DE4222324C2 - Optical glass with negative anomalous partial dispersion in the blue region of the spectrum, refractive indices n¶d¶ between 1.59 and 1.66 and payoff between 40 and 50 - Google Patents

Description

The invention relates to a chemically resistant, optical glass with high crystallization stability based on a lead borate glass with negative anomalous partial dispersion in the blue region of the spectrum, a refractive index n _d between 1.59 and 1.66 and a Abbe number v _d between 40 and 50.

The refractive index of an optical glass depends on the wavelength of the refracted light, whereby the refractive index decreases with increasing wavelength. The degree of dependence is different for each type of glass. For characterizing the dispersion of an optical glass, that is, the dependence of the refractive index on the wavelength, in general, the refractive index n _d at the wavelength 587.56 nm (sodium d-line) and Abbezahl

where n _{F is} the refractive index at the wavelength 486.13 nm (hydrogen line) and n _C at the wavelength 656.27 nm (helium line).

The partial dispersion P _{x, y of} a glass for light of two different wavelengths x and y can be understood by the general relationship

represent. Here, n _x and n _y denote the refractive indices of the light at the wavelengths x and y, respectively. In the conventional flint and crown glasses, the relationship between the partial dispersibility P _{x, y} and the Abbe number of the respective glass shows a linear relationship of the general equation

P _{x, y} = A _{x, y} + B _{x, y} · v _d

where A _{x, y} and B _{x, y} are different constants depending on the choice of the wavelengths x and y. Glasses that obey this equation are called "standard glasses" and form the majority among the flint and crown glasses. The equation underlying the equation is called "normal straight".

From this line deviating glasses have a anomalous partial dispersion. As a measure of deviation becomes the ordinate difference .DELTA.P is given, that is the vertical distance of the value for the partial Dispersibility of a glass with anomalous Partial dispersion from the normal straight line.

The dependence of the refractive index of the Light wavelength requires that an axial Object point when calculating a series of images has. This phenomenon is called as Longitudinal chromatic aberration. This chromatic mistake is regardless of the lens shape and depends only on the Focal length, the lens position in the beam path and the dispersion of the glass used in each case. By combining several lenses into one system can about the different focal lengths and Pay off the lenses used Color longitudinal errors are corrected.

When combining two lenses different Composition can be a chromatic error eliminate for at least two discrete wavelengths  (Achromasie). For other than this discreet However, wavelengths remain residual color error, the so-called "secondary spectrum", left over. It is however, it is known that by additional use of Glasses whose partial dispersion is from the normal straight line deviates, that is from glasses, which is anomalous Partial dispersion, the "secondary spectrum" can largely be corrected. It will be Preferably such glasses are used, the one especially high deviation from the normal line demonstrate.

In the prior art are glasses with both high positive as well as negative partial dispersion known. Positive partial dispersion is present when the Value for the deviation from the normal straight line ΔP gives positive values, a negative partial dispersion if the value for ΔP <0. For the correction of Secondary spectra in lens systems often become Glasses with negative partial dispersion needed which additionally have relatively high refractive indices have to.

In DE-OS 22 62 364 glasses are anomalous Partial dispersion in the longwave region of the spectrum described. The glasses show at Abbezahlen in Range of 30-55 refractive indices of 1.54-1.71 and are lead borate glasses in their chemical nature with an essential addition to a minimum salary of 1 mol% of the components SiO₂ + Na₂O. These Components serve to the anomaly of the partial Dispersion in the range of long wavelengths too achieve. This is possible because the density of the Glass is reduced by the addition. moreover are the glasses La₂O₃-free.  

In Japanese Scripture 7 634 409 and DE-OS 21 39 855 lead borate glasses are described, in which to achieve the anomalous partial dispersion in long-wave, in particular infrared range Minimum content of 1.3% by weight of the components La₂ O ₃ + CaO was used, wherein CaO in the essentially increases the anomaly while La₂O₃ the increase in acid resistance is used. Here but no SiO₂ is present, are the chemical Resistance set relatively tight limits.

From DE-OS 19 44 284 and DE-OS 15 96 807 are glasses on lead borate glass known with Anomalous partial dispersion, which due to high SiO₂ additives and for lead borate glasses relative low B₂O₃ content extremely high chemical Have durability. But such glasses have relatively high melting temperatures, the Glasses during melting the melting vessels what ultimately leads to a deterioration of the Transmission and thus the quality of the optical Glasses leads.

However, there is still a lack of optical Glasses with negative anomalous partial dispersion in the blue part of the spectrum, which is at relative high refractive index by a dispersion behavior with Pay off over 40, and those for the Use to correct the secondary spectrum in Lens systems have sufficient resistance both against chemical attack as well as against devitrification feature.

The invention is therefore based on the object to provide a chemically resistant, optical glass high crystallization stability based on a lead borate system with negative anomalous partial dispersion in the blue region of the spectrum available, which has a refractive index n _d in the range between 1.59 and 1.66 and has a Abbezahl in the range between 40-50.

