Optical design and spectral calibration of the visible-near infrared irradiance monitor

The climate on Earth is affected by the Sun. It is thus necessary to determine the solar spectral irradiance (SSI) with high precision on the ground for the improved accuracy of climate models used for Earth. To achieve this purpose, a solar spectral irradiance monitor (VISM) with a wavelength ranging from 380 to 2400 nm is developed. In this work, the optical design and spectral calibration of the VISM are explored, with two optical paths designed to determine the SSI in the wide spectral region. One of them is the main optical path through which the sunlight is dispersed and imaged by a single Fery prism. A laser and a concave mirror are used in the other optical path to determine the rotated angle of the Fery prism, which is crucial to determining the spectral information. Furthermore, a new spectral calibration method is developed for VISM. According to this method, the dispersion equation of the prism is combined with the results of three characteristic wavelengths to determine the relationship between the dispersed wavelengths and the pixels in the detector for the whole spectrum. An apparatus is set up in the laboratory for spectral calibration. It is revealed that the uncertainty of spectral calibration is about 0.3 nm, which exceeds the design requirement. When the designed VISM works outdoor, the SSI can be determined readily. It is shown that the optical design and spectral calibration proposed in this study for VISM are correct. (c) 2024 Society of Photo-Optical Instrumentation Engineers (SPIE)