Abstract

The use of infrared-absorption techniques has become increasingly more prevalent and, with continued improvements in instrumentation, it has become possible to examine a wide variety of materials by transmission–absorption methods. Double-beam operation, increased detector sensitivity, scale expansion, and other instrumental improvements leading to high sensitivity and signal-to-noise ratios also lead to the spectral study of samples of very low transmittance, making such studies feasible if not facile. We have made such measurements with a variety of samples having transmittances below about 1%, using Perkin-Elmer model 521 and model 621 spectrophotometers, and have found that the apparent transmittance of a sample may be anomalously high if the sample is optically thick. The anomaly is caused by the emission of the sample. Similar emission effects have been mentioned, but seem to be generally disregarded. Typical effects are shown by the transmission spectra in Fig. 1, paper samples being used as examples because of the case of changing the sample thickness.

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