Ratio analysis of reflectance spectra (RARS): An algorithm for the remote estimation of the concentrations of chlorophyll A, chlorophyll B, and carotenoids in soybean leaves

An algorithm utilizing reflectance spectra bands in the photosynthetically active radiation (PAR) region of the solar spectrum was developed for the remote estimation of the concentrations of chlorophyll a, chlorophyll b, and carotenoids in soybeans. The defining of specific bands in the reflectance spectrum that corresponded to absorption bands of the individual pigments was basic to the development of the algorithm. The detection of these bands was rendered difficult by the lack of detail in reflectance spectra. It was therefore necessary to manipulate the reflectance spectra so that absorption bands due to specific pigments could be detected and their spectral maxima defined. It was found that by dividing soybean reflectance spectra by an arbitrarily selected reference soybean reflectance spectrum, ratio spectra were obtained in which the absorption bands could be distinctly seen and their wavelength defined. These ratio spectra allowed the defining of those bands corresponding to the absorption bands of chlorophyll a chlorophyll b, and carotenoids. The strong linear relationships of certain combinations of the bands in the ratio spectra to the concentrations of the photosynthetic pigments made it possible to develop a ratio analysis of reflectance spectra algorithm (RARS) by which the concentrations of these pigments could be calculated from the reflectance spectra. The measurements necessary for the development of RARS were made using soybeans which were grown at different nitrogen levels in order to obtain a range of reflectance spectra. A test of the RARS algorithm using other soybean plants showed very good agreement between measured pigment values and those calculated using RARS.