Optical fluorophosphate glass with anomalous positive incremental dispersion and process for making it

Abstract

An optical fluorophosphate glass with an anomalous positive incremental dispersion +.DELTA..nu..sub.e between 11.8 and 12.5, a refractive index n.sub.e between 1.53 and 1.55 and an Abbe coefficient .nu..sub.e between 72.8 and 73.5 has the following atomic composition (in atom %): Mg: 2.0-3.5, Ca: 2.5-4.5, Sr: 13.0-16.0, Ba: 22.0-25.0, Al: 4.5-6.0, K: 0.1-0.3, P: 9.0-12.0, O: 17.0-20.0 and F: 20.0-23.0, the F/P ratio lying between 1.8 and 2.4 and the sum of the alkaline earths between 39.5 and 49. The new glasses are melted from a material having the following initial composition (in wt. %): Ca(PO.sub.3).sub.2 : 1.0-2.5, Ba(PO.sub.3).sub.2 : 23.0-25.5, Al(PO.sub.3).sub.3 : 12.0-14.5, MgF.sub.2 : 6.5-8.5, CaF.sub.2 : 5.5-8.0, SrF.sub.2 : 16.0-19.0, AlF.sub.3 : 10.0-12.5, SrO: 2.0-3.0, BaO: 12.5-14.5 and K.sub.2 TiF.sub.6 : 0.2-0.7, the alkaline earth fluorides content lying between 28.0 and 35.5 and the total content of all fluorides used between 38.2 and 48.7.

Description

The application relates to an optical fluorophosphate glass having anomalous positive partial dispersion, in which the tendency toward crystallization is extremely low.

An optical glass based on metaphosphates and fluorides of alkaline-earth metals and aluminum with the following composition is already known:

______________________________________Ca(PO.sub.3).sub.2 : 2.53% by weightBa(PO.sub.3).sub.2 : 18.01% by weightAl(PO.sub.3).sub.3 : 22.94% by weightMgF.sub.2 : 9.70% by weightCaF.sub.2 : 7.40% by weightSrF.sub.2 : 21.80% by weightAlF.sub.3 : 6.69% by weightBaO: 9.93% by weightKHF.sub.2 : 1.00% by weight______________________________________ The elemental composition of this known glass is as follows (in weight-%): Mg: 3.78; Ca: 4.31; Sr: 15.20; Ba: 17.29; Al: 4.49; K: 0.50; P: 12.66; 0: 20.62 and F: 21.15.

This known glass has a refractive index n.sub.e of 1.544 and an Abbe number .nu..sub.e of 73.1. However, melting is only possible with great technological difficulty. The yield is extremely low, due, inter alia, to the high tendency of this glass towards crystallization.

It is therefore an object of the present invention to provide a glass with analogous optical properties, i.e. with an identical refractive index n.sub.e and an identical Abbe number .nu..sub.e, in which the above mentioned technological difficulties in melting are avoided. It is a further object to indicate a process for crystallization-free melting of the glass in large units.

This object is achieved according to the invention by providing a fluorophosphate optical glass produced by melting a mixture having the following composition (in % by weight):

______________________________________ Ca(PO.sub.3).sub.2 1.0-2.5 Ba(PO.sub.3).sub.2 23.0-25.5 Al(PO.sub.3).sub.3 12.0-14.5 MgF.sub.2 6.5-8.5 CaF.sub.2 5.5-8.0 SrF.sub.2 16.0-19.0 AlF.sub.3 10.0-12.5 SrO 2.0-3.0 BaO 12.5-14.5 K.sub.2 TiF.sub.6 0.2-0.7______________________________________ wherein the alkaline earth metal fluoride content is between 28 and 35.5% by weight; the total content of all fluorides is between 38.2 and 48.7% by weight, and the F/P ratio is between 1.8 and 2.4; said glass having a refractive index n.sub.e between 1.53 and 1.55, an Abbe number .nu..sub.e between 72.8 and 73.5, and an anomalous positive partial dispersion .DELTA..nu..sub.e between +11.8 and +12.5.

