KEYWORDS: Reflectivity, Temperature metrology, Sensors, FT-IR spectroscopy, High dynamic range imaging, Mirrors, Reflectometry, Signal detection, Aluminum, Data modeling
We have developed a system to measure the directional thermal emission from a surface, and in turn, calculate its
emissivity. This approach avoids inaccuracies sometimes encountered with the traditional method for calculating
emissivity, which relies upon subtracting the measured total reflectivity and total transmissivity from unity. Typical total
reflectivity measurements suffer from an inability to detect backscattered light, and may not be accurate for high angles
of incidence.
Our design allows us to vary the measurement angle (θ) from near-normal to ~80°, and can accommodate samples as
small as 7 mm on a side by controlling the sample interrogation area. The sample mount is open-backed to eliminate
shine-through, can be heated up to 200 °C, and is kept under vacuum to avoid oxidizing the sample. A cold shield
reduces the background noise and stray signals reflected off the sample. We describe the strengths, weaknesses, trade-offs,
and limitations of our system design, data analysis methods, the measurement process, and present the results of our
validation of this Variable-Angle Directional Emissometer.
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.
Conference Committee Involvement (6)
Reflection, Scattering, and Diffraction from Surfaces IV
17 August 2014 | San Diego, California, United States
Reflection, Scattering, and Diffraction from Surfaces III
14 August 2012 | San Diego, California, United States
Reflection, Scattering, and Diffraction from Surfaces II
2 August 2010 | San Diego, California, United States
Reflection, Scattering, and Diffraction from Surfaces
11 August 2008 | San Diego, California, United States
Optical Diagnostics
3 August 2005 | San Diego, California, United States
Optical Diagnostic Methods for Inorganic Materials III
6 August 2003 | San Diego, California, United States
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