The problem is solved by a glass with a Component composition according to the characterizing one Part of claim 1. A preferred Embodiment represents a glass with the Component composition according to characterizing Part of claim 2.

PbO and B₂O₃ form the backbone of Glassystems, with essential parts of Al₂O₃, SiO₂ and La₂O₃, Al₂O₃ serves the Increase in chemical resistance. alumina was chosen as a component, as it is the one increased chemical resistance in a clear way, on the other hand no harmful effect on a negative deviation of the partial dispersion of the Normal straight shows. The content of the invention Aluminum oxide should not be exceeded otherwise the devitrification stability of the Glasses and the melting temperature of the glasses clearly increases. However, the proportion of aluminum oxide falls below, so the chemical resistance decreases clear from the glasses.

The component SiO₂ also serves as Al₂O₃ Achieving a chemical resistance of the Glasses. SiO can, within certain limits against the replaced the batch price significantly increasing GeO₂ be without disadvantages. The upper limit the SiO content should not be exceeded otherwise the melting temperatures will go so far increase, that a strong attack of the melt on the melting vessels takes place, whereby the transmission  is significantly reduced what the optical Quality of the glasses impaired.

Another essential part of glasses of the invention forms the addition of La₂O₃. The component increases the chemical Stability also clear, being in the Contrary to the Al₂O₃ and SiO₂ but one has an increased effect on the refractive power. The Upper limit of the content of La₂O₃ should but are not exceeded because the viscosity of the Glasses with increasing lanthanum oxide content sinks and negative influences on the producibility of the Glasses are to be expected. La₂O₃ can without further exchanged by up to 2 wt .-% Y₂O₃ become.

To improve the meltability, the Glasses of alkali metal oxides are added, but these lower both the refractive power and the negative Divergence of the partisan, in Amounts up to a maximum of 1% by weight.

For setting the optical constants, refractive index and Abbezahl, the glasses in addition shares be added by alkaline earth oxide and ZnO. There at higher concentrations of these additives the devitrification tendency is observed, the Total content of the eralkalioxides and ZnO 10% by weight do not exceed. In addition, there is the Danger that the desired anomalous partial dispersion get lost. In additions of these components within the limits of the invention, however these additives have the effect of reducing the devitrification.

To increase the refractive power can also oxides such as Ta₂O₅, TiO₂, ZrO₂, HfO₂, etc. added  become. Within the limits of the invention they also have a positive effect on the chemical Stability of the glasses. When crossing the limits set according to the invention for this Oxides they act again in the sense of an increase in Devitrification. Will Bi₂O₃ in shares up added to 9% by weight, then none Impairments for the negative partial dispersion to observe.

In the following table 5 are exemplary Compositions for glasses according to the invention the associated parameters refractive index, Abbezahl, Partial dispersion in the blue area and anomaly of the Partial dispersion reproduced.

The partial dispersibility in the blue part of the Spectrum is through the expression

where n _{g is} the refractive index at the wavelength 435.83 nm. The normal straight line, which passes through the glasses K7 and F2 (Schott catalog "optical glasses", Schott Glaswerke Mainz, 1980), obeys the blue spectrum of the equation

P _{g, F} = 0.6438 - 0.001682 · v _d .

The glasses according to the invention are as follows manufactured:

The raw materials (oxides, carbonates, etc.) will be weighed and mixed well. The mixture is added  1300 ° C melted, then purified and good homogenized. The casting takes place at 1200 ° C in preheated molds.

table

Claims (2)

1. Chemically stable optical glass high crystallization stability based on lead borate glass with negative anomalous partial dispersion in the blue region of the spectrum, a refractive index n _d between 1.59 and 1.66 and Abbezahlen v _d between 40 and 50, characterized in that it following weight percentages Composition comprising: SiO₂ 0.5-10.5 GeO₂ 0-2 ΣSiO₂ + GeO₂ 0.5-12 B₂O₃ 41-53 Al₂O₃ 3-18 Li₂O 0-1 Na₂O 0-1 K₂O 0-1 ΣAlkalioxide 0-1 MgO 0-6 CaO 0-6 SrO 0-6 BaO 0-6 ZnO 0-4 ΣMgO + CaO + SrO + BaO + ZnO <10 La₂O₃ 2-9 Y₂O₃ 0-2 ZrO₂ 0-5 HfO₂ 0-3 TiO₂ 0-4 Nb₂O₅ 0-5 Ta₂O₅ 0-2 WO₃ 0-2 Bi₂O₃ 0-9 PbO 23-38 as well as according to the prior art known auxiliaries for purification in amounts up to 1 wt .-%. 2. Optical glass according to claim 1, characterized characterized in that it is Al₂O₃ in the range of Contains 6-18 wt .-%.