerably increased as compared with the oxide ions. Such a glass tends to be crystallized more than ordinary optical glasses and is so unstable that the production thereof on a commercial scale, as having uniform properties, is very difficult. In view of the situation, several fluorophosphate glasses have thus been reported as disclosed in Japanese Patent Publication Nos. 34768/1979 and 14378/1983 and Japanese Patent Application OPI Nos. 43112/1975, 81042/1985 and 210545/1985. However, these glasses have such problems that large amounts of volatile components contained therein evaporate during melting, thus resulting in a fluctuation of the refractive index, and the production of those containing relatively expensive and special components, on a commercial scale, meets with an economical problem. Furthermore, the above proposed glasses are not satisfactory in the effect for crystallization. Therefore, a great deal of efforts are required so as to obtain a sufficiently stable and uniform glass. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical glass with a low refractive index and low dispersion. It is another object of the present invention to provide a very stable and uniform fluorophosphate optical glass. These objects can be attained by a fluorophosphate optical glass having optical constants, a refractive index (nd) of 1.42 to 1.47 and an Abbe number (νd) of 90 to 97 and comprising 3.0 to 8.0 % (by mole) of P₂O₅. 0.1 to 2.0 % of Al₂O₃, 1.0 to 7.0 % of BaO, 35.5 to 41.0 % of AlF₃, 8.0 to 13.0 % of MgF₂, 16.0 to 26.0 % of CaF₂, 15.0 to 21.0 % of SrF₂, 3.5 to 10.0 % of BaF₂ and 1.0 to 6.0 % of NaF. DETAILED DESCRIPTION OF THE INVENTION The inventors have made various efforts to overcome the disadvantages of the prior art, as described above, and consequently, have found a very stable fluorophosphate glass having optical constants, i.e. a refractive index nd of 1.42 to 1.47 and an Abbe number νd of 90 to 97 in P₂O₅-Al₂O₃-BaO-AlF₃-RF₂-NaF (R = Mg + Ca + Sr + Ba) systems. The present invention is based on this finding. Accordingly, the optical glass of the present invention has a chemical composition (% by mole) comprising 3 to 8 % P₂O₅, 0.1 to 2 % Al₂O₃, 1 to 7 % BaO, 35.5 to 41 % AlF₃, 8 to 13 % MgF₂, 16 to 26 % CaF₂, 15 to 21 % SrF₂, 3.5 to 10 % BaF₂ and 1 to 6 % NaF, and can readily be prepared in conventional manner by mixing and melting the corresponding raw materials in such a proportion as to give the above described contents. The reasons for limiting thereto the proportions of the components are as follows: P₂O₅ is a component essential for forming the glass and is in a proportion of 3.0 to 8.0 % (by mole), preferably 3.5 to 6.5 %, since if more than 8.0 %, the optical constants of the resulting glass are outside the above described and aimed range, while if less than 3.0 %, the glass is hard to be formed. Al₂O₃ acts as a component for increasing the viscosity of the glass melt and is in a proportion of 0.1 to 2.0 %, preferably 0.2 to 1.5 %, since if the proportion if outside the above described range, its effect is similar to that of P₂O₅. BaO is an essential component for improving the melting property of the glass and is in a proportion of 1.0 to 7.0 %, preferably 1.6 to 5.0 %, since if the proportion is outside the above described range, the intended optical constants cannot be obtained. AlF₃ is an essential component for forming the glass and acting as a source of fluoride ion and is in a proportion of 35.5 to 41.0 %, preferably 35.5 to 39.0 %, since if the proportion is outside this range, the crystallization tendency of the glass is increased. MgF₂ is in a proportion of 8.0 to 13.0 %, preferably 9.0 to 12.0 %, since if more than 13 %, the crystallization tendency is increased, while if less than 8 %, the melting property is deteriorated. CaF₂ is in a proportion of 16.0 to 26.0 %, preferably 17.0 to 25.0 %, since if more than 26 %, the crystallization tendency is increased, while if less than 16 %, the intended optical constants cannot be satisfied. SrF₂ is in a proportion of 15.0 to 21.0 %, preferably 15.0 to 20.0 %, since if more than 21 %, the crystallization tendency is increased, while if less than 15 %, the melting property is deteriorated. BaF₂ is in a proportion of 3.5 to 10.0 %, preferably 5.0 to 9.0 %, since if more than 10 %, the intended optical