On the measurement of infrared emission spectra using grating spectrometers

Summary

The practical limits for IR emission measurements with conventional grating spectrometers have been checked using thin solid films on metal substrates as samples. Favourable conditions provided (large sample area, appropriate sample thickness), good quality emission spectra, which are in excellent agreement with corresponding absorption spectra, can be obtained without too much experimental requirements (temperatures below 100° C, no special optics) and within a reasonable time (less than 1 h). Because of the restricted instrument sensitivity, however, lower limits for the experimental parameters (temperature, sample area, thickness) rapidly are reached, below which the recorded emission spectra become useless. These limits are about 40° C for the sample temperature, 0.5 cm² for the sample area and 1μm for the thickness of a polystyrene sample. For the sample thickness there exists an upper limit too, since with increasing thickness the emissivity converges to (1-reflectivity) within the whole frequency range, similar to absorption spectra, which converge to zero transmission. The upper limits for the permissible thicknesses are very similar for emission and absorption, being about 100μm for polystyrene.