Calibrating spectrometers for measurements of the spectral irradiance caused by solar radiation

Measuring the spectral irradiance of solar radiation is required in many fields of science and technology. In this work, we present an in-depth discussion of the measuring procedure and required corrections for such measurements. We also describe our measurement uncertainty analysis, which is based on a Monte-Carlo procedure in accordance with the Guide to the expression of uncertainty in measurement (JCGM, Paris, 2008). For this purpose, fifteen uncertainty sources are identified, analyzed and described analytically. As a specific application example, we describe the instrumentation and procedure for determining the spectral irradiance of a solar simulator at the ISO/IEC 17 025 accredited solar cell calibration laboratory ISFH CalTeC and the corresponding measurement uncertainty analysis. Moreover, we provide a Python implementation for this calculation along with the paper. We show that for state-of-the-art instrumentation, significant uncertainty contributions arise from the reference lamp (primary calibration standard), stray light and signal-to-noise ratio. If sharp spectral features are present (which is common, e.g. for Xenon lamps), spectral bandwidth and wavelength uncertainty also contribute significantly to the overall uncertainty.