The glasses according to the invention have the following elemental composition: (in wt. %)

TABLE 1______________________________________(in atom-%):Range of the Preferred Preferred singleinvention range example______________________________________Mg 2.0-3.5 2.0-3.0 2.93Ca 2.4-4.5 3.0-4.0 3.88Sr 13.0-16.0 14.0-15.0 14.51Ba 22.0-25.0 23.0-24.0 23.55Al 4.5-6.0 5.0-6.0 5.05K 0.1-0.3 0.1-0.2 0.16P 9.0-12.0 10.0-11.0 10.51O 17.0-20.0 18.0-19.0 18.14F 20.0-23.0 21.0-22.0 21.27n.sub.e 1.53-1.55 1.54-1.55 1.544.nu..sub.e 72.8-73.5 73.0-73.2 73.10.DELTA..nu..sub.e 11.8-12.5 12.1-12.2 12.17______________________________________ In Table 2 below, the ranges of the invention are indicated for the initial compositions: (in wt. %) TABLE 2______________________________________(in % by weight): Preferred Preferred Range of the interval of individual invention the invention example______________________________________Ca(PO.sub.3).sub.2 1.0-2.5 1.6-1.8 1.7Ba(PO.sub.3).sub.2 23.0-25.5 24.0-25.0 24.2Al(PO.sub.3).sub.3 12.0-14.5 13.0-14.0 13.9MgF.sub.2 6.5-8.5 7.0-8.0 7.5CaF.sub.2 5.5-8.0 6.0-7.0 6.9SrF.sub.2 16.0-19.0 17.0-18.0 17.9AlF.sub.3 10.0-12.5 11.0-12.0 11.3SrO 2.0-3.0 2.3-2.5 2.4BaO 12.5-14.5 13.0-14.0 13.7K.sub.2 TiF.sub.6 0.2-0.7 0.4-0.6 0.5______________________________________ The initial quantity is molten in a manner known per se in a platinum crucible. The glass molten from the initial quantity is colorless, ream-free, stable to crystallization and can be produced in significantly higher yield than the known glass. After slow cooling, the individual example shown has the following optical data: n.sub.e =1.544 .nu..sub.e =73.10 .theta.'.sub.g =0.4804 +.DELTA..nu..sub.e =12.17

The symbols here have the following meanings:

n.sub.e =refractive index .nu..sub.e =Abbe number (reciprocal of the dispersion) .theta.'.sub.g =(actual) anomalous partial dispersion, where the following applies: ##EQU1## In this equation, the subscripts have the following meanings: g=blue mercury line (435.84 nm); F'=blue cadmium line (479.99 nm); C'=red cadmium line (643.85 nm); +.DELTA..nu..sub.e =positive deviation from the so-called "standard straight line", as shown in diagrammatic form and described in greater detail in, for example, the drawing of German Patent 1,496,563 and corresponding U.S. Pat. No. 3,451,829. This difference is known as the "positive anomalous partial dispersion value". The glass obtained has the transmission characteristics shown in Table 3 below: TABLE 3______________________________________.lambda.[nm] .tau.;[5nm] .tau.;[25nm]______________________________________ 1014.0 0.998 0.990700 0.999 0.995660 0.999 0.995620 0.999 0.995580 0.998 0.993 546.1 0.999 0.997500 0.997 0.988460 0.996 0.983 435.8 0.994 0.973420 0.993 0.968 404.7 0.992 0.963400 0.991 0.958390 0.989 0.948380 0.985 0.927370 0.976 0.884365 0.963 0.829350 0.883 0.537 334.1 0.612 0.086320 0.158 --310 0.045 --______________________________________ In this table, the symbols have the following meanings: .lambda.: the measurement wavelength used [nm]; .tau..sub.i (5 mm): the net transmittance of a glass plate of thickness 5 mm; .tau..sub.i (25 mm): the net transmittance of a glass plate of thickness 25 mm. Further physico-chemical properties of the glass obtained are shown below: linear thermal expansion coefficient: .alpha..sub.20/300.degree. C. =13.43.multidot.10.sup.-6 /K; Transformation temperature T.sub.g =502.degree. C.; Density .rho.=3.64 g/cm.sup.3 ; Knoop hardness HK=408. The glass according to the invention can be press-molded and is low-fluorescence and can be used for highly corrected objectives.

