A global spectral irradiance intercomparison using spectroradiometers was organized by the National Renewable Energy Laboratory's (NREL's) Solar Radiation Research Laboratory. The intercomparison was performed both indoors and outdoors on September 17, 2013. Five laboratories participated in the intercomparison using 10 spectroradiometers. A coordinated measurement setup and a common platform were employed to compare spectral irradiances under both indoor and outdoor conditions. The intercomparison was aimed at understanding the performance of the different spectroradiometers and sharing knowledge in making spectral irradiance measurements. At NREL's Optical Metrology Laboratory, the intercomparison is part of an internal performance-based quality-control check to monitor the legitimacy of a measurement and calibration undertaken by a laboratory to demonstrate compliance with International Standards Organization/International Electrotechnical Commission (ISO/IEC) 17025 accreditation requirements.The indoor performance comparison showed that all of the participating spectroradiometers had satisfactory statistical results (±1) compared to the NREL reference instrument. However, each laboratory's instruments behaves differently with respect to the statistical limit, and such differences could be related to various reasons-for example, differing calibration setups from one laboratory to another, differing environmental conditions inside laboratories, whether a primary or secondary spectral irradiance calibration lamp was used for the calibration, instrument age, and the amount of time since the last calibration.The outdoor intercomparison showed up to ±10% deviation relative to the average spectral irradiance measured by the participating spectroradiometers. Differing scan rates, sizes of the entrance optics, or fast-changing atmospheric conditions could be reasons for such deviations. Mean bias error (MBE) and root mean square error (RMSE) were calculated representing average differences from the three outdoor runs and results from the aggregation of hundredwavelength bins. Almost all instruments were within +10% MBE and 10% RMSE.Simulations using the Simple Model of the Atmospheric Radiative Transfer of Sunshine (SMARTS) were applied to the outdoor intercomparison as an explanatory tool and to understand how well the SMARTS-modeled spectra compare to various types of spectroradiometers considering the model's spectral resolution compared to the spectroradiometers under scrutiny. Running the smoothing postprocessor of the SMARTS model was therefore necessary to downgrade the resolution of its spectra and make them match that of any specific instrument based on the shape of its passband (e.g., Gaussian), its width (as measured by the full width at half maximum), and its wavelength step (e.g., 5 nm). The SMARTS model in the outdoor intercomparison provides relevant information when predicting clear-sky solar spectral irradiance under varying atmospheric conditions. The output from the model compared well to the outdoo...
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