We describe a new method for obtaining the optical constants of a material based upon measurement of the relative derivative of reflectance with angle of incidence (1/R)(dR/dθ). The systematic optical errors in this system were reduced to about 5 × 10−4 rad−1. The relative accuracy in (1/R)(dR/dθ) was then typically five times better than the relative accuracy in the optical absorptance (1 − R0) measured in a standard reflectometer. Systematic calibration errors were comparable with those for a reflectometer and were about 0.6%. Used in conjunction with a normal incidence reflectometer, the system provides the optical constants of materials in regions where the reflectivity is not high. When the reflectivity approaches unity, as it does for metals in the ir, the measured quantity (for light of both polarizations) becomes proportional to (1 − R0) and may be used to measure the absorptance of a sample with an accuracy of about 3%.
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