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OPTICAL PROPERTIES
Optical transmission properties provide a means for distinguishing among various types of vitreous silica as the degree of transparency reflects material purity and the method of manufacture. Specific indicators are the UV cutoff and the presence or absence of bands at 245 nm and 2.73 micrometers. The UV cutoff ranges from about 155 to 175 nm for a 10 mm thick specimen and for pure fused quartz is a reflection of material purity. The presence of transition metallic impurities will shift the cutoff toward longer wavelengths. When desired, intentional doping, e.g., with Ti in the case of Type 219, may be employed to increase absorption in the UV. The absorption band at 245 nm characterizes a reduced glass and typifies material made by electric fusion. If a vitreous silica is formed by a "wet" process, either flame fusion or synthetic material, for example, the fundamental vibrational band of incorporated structural hydroxyl ions will absorb strongly at 2.73 micrometers.
UV Cutoff
As the transmission curve illustrates, GE Type 214 fused quartz has a UV cutoff
(1 mm thickness) at < 160 nm, a small absorption at 245 nm and no appreciable
absorption due to hydroxyl ions. Type 219, which contains approximately 100
ppm Ti, has a UV cutoff at about 230 nm for a 1 mm thick sample.
High IR Transmission
The IR edge falls between 4.5 and 5.0 micrometers for a 1 mm thick sample. Type
(214/124 electrically) fused quartz is a very efficient material for the transmission
of infrared radiation. Its infrared transmission extends out to about 4 micrometers
with little absorption in the "water band" at 2.73 micrometers. This makes GE
electrically fused quartz different than flame fused quartz (often referred
to as "wet" quartz). This difference is seen in the transmission for the IR
range. The IR Transmission figure illustrates this difference. Conversion to
other thicknesses can be accomplished with the following formula:
T = (1-R)2 e -at
Where:
T = percent transmission expressed as a decimal.
R = surface reflection loss for one surface.
e = base of natural logarithms
a = absorption coefficient, cm-1
t = thickness, cm
