Glass of abnormal dispersion
Glasses with abnormal dispersion or abnormal partial dispersion are optical glasses with unusual dispersion . This means that the course of the refractive index over the wavelength of the light differs significantly from that of most other optical glasses. A distinction is made among other things with long crown glasses , which have a relatively large partial dispersion in the short-wave range compared to other glasses with roughly the same refractive index and Abbe number , and short flint glasses , the dispersion of which is relatively low at short wavelengths. Such glasses are used in lenses for apochromats in order to reduce the chromatic aberration of these objectives (more precisely: to reduce or eliminate the secondary spectrum).
"Special low dispersion glass" ( SLD glass ), "extraordinary low dispersion glass" ( ELD glass ) are glasses with particularly low dispersion. Other glasses in this category are "extra-low dispersion glass" ( ED glass ) and "ultra-low dispersion glass" ( UL glass ).
Prior to the availability of these glasses, lenses were made from crystalline calcium fluoride . However, the low power of calcium fluoride required strongly curved lenses, which therefore had a strong spherical aberration . Fluoride was also not dimensionally stable and fragile.
An alternative to calcium fluoride are fluorophosphate glasses, in which part of the fluoride is stabilized by metaphosphates with the addition of titanium dioxide .
The high production costs of these glasses result from the necessary purity of the substances used for production and the difficulties in production already mentioned.
Individual evidence
- ↑ Archived copy ( memento of the original from March 12, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
