FOURIER TRANSFORM SYSTEM FOR CHARACTERIZATION OF INFRARED SPECTRAL EMITTANCE OF MATERIALS

摘要

To meet the existing demand for measurements of emittance of opaque and semi-transparent materials, a new facility is being built at the National Institute of Standards and Technology (NIST) employing a bench-top Fourier transform infrared (FT-IR) spectrometer. This facility will complement existing capabilities for characterisation of sample optical properties, which now encompass specular and directional-hemispherical reflectance and transmittance in the mid-IR. The first stage of the project is focused on opaque samples at medium temperatures, with semi-transparent sample capability. Expansion of the temperature range will follow later. The approach selected for direct emittance characterization at temperatures above 500 K is to some extent similar to that employed in existing NIST facilities, sharing the FT-IR detection technique and integrating sphere based measurements of hemispherical-directional reflectance. The reflectance measurements are performed in a single spectral band to allow a radiometric determination of the temperature of the sample surface. Design and modelling of the spherical laser reflectometer is the subject of a separate paper presented at this conference. A set of reference blackbodies, which contains four heat pipe furnaces and several interchangeable fixed-point cells, is to be received soon. An all-reflective off-axis optical train, designed for low emittance and scatter, has been received and is being characterized. To allow direct measurements of emittance at the temperatures below 500 K, a cryogenically cooled "radiometric zero" trap and background control measures are being envisioned. Anticipated performance of the new facility, the optical set up and the principles of measurement, as well as further development prospects are discussed. Along with a description of the developed methodology of directional emittance capability establishment, a concept of a "controlled temperature emittance standard" (CTES) is proposed. It is expected that such a standard will significantly reduce the loss of accuracy at the end of the calibration chain for a number of users with specialized multi-band thermometers.