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Fresnel reflectivity suppression using random nano-structured surfaces (rARSS) has been investigated by numerous groups in recent years. A variety of measurement methods are used, from simple specular reflection to directional transmission and combinations thereof. In most cases, there is the assumption that the incident irradiance is only divided between specular reflection (R) and on-axis transmission (T), ignoring wavelength-dependent bidirectional scatter radiance distributions that attenuate both on-axis intensities. Broadband spectroscopic measurements over limited surface areas are common, assuming that there is good structure uniformity across the optical surfaces under test. Process development of rARSS requires repetitive functional testing and measurement, which at times is a result of intrusive techniques, such as electron microscopy of sample crossections, albeit in limited surface areas. We explored a double amplitude-division interferometric non-invasive technique, based on a Fabry-Perot etalon/Michelson interferometer (FPE/MI) combined instrument, to extract the absolute specular reflectance and on-axis transmission of single optical surfaces in the midwave to longwave infrared (2.0-12 μm), without sacrificing the samples. The technique was applied to before-and-after rARSS addition on infrared transmissive silicon, gallium arsenide, and germanium windows. The entire FPE/MI test system losses were taken into account, which can be considerable due to differences on sample setup conditions. We determined the interference ordinal-area of the FPE resonances within the MI scanning sub-bands, and extracted single surface values for R and T, which agree with independent spectroscopic measurements. The technique can be applied to large surface areas, limited only by the size of the MI aperture.
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