Spectral emissivity of tungsten: analytic expressions for the 340-nm to 26-μm spectral region

Abstract

To correct emission spectra a standard radiance source is often used to determine the spectral responsivity of the detection system. In the near-UV, visible, and near-IR spectral regions the most common radiance standard is a tungsten strip lamp calibrated by a standards laboratory. For day-to-day experiments where slightly less accuracy is acceptable, a less expensive uncalibrated lamp is useful. In this case, the radiant temperature T/sub r/ of the lamp is measured with an optical pyrometer, generally at a single wavelength such as 650 nm, and the source spectral radiance L(lambda) is calculated from L(lambda) = tau(lambda)epsilon(lambda,T)L/sub B/(lambda,T). The transmittance of the source is tau(lambda), the spectral emissivity is epsilon(lambda,T), and L/sub B/(lambda,T) is the spectral distribution of blackbody radiation, Planck's radiation law. To obtain the true temperature T, Wien's approximation is employed. To conveniently calibrate a system, especially one which utilizes a microcomputer, it is advantageous to have analytic expressions for the spectral emissivity of tungsten. Although Larrabee has published such expressions, they are limited to the 450-800-nm spectral region. To obtain analytic expressions from 340 nm to 2.6 ..mu..m they have used the measurements of DeVos. Although DeVos's results differ by 2% from those of Larrabee, this differencemore » is assumed to be acceptable.« less