Claims

1. Optical fluorophosphate glass produced by melting a mixture having the following composition (in % by weight):

______________________________________ Ca(PO.sub.3).sub.2 1.0-2.5 Ba(PO.sub.3).sub.2 23.0-25.5 Al(PO.sub.3).sub.3 12.0-14.5 MgF.sub.2 6.5-8.5 CaF.sub.2 5.5-8.0 SrF.sub.2 16.0-19.0 AlF.sub.3 10.0-12.5 SrO 2.0-3.0 BaO 12.5-14.5 K.sub.2 TiF.sub.6 0.2-0.7______________________________________

wherein the alkaline earth metal fluoride content is between 28 and 35.5% by weight; the total content of all fluorides is between 38.2 and 48.7% by weight, and the F/P ratio is between 1.8 and 2.4; said glass having a refractive index n.sub.e between 1.53 and 1.55, an Abbe number .nu..sub.e between 72.8 and 73.5, and an anomalous positive partial dispersion .DELTA..nu..sub.e between +11.8 and +12.5.

2. Glass according to claim 1, wherein said glass is produced by melting a mixture having the following composition (in % by weight):

______________________________________ Ca(PO.sub.3).sub.2 1.6-1.8 Ba(PO.sub.3).sub.2 24.0-25.0 Al(PO.sub.3).sub.3 13.0-14.0 MgF.sub.2 7.0-8.0 CaF.sub.2 6.0-7.0 SrF.sub.2 17.0-18.0 AlF.sub.3 11.0-12.0 SrO 2.3-2.5 BaO 13.0-14.0 K.sub.2 TiF.sub.6 0.4-0.6______________________________________

wherein the alkaline earth metal fluoride content is between 30 and 33% by weight; the total content of all fluorides is between 41.4 and 45.6% by weight, and the F/P ratio is between 1.9 and 2.2; said glass having a refractive index n.sub.e between 1.54 and 1.55, an Abbe number .nu..sub.e between 73.0 and 73.2, and an anomalous positive partial dispersion .DELTA..nu..sub.e between +12.1 and +12.2.

3. Glass according to claim 2, wherein said glass is produced by melting a mixture having the following composition (in % by weight):

______________________________________ Ca(PO.sub.3).sub.2 1.7 Ba(PO.sub.3).sub.2 24.2 Al(PO.sub.3).sub.3 13.9 MgF.sub.2 7.5 CaF.sub.2 6.9 SrF.sub.2 17.9 AlF.sub.3 11.3 SrO 2.4 BaO 13.7 K.sub.2 TiF.sub.6 0.5______________________________________

wherein the alkaline earth metal fluoride content is 32.3% by weight; the total content of all fluorides is 44.1% by weight; and the F/P ratio is 2.0; said glass having a refractive index n.sub.e of 1.544, an Abbe number .nu..sub.e of 73.1, and an anomalous positive partial dispersion .DELTA..nu..sub.e of +12.17.

4. Glass according to claim 3, wherein said glass has the following elemental composition (in % by weight):

______________________________________ Mg 2.93 Ca 3.88 Sr 14.51 Ba 23.55 Al 5.05 K 0.16 P 10.51 O 18.14 F 21.27.______________________________________

5. Glass according to claim 4 having a linear thermal expansion coefficient .alpha..sub.20/300.degree. C. of 13.43.multidot.10.sup.-6 /K, a transformation temperature T.sub.g of 502.degree. C., a density of 3.64 g/cm.sup.3, and a Knoop Hardness HK of 408.