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Optical Materials 217

optical glass. For this reason it is frequently used in spectrophotometers,
infrared equipment, and ultraviolet devices. The excellent thermal
stability of fused quartz is responsible for its use where extremely
precise reflecting surfaces are required. Large mirrors and test
plates are frequently made from fused quartz for this reason. As pre-
viously mentioned, pure fused quartz is highly resistant to radiation
browning. The index of refraction and transmission of fused quartz
are given in Fig. 10.6. Note that the absorption bands indicated are
not of the type indicated in Fig. 10.1, but are due to impurities and
are thus subject to elimination, as indicated by the range of trans-
missions given.
A new class of materials, which are partially crystallized glasses, is
available for use as extremely thermally stable mirror substrates, since

Figure 10.6 Optical characteristics of fused quartz.