Instantaneous Measurement of Reflectance Spectra in the Open Field using Diode Array Spectrometers

A sensor and a calibration procedure for instantaneous measurements of reflectance spectra in the open field have been developed that eliminates the need for frequent irradiance reference recordings. The presented method for measuring reflectance was based on simultaneous irradiance and radiance recordings using two diode array spectrometers with separate upwards and downwards directed collimators. The wavelength range of the system was 360–900 nm. Formulae for calculating reflectance including radiometric calibration and correction for wavelength matching between the two spectrometer modules were developed. Measurements of reflectance spectra, made in manually thinned spring wheat, were consistent with results found in the literature. The developed formulae made it possible to adjust the utilisation of the range of measurement through altering the integration time. An iterative procedure for the automatic change of integration time ensured 90–100% utilisation of the range of measurement. No significant erroneous behaviour of the reflectance spectra could be found using the correction for wavelength matching between irradiance and radiance recordings. The system was found to be fast and no moving components were used. The developed formulae improved the procedure for measuring reflectance spectra by the ability of optimising the utilisation of the range of measurement.