Improvements in spectroradiometric measurements and applications

Irradiance refers to the density of the radiant flux of electromagnetic radiation incident on a surface. Spectrally resolved measurements of irradiance are required in many fields, such as in solar ultraviolet radiation monitoring, in ozone retrieval, and in photometry. Spectral irradiance is commonly measured using scanning or array spectroradiometers equipped with a mono-chromator. In order to fulfill the accuracy requirements for a given application, several instrument properties need to be considered, including the quality of the entrance optics, the linearity characteristics of the instruments, and the stray light. This thesis describes a ray-tracing method for simulating radiation transport in the entrance optics which can be used to aid in the optimization of the angular response of the instruments. The quality of entrance optics is especially important in global measurements of solar irradiance. The thesis also presents the nonlinearity characterization results for two array spectroradiometers. Significant nonlinearities were detected for the studied spectroradiometers. Furthermore, in this thesis, the stray light properties of Brewer spectrophotometers, used globally in solar ultraviolet and ozone monitoring, were studied through direct measurements, filter characterizations, and simulations. Stray light from wavelengths outside the nominal measurement range of the instruments were found to cause significant issues in the solar ultraviolet spectral irradiance measurements. Photometry is a field of science that focuses on measuring the brightness perception in humans caused by radiation. Spectral irradiance measurements are commonly used in photometry to determine spectral correction factors. Spectral irradiance data can also be used directly to calculate the corresponding photometric quantity, illuminance. However, this technique is rarely used in applications with high accuracy requirements, due to the uncertainties of absolute spectral irradiance determination. Instead, filtered broadband detectors, photometers, are used in the measurements. This thesis presents a method for determining the illuminance of white LED sources using a combination of relative spectral irradiance measurement and a highly accurate unfiltered broadband detector. This method can improve the uncertainty of illuminance measurements of white LEDs by eliminating the need for a separate spectral responsivity characterization of the photometer.