Page 233 - Modern Optical Engineering The Design of Optical Systems

214 Chapter Ten

glasses and all the rare earth glasses are processed in platinum cru-
cibles, since the highly corrosive molten glass tends to attack the walls
of a clay pot and the dissolved pot materials affect the glass character-
istics. In extremely large volume production, a continuous process is used,
with the raw materials going in one end of the furnace and emerging as
extruded strip or rod glass at the other end. Raw glass is frequently
pressed into blanks, which are roughly the size and shape of the finished
element. The final stage before the glass is ready for use is annealing.
This is a slow cooling process, which may take several days or weeks,
and which relieves strains in the glass, assures homogeneity of index,
and brings the index up to the catalog value.
The characteristics of optical glass vary somewhat from melt to melt
(because of variations in composition and processing) and also due to
variations in annealing procedures. Ordinarily the lower index glasses
(to n 1.83), are supplied to a tolerance of ±0.0005 on the catalog value
of n d ; the higher index glasses may vary ±0.0016 from the nominal
index. Similarly the V-value may vary from the catalog value by about
0.3 percent. Most glass manufacturers will select glass to closer toler-
ances at an increased price.
Optical glass may be obtained in hundreds of different types; complete
information is best obtained from the manufacturer’s catalog.
Figure 10.5 gives an indication of the spectral transmission of optical
glasses. In general, most optical glasses transmit well from 0.4 to 2.0 m.
The heavy flints tend to absorb more at the short wavelengths and
transmit more at the long wavelengths. The rare earth glasses also
absorb in the blue region. Since the transmission of a glass is affected
greatly by minute impurities, the exact characteristics of any given
glass may vary significantly from batch to batch, even when made by
the same manufacturer. In general the transmission values tend to
improve as the purity of the raw materials is improved over time.
Most optical glasses turn brown (or black) when exposed to nuclear
radiation because of increased absorption of the short (blue) wavelengths.
To provide glasses which can be used in a radiation environment,
the glass manufacturers have developed “protected” or “nonbrowning”
glasses containing cerium. These glasses will tolerate radiation doses
to the order of a million roentgens. Fused quartz glass, which is dis-
cussed in the next section, is almost pure SiO 2 and is extremely resis-
tant to radiation browning.
Although not strictly “optical glass,” ordinary window glass and
plate glass are frequently used when cost is an important factor. The
index of window glass ranges from about 1.514 to about 1.52, depend-
ing on the manufacturer. Ordinary window glass is slightly greenish,
due primarily to modest amounts of absorption in the red and blue
wavelengths; the red absorption continues to about 1.5 m. Window