Page 104 - Chalcogenide Glasses for Infrared Optics

82 Cha pte r T h ree

Insulation

Heater control Equalization tube
thermocouple

Capillary
Heaters
Rotation Rotation
axis axis

Dip coil

Thermocouple #2

TI 1173 melt Thermocouple #1

FIGURE 3.10 High-temperature chalcogenide glass viscometer.

composition from the glass, which means the refractive index changes
slightly. The question becomes, How slightly? Now as the glass is
processed into the form of blanks, one is left with a certain amount of
scrap that is valuable because it contains germanium. The desire is to
clean up the scrap and reprocess it. When a boule of glass is com-
pounded, scrap may be mixed with fresh elements as a cost-reducing
means. A large producer such as TI desires to reduce cost but without
degrading the system performance due to index change. The decision
was made to measure the change when glass is recast. Three prisms
were fabricated from standard glass production. Using the TI infrared
refractometer, the index of all three was measured over the range of
2.5 to 14 µm. The values for all three were averaged and found to
agree with one another over the wavelength range by ±0.0002. The
process was repeated with three prisms made from glass recast once.
An average value for the recast glass prisms was calculated and did
not agree as well as the standard glass production ±0.0010. When the