Ellipsoidal mirror reflectometer

Abstract

A new ellipsoidal mirror reflectometer is described, in which radiant flux from an infrared monochromator is focused on a specimen placed at the first focal point of the ellipsoid and a thermopile detector is placed at the second. Errors associated with angular and areal variations in sensitivity of the detector and with aberrations in the optics were largely eliminated through use of a small averaging sphere placed over the detector. Losses caused by the presence of the entrance hole in the ellipsoidal mirror and from mirror absorption were evaluated both theoretically and experimentally. Corrections for these losses permitted absolute reflectance to be obtained for both diffuse and partially diffuse reflecting specimens. In addition, the unique optics of the ellipsoidal mirror provide more versatility than is available in previous reflectometers. This versatility includes the ability to accurately measure directional-hemispherical, specular, nonspecular, and directional-annular cone reflectance. An analysis of the accuracy of the instrument indicates that an accuracy of better than one percent is possible for all engineering materials. The use of the sulfur averaging sphere also allowed the construction of a simple accurate specular reflectometer for calibration of the mirror reference standards used in these measurements.