While most basic solar simulators use a single xenon lamp as their light source, advanced solar simulators commonly use multiple light sources (referred to as 'multi-source') in order to achieve a close spectral match to the reference spectrum. This is generally done by filtering and merging the output of two types of source, usually an arc source (metal halide or xenon) for the visible and a QTH source for the NIR-LWIR ranges.
The term 'close-match' to describe a solar simulator that attempted to move beyond a single-source design and accurately reproduce the solar reference spectrum was first used in 1997 by Dr Williams from 'TS-Space Systems LTD' when the results from the first ever close-match solar simulator were presented. The research compared the results of testing multi-junction solar cells using a close-match spectrum to the results from using another, basic solar simulator and showed measurement variations of up to 20% in some cases.
 Wilkinson, V. A.; Goodbody, C.; Williams, W. G., "Measurement of multijunction cells under close-match conditions," Photovoltaic Specialists Conference, 1997., Conference Record of the Twenty-Sixth IEEE , vol., no., pp.947,950, 29 Sep-3 Oct 1997 doi: 10.1109/PVSC.1997.654244
 "Solar Cell Calibration and Measurement Techniques" NASA Technical Memorandum 113155 November 1997 IECEC–97534
 Jeffrey H. Warner ; Robert J. Walters ; Scott R. Messenger ; Justin R. Lorentzen ; Geoffrey P. Summers, et al."Measurement and characterization of triple junction solar cells using a close matched multizone solar simulator", Proc. SPIE 5520, Organic Photovoltaics V, 45 (November 3, 2004); doi:10.1117/12.559734; http://dx.doi.org/10.1117/12.559734
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