Contributed Review: Absolute spectral radiance calibration of fiber-optic shock-temperature pyrometers using a coiled-coil irradiance standard lamp
We describe an accurate and precise calibration procedure for multichannel optical pyrometers such as the 6-channel, 3-ns temporal resolution instrument used in the Caltech experimental geophysics laboratory. We begin with a review of calibration sources for shock temperatures in the 3000-30 000 K range. High-power, coiled tungsten halogen standards of spectral irradiance appear to be the only practical alternative to NIST-traceable tungsten ribbon lamps, which are no longer available with large enough calibrated area. However, non-uniform radiance complicates the use of such coiled lamps for reliable and reproducible calibration of pyrometers that employ imaging or relay optics. Careful analysis of documented methods of shock pyrometer calibration to coiled irradiance standard lamps shows that only one technique, not directly applicable in our case, is free of major radiometric errors. We provide a detailed description of the modified Caltech pyrometer instrument and a procedure for its absolute spectral radiance calibration, accurate to ±5%. We employ a designated central area of a 0.7× demagnified image of a coiled-coil tungsten halogen lamp filament, cross-calibrated against a NIST-traceable tungsten ribbon lamp. We give the results of the cross-calibration along with descriptions of the optical arrangement, data acquisition, and processing. We describe a procedure to characterize the difference between the static and dynamic response of amplified photodetectors, allowing time-dependent photodiode correction factors for spectral radiance histories from shock experiments. We validate correct operation of the modified Caltech pyrometer with actual shock temperature experiments on single-crystal NaCl and MgO and obtain very good agreement with the literature data for these substances. We conclude with a summary of the most essential requirements for error-free calibration of a fiber-optic shock-temperature pyrometer using a high-power coiled tungsten halogen irradiance standard lamp.
© 2015 AIP Publishing LLC. Received 23 January 2015; accepted 16 September 2015; published online 19 October 2015. Thanks are due to the late Professor Tom Ahrens, the founder of the Lindhurst Laboratory of Experimental Geophysics at Caltech, for inviting one of us (O.V.F.) to join the research staff of this laboratory in 2006, for many interesting suggestions and advice, and for his broad interest in nontraditional methods in shock-wave physics including the topic of pyrometer calibration. Our LLNL collaborators, Dr. Neil Holmes and Dr. Jeff Nguyen, are gratefully acknowledged for their pyrometer and Pockels cell that largely extended the transient response characterization of our photodetectors. Professor George Rossman of Caltech helped with operating his visible range and FTIR spectrometers and shared his extensive knowledge about water contamination of minerals. Former technical staff members of the Lindhurst Lab Mike Long, Papo Gelle, and Russ Oliver provided expert assistance with the two-stage light-gas gun experiments. O.V.F. thanks his wife Elena who reviewed the very first draft of this manuscript and made the whole narrative substantially smoother. Valuable comments and suggestions by anonymous reviewers that helped improve the manuscript and made it more accessible are really appreciated. The financial support came from the U.S. NSF, Award No. EAR-1426526.
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