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Optical Materials 211

Figure 10.3 gives some indication of the variety of the available opti-
cal glasses. Each point in the figure represents a glass whose n d is plot-
ted against its V-value; note that the V-values are conventionally plotted
in reverse, i.e., descending, order. Glasses are somewhat arbitrarily
divided into two groups, the crown glasses and the flint glasses, crowns
having a V-value of 55 or more if the index is below 1.60, and 50 or more
for an index above 1.60; the flint glasses are characterized by V-values
less than these limits. The “glass line” in Fig. 10.3 is the locus of the ordi-
nary optical glasses made by adding lead oxide to crown glass. These
glasses are relatively cheap, quite stable, and readily available.
The addition of lead oxide to crown glass causes its index to rise, and
its V-value to decrease, along the glass line. Immediately above the
glass line are the barium crowns and flints; these are produced by the
addition of barium oxide to the glass mix. In Fig. 10.3 these are iden-
tified by the symbol Ba for barium. This has the effect of raising the
index without markedly lowering the V-value. The rare earth glasses
are a completely different family of glasses based on the rare earths
instead of silicon dioxide (which is the major constituent of the other
glasses). These are identified by the symbol La in Fig. 10.3, signifying
the presence of lanthanum.
The table of Fig. 10.4 lists the characteristics of the most common
optical glass types. Each glass type in the table is available from the
major glass manufacturers, so that all types listed are readily obtain-
able. The index data given are taken from the Schott catalog; the
equivalent glasses from other suppliers may have slightly different
nominal characteristics.
Recently the glass manufacturers have reformulated many optical
glasses in order to eliminate toxic ingredients such as lead, cadmium,
and arsenic. For the most part, the new formulations have refractive
properties which are well within manufacturing tolerances of the
glasses they replace, and their physical properties are often an
improvement over the original glasses. Not all glasses have been refor-
mulated, and many of the glasses for which lead is essential are still
produced. There are a few glasses which have “disappeared”. Since the
field is still, at least in part, in a state of flux, we have elected not to
attempt to update Figs. 10.3 and 10.4. In any case, one should always
consult the vendor’s current catalog or information for material data,
since catalogs become obsolete almost as fast as textbooks.
Formerly, optical glass was made by heating the ingredients in a large
clay pot, or crucible, stirring the molten mass for uniformity, and care-
fully cooling the melt. The hardened glass was broken into chunks
which were then sorted to select pieces of good quality. Currently the
molten glass is more likely to be poured into a large slab mold; this gives
better control over the size of the pieces of glass available. Many barium