Technique for measuring the reflectance of irregular, submillimeter-sized samples

TLDR

The authors describe a modified Michelson interferometer or grating spectrometer that mounts irregular sub‑millimeter samples and a reference mirror on non‑reflecting cones, swaps their positions by a 90° rotation, corrects scattering through in‑situ metal evaporation, and estimates diffraction effects using Mie theory. Calculations show that for frequencies above ≈ 40 cm⁻¹ and ≈ 1 mm sample diameters with a 30° detector field of view, the ratio of backscattered intensities provides a good approximation of specular reflectance.

Abstract

Details are given of a technique for measuring the reflectance at near-normal incidence of small, irregular, submillimeter-sized samples from the far IR (40 cm(-1)) to the visible (40000 cm(-1)) between 10 and 300 K by using a modified Michelson interferometer or grating spectrometer. The sample and a reference mirror are mounted on nonreflecting cones. At the focus the size of the beam is larger than either the sample or the reference, so that the entire area of the sample is utilized. The positions are interchanged by a 90° rotation by using preset mechanical stops. The scattering caused by geometrical effects is corrected for by the in situ evaporation of gold or aluminum onto the sample. The effect of diffraction is estimated from Mie theory by assuming the sample and reference to be spheres. For frequencies above ≈ 40 cm(-1) and sample diameters of ≈ 1 mm with a detector field of view of 30°, the calculations show that the ratio of the backscattered intensities gives a good approximation of the specular reflectance.