constants can hardly be obtained, while if less than 3.5 %, glass formation is difficult. NaF is an essential component for lowering the melting temperature of the glass melt and effectively preventing evaporation and is in a proportion of 1.0 to 6.0 %, preferably 1.0 to 4.0 %, since if more than 6 %, the crystallization tendency is increased, while if less than 1 %, its effects are insufficient. All of the alkali metal and alkaline earth metal fluorides are used for the purpose of improving the melting property of the glass. A glass having a composition in the above described range according to the present invention has similar optical constants to those of crystalline calcium fluoride and can be obtained in relatively stable manner, leading to the production on a commercial scale. Production of this glass 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 to 1100 °C and then subjecting the melt to casting in conventional manner. The fluorophosphate optical glass of the present invention can more readily be obtained as the so-called glass and can be produced on a commercial scale. Therefore, this glass is promising as a low refractive index and low dispersion optical material serving for crystalline calcium fluoride and useful as optical parts such as camera lens. The following examples are given in order to illustrate the present invention in greater detail. Example 1 Compounds shown in Table 1 were used as raw materials, mixed in a proportion by weight as Sample No. 1, melted at a temperature of 1000 to 1100 °C and formed by pouring the resulting melt in a mold of carbon. A glass was thus obtained in relatively stable manner. Examples 2 to 8 Example 1 was repeated except using compounds in proportions shown in Table 1, Sample Nos. 2 to 8, thus obtaining glasses. The compositions and optical constants, i.e. refractive indexes and Abbe numbers of the glasses prepared in Example 1, Sample No. 1 and Examples 2 to 8, Sample Nos. 2 to 8 are shown in Table 2: Comparative Example 1 Example 1 was repeated except using raw materials to give a composition of 9.9 % by mole of P₂O₅, 2.9 % Al₂O₃, 1.2 % BaO, 23 % AlF₃, 13.2 % MgF₂, 21.3 % CaF₂, 22 % SrF₂, 6 % BaF₂ and 0.5 % NaF. The resulting glass showed optical constants, nd = 1.46715 and νd = 88.1 It is to be noted from the examples that ingredients for the glass such as P₂O₅ and Al₂O₃ may be employed in more complex forms, such as Al(PO₃)₃. The invention also includes a process for producing fluorophosphate optical glass described herein. Further the invention includes lenses for an optical device comprising the new glass and also cameras and other optical devices comprising such lens.

Claims: A fluorophosphate optical glass having the optical constants, i.e. a refractive index (nd) of 1.42 to 1.47 and an Abbe number ( ν d) of 90.5 to 97, and comprising 3 to 8 mole % of P₂O₅, 0.1 to 2 mole % of Al₂O₃ 1 to 7 mole % of BaO, 35.5 to 41 mole % of AlF₃, 8 to 13 mole % of MgF₂, 16 to 26 mole % of CaF₂, 15 to 21 mole % of SrF₂, 3.5 to 10 mole % of BaF₂ and 1 to 6 mole % NaF. A fluorophosphate optical glass having the optical constants, i.e. a refractive index (nd) of 1.42 to 1.47 and an Abbe number ( ν d) of 90.5 to 97, and comprising 3.5 to 6.5 mole % of P₂O₅, 0.2 to 1.5 mole % of Al₂O₃, 1.6 to 5.0 mole % of BaO, 35.5 to 39.0 mole % of AlF₃, 9.0 to 12.0 mole % of MgF₂, 17.0 to 25.0 mole % of CaF₂, 15.0 to 20.0 mole % of SrF₂, 5.0 to 9.0 mole % of BaF₂ and 1.0 to 4.0 mole % of NaF. A process for producing the fluorophosphate optical glass as claimed in claim 1 or claim 2 wherein the specified raw materials were mixed in appropriate proportions by weight, thereafter melted at a temperature of 1,000 to 1,100 °C and the resulting glass formed by pouring the melt into a mould of carbon. An optical part formed from a fluorophosphate optical glass as claimed in claim 1 or claim 2 or when produced by the process claimed in claim 3. A lens for an optical device comprising one or more optical parts as claimed in claim 4. 6. A camera or other optical device comprising a lens as claimed in claim 5.

Inventors: BABA NOBUYOSHI, NAGAHAMA SHINOBU, SAWANOBORI NARUHITO, BABA, NOBUYOSHI, NAGAHAMA, SHINOBU, SAWANOBORI, NARUHITO
Applicants: SUMITA OPTICAL GLASS, SUMITA OPTICAL GLASS INC.
Application year: 1987
Main classification: C03C3
IPC codes: C03C3