Goniophotometer for Measurements of Spectral Reflectance Coefficients and Transmission Spectra

Abstract

We describe the design of an automated laboratory goniophotometer CHERRY (Chlorophyll Estimation and Reflectance Registration sYstem) intended for measuring the spectral reflectance coefficients of coniferous needles in the spectral range 400-2500 nm and the quantitative analysis of the content of colored pigments in needles in the spectral range 400-700 nm. The CHERRY goniophotometer permits to carry out two types of measurements: goniometric ones, in which the spectral reflