Low Dispersion Glass - Variants

Variants

Some glasses have a peculiar property called anomalous partial dispersion. Their use in long focal length lens assemblies was pioneered by Leitz. Before their availability, calcium fluoride in the form of fluorite crystals were used as material for these lenses; however the low refraction index of calcium fluoride required high curvatures of the lenses, therefore increasing spherical aberration. Fluorite also has poor shape retention and is very fragile. Abnormal dispersion is required for design of apochromat lenses.

Glass with addition of thorium dioxide has high refraction and low dispersion and was in use since before World War II (WW2), but its radioactivity led to its replacement with other compositions. Even during WW2, Kodak managed to make high-performance thorium-free optical glass for use in aerial photography, but it was yellow-tinted. In combination with black and white film, the tint was actually beneficial acting as a photographic filter improving contrast.

Leitz laboratories discovered that lanthanum(III) oxide can be a suitable thorium dioxide replacement. Other elements however had to be added to preserve the amorphous character of the glass and prevent crystallization that would cause striae defects.

After 1930, George W. Morey introduced the lanthanum oxide and oxides of other rare earth elements in borate glasses, greatly expanding the available range of high-index low-dispersion glasses. Borate glasses have lower wavelength-refraction dependence in the blue region of spectrum than silicate glasses with the same Abbe number. These so-called "borate flint" glasses, or KZFS, are however highly susceptible to corrosion by acids, alkalis, and weather factors. However borate glass with more than 20 mol.% of lanthanum oxide is very durable under ambient conditions. The use of rare earths allowed development of high-index low-dispersion glasses of both crown and flint types.

Another high-performance glass contains high content of zirconium dioxide; however its high melting point requires use of platinum lined crucibles to prevent contamination with crucible material.

A good high-refraction replacement for calcium fluoride as a lens material can be a fluorophosphate glass. Here, a proportion of fluorides is stabilized with a metaphosphate, with addition of titanium dioxide.

Several of the mentioned high-performance glasses are expensive because highly pure chemicals must be produced in substantial quantities.

Read more about this topic:  Low Dispersion Glass

Famous quotes containing the word variants:

    Nationalist pride, like other variants of pride, can be a substitute for self-respect.
    Eric Hoffer (1902–1983)