Abstract
We developed a cost-effective Fast Response Optical Spectroscopy Time synchronized instrument (FROST). FROST can measure 18 light spectra in 18 wavebands ranging from 400 to 950 nm with a 20 nm full width half maximum bandwidth. The FROST 10 Hz measurement frequency is time-synchronized by a Global Navigation Satellite System (GNSS) timing pulse and therefore multiple instruments can be deployed to measure spatial variation of solar radiation in perfect synchronization. We show that FROST is capable of measuring broadband shortwave, global horizontal, total irradiance (GHI) despite its limited spectral range. It is very capable of measuring Photosynthetic Active Radiation (PAR) because 11 of its 18 wavebands are situated within the 400 to 700 nm range. A digital filter can be applied to these 11 wavebands to derive the Photosynthetic Photon Flux Density (PPFD) and retain information of the spectral composition of PAR radiation. The 940 nm waveband can be used to derive information about atmospheric moisture. We showed that the silicon sensor has undetectable zero offsets for solar irradiance settings and that the temperature dependency as tested in an oven between 15 °C and 46 °C appears very low (-250 ppm K-1). For solar irradiance applications, the main uncertainty is caused by our Poly Tetra Fluor Ethylene (PTFE) diffuser (Teflon), a common type of diffuser material for cosine-corrected spectral measurements. The oven experiments showed a significant jump in PTFE transmission of 2 % around 21 °C. The FROST total cost (<€200) is much lower than current field spectroradiometers, PAR sensors or Pyranometers, and includes a mounting tripod, solar power supply, datalogger and GNSS and waterproof housing. The FROST is a fully stand-alone measurement solution. They can be deployed anywhere with their own power supply and can be installed in vertical in-canopy profiles as well. This low cost makes it feasible to study spatial variation of solar irradiance using large grid high-density sensor set-ups or to use FROST to replace existing PAR sensor for detailed spectral information.
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