Optical properties of fused Quartz
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 characterises 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 in Figure 24 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. The IR edge falls between 4.5 and 5.0 micrometers for a 1mm thick sample. Tables XIV and XV detail the percent transmittance for Types 214 and 124 fused quartz, including the losses caused by reflections at both surfaces. Values represent a 1mm thick Type 214 sample and a 10 mm thick Type 124 sample.
Type 124 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. Conversion to other thicknesses can be accomplished with the following formula:
T = (1-R) 2e - 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
(Data for GE Fused Quartz)
SOME PHYSICAL CONSTANTS OF CLEAR FUSED QUARTZ
Density: 2.2g./c.c.
Hardness: 4.9(Mohs')
Thermal Conductivity: .0033 g. cal./cm.²/sec./°C/cm.
Specific Heat: .18 g. cal./gm.
Softening Point: (approx.) 1665°C
Annealing Point: (approx.) 1140°C