High refractive glass
Under highly refractive glass refers to varieties of optical glass that is both a high refractive index and a high Abbe number (and therefore low dispersion have). In the Abbe diagram, these glasses are on the left above the narrow area in which the normal crown and flint glasses are located.
The development of barium- containing crown glass by Schott in 1886 made it possible for the first time to construct anastigmatic lenses without a field curvature. Its refractive index is higher than that of normal crown glass and also that of many flint glasses. By combining such a high refractive crown glass with lower refractive flint glass, you can eliminate the color error ( chromatic aberration ) and at the same time make the Petzval sum zero, which is a prerequisite for correcting astigmatism and field curvature .
In general, it is helpful for the correction of aberrations to have glasses with various combinations of refractive index and Abbe number available, especially glasses with high refractive index. Over time, glass manufacturers developed numerous types of glass in order to expand the area covered in the Abbe diagram to the top left.
Glass with portions of thorium dioxide was a step forward and was used before 1940, but thorium is radioactive , which is particularly harmful to the health of lens grinders and also causes the thorium-containing glass to turn yellowish over time (several years). That is why they looked for alternatives: Around 1940, the Kodak company succeeded in developing a thorium-free glass, which, however, was poorly translucent in the short-wave range and therefore had a yellow color from the outset.
At the Leitz company it was discovered that lanthanum oxide is suitable as a substitute for thorium oxide when other additives are used to avoid crystallization. Another glass with a high refractive index has a large proportion of zirconium dioxide ; Due to its high melting point, it has to be made in platinum vessels .