Fourier Instrumentation for Radiation Thermometry

Franklin J. Dunmore
Optical Technology Division, NIST

A miniature Fourier transform spectrometer (MFTS) has been tested as a device for remotely measuring the temperature of a high-stability/emissivity blackbody. The commercially manufactured device is based on the novel design of a polarizing Wollaston prism spatial domain interferometer, with a Si diode array detector, and without any moving parts. The measurement of temperature using Planck's law exhibited a consistent nonlinear effect. This results in an error of approximately 1% for measurement of temperatures of 500 K and above. Initially, a calibration curve was generated by fitting a polynomial to the results obtained over a narrow spectral range at a set of blackbody temperatures, to reduce the measurement error. Work is in progress to directly calibrate the detector array linearity. This is used to correct the interferogram in the processing software prior to Fourier transformation. This improved calibration procedure has resulted in a factor of 3 reduction of the temperature measurement error. To reduce radiation source nonuniformity and drift for standard (Michelson) Fourier transform spectroscopy (FTS) in the infrared and visible, the standard globar and lamp sources have been replaced with a high-temperature/stability blackbody with 0.999 emissivity. In addition to detector nonlinearity and the instability of mirror alignment during travel in the FT instrument, temperature drift is one of the important sources of measurement error in standard FTS. FTS's quantitative measurement capabilities will be advanced by the reduction of these error sources. An additional goal of this project is the realization of a Planck's Law-based Kelvin temperature scale below the Silver melting point (1234.96 K), and improvement in its accuracy at higher temperatures not covered by the present temperature scale.