R ECENTLY sapphire has found increased applications as a window material for the transmission of visible and in frared radiant energy. The material has been used for windows in absorption cells, light sources, detectors, and domes of aircraft. Its popularity is due mainly to its relatively high transparency in the 0.3-6 ฮผ region of the spectrum, its extreme hardness (1500-2000 Knoop), and to its high melting point (2030ยฐC). Synthetic sapphire is now available in a great variety of shapes and sizes, and has been successfully incorporated into many types of optical equipment.
At elevated temperatures the transmittance of sapphire at its long-wavelength limit is much reduced, and its useful range becomes restricted to wavelengths below approximately 4 ฮผ. The strong emission which occurs beyond this limit severely restricts its use for certain applications. Since only a few measurements were available in the literature 1 at the long-wavelength limit, it was decided to determine the transmittance of sapphire in the entire 1-6 ฮผ region at temperatures between 20ยฐ and 1000ยฐC.
The measurements were taken by placing a window of synthetic sapphire 2 in a quartz tube furnace and observing its spectral transmittance at normal incidence as a function of temperature through the use of an infrared spectrometer. The window had a diameter of 30 mm and was 8 mm thick. Chopped radiation from a Globar source was focused on the sample and then refocused on the entrance slit of a Perkin-Elmer Model 12G spectrometer. The temperature was controlled by means of a chromel-alumel ther mocouple placed inside the furnace next to the sample. The whole optical system was flushed with dry nitrogen during observations in order to eliminate atmospheric absorption effects. The results of these measurements, with external transmittance plotted