Design, Construction and Calibration of a Low-Cost Spectrometer for Solar Spectrum Measurements in the Visible Region and UV Radiation

This article presents the design, construction, and calibration of a portable instrument capable of measuring the solar spectrum in the visible region (430–650 nm), as well as UVA, UVB, and UVC radiation, while also determining the ultraviolet radiation index. The hardware was constructed by integrating a Raspberry Pi 4B, a Raspberry Pi HQ camera coupled to a spectroscope for solar spectrum analysis, and an AS7331 sensor for UV radiation measurement. The control software was developed in Python and includes a graphical user interface that displays the solar spectrum and UVA, UVB, and UVC radiation levels, in addition to calculating the ultraviolet index from UVB-band measurements. The instrument was calibrated using a mercury (Hg) spectral lamp, a reference spectrometer, and commercial UVA and UVB radiation meters. The prototype spectrometer was calibrated and adjusted in the city of Arequipa. During this stage, the equations required to convert the digital signals from the sensors into radiometric units were determined, and the measurement uncertainty of the instrument was obtained. Finally, the instrument was evaluated under field conditions at three locations situated at different altitudes: Camaná (0 m a.s.l.), Arequipa (2330 m a.s.l.), and Sumbay (4100 m a.s.l.). The spectrometer recorded irradiance values of 111.2 W/m2, 670.7 W/m2, and 643.1 W/m2 in Camaná, Arequipa, and Sumbay, respectively, and exhibited a maximum deviation of 15.8 W/m2 from the measurements obtained with the reference instrument. Regarding UVA, UVB, and UVC radiation, the performance of the pilot instrument was evaluated against a Black Comet reference spectrometer, showing maximum differences of 0.27 mW/cm2, 0.02 mW/cm2, and 0.01 mW/cm2, respectively. On the other hand, the maximum differences found when compared to commercial UVA and UVB meters were 0.24 mW/cm2 and 0.06 mW/cm2, respectively. These absolute deviations translate to a highly acceptable relative uncertainty of approximately 12% when the instrument is deployed in high-altitude environments. In addition, the ultraviolet radiation index was found to reach values of up to 21 in Sumbay, posing a risk to the population of the region. The results showed that the solar spectrum and UV radiation measurements obtained with the constructed instrument were proportional to those recorded by the reference spectrometer and commercial UV meters, demonstrating its reliability and feasibility as a measurement instrument.