Low dispersion glass

Glasses with anomalous dispersion or anomalous partial dispersion, optical glasses with unusual dispersion. That is, the profile of the refractive index versus wavelength of the light is significantly different from that of most other optical glasses from. One of the distinctions Langkrongläser that hold in the short wavelength region, a relatively large partial dispersion, compared with other glasses with approximately the same refractive index and Abbe number, as well as short flint glasses whose dispersion is relatively low at short wavelength. Such glasses are used in lenses for Apochromats to reduce the chromatic aberration of these lenses (or more precisely to reduce or eliminate secondary spectrum ).

With "Special Low Dispersion glass" (SLD glass), "extraordinary low dispersion glass" (ELD glass) are referred to with a particularly low dispersion glasses. Other lenses in this category are "extra - low dispersion glass" (ED glass), "ultra- low dispersion glass" ( glass UL ).

Prior to the availability of these glasses lenses were made ​​of crystalline calcium fluoride. However, the low power of calcium fluoride strongly required curved lenses, which therefore had a strong spherical aberration. Fluoride was also not stable enough and fragile. Glass with shares of thorium dioxide had the desired high power and low dispersion and was used before 1940, but the hazards arising from the thorium radioactivity led to the search for alternatives: The Kodak company succeeded in 1940 to develop a thorium-free glass, but which had a yellow color.

The company Leitz was discovered that lanthanum is if you use other additives to prevent crystallization as a replacement of thorium oxide. Another glass with outstanding features has a large content of zirconium dioxide, it must be manufactured in platinum vessels due to the high melting point.

An alternative to calcium fluoride are fluorophosphate glasses, in which a part of the fluoride is stabilized by the addition of titanium dioxide under metaphosphates.

The high manufacturing cost of these lenses results from the required purity of the substances used for the preparation and the aforementioned difficulties in manufacturing.