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14 November 2003 Correction for angular spread in HDR determination of IR optical constants
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Hemispherical Directional Reflectometer (HDR) measurements provide broadband IR data for oblique polarized reflection as well as normal-incidence transmission. Tests on thin polymer films in low-loss wavelength ranges typically show fringes conforming to Fresnel reflection/transmission. Hence, HDR experiments are a promising approach to determining optical constants for organic materials. The same experiments also quantify operational features of the HDR for applications to inorganic materials. Exploration of the HDR as a device for determining the optical constants of thin films shows that accuracy of data-reduction algorithms for n and k can be improved by simulating a feature that arises from limits on angular resolution achievable in the HDR configuration. In particular, an overhead mirror used to collect IR radiation scattered from the sample film subtends a non-negligible angle. This effect causes measured reflection extrema to be "damped" relative to rigorous calculations assuming incidence at a discrete angle. The transmission and polarized reflection actually observed are well simulated by a simple algorithm that averages power scattering over an angular spread corresponding to the size of the mirror. An algorithm incorporating corrections for angular spread has been developed to determine film thickness as well as optical constants on low-loss ranges, and has been validated by application to a polyethylene film.
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H. M. Graham and Harris G. Carter Jr. "Correction for angular spread in HDR determination of IR optical constants", Proc. SPIE 5192, Optical Diagnostic Methods for Inorganic Materials III, (14 November 2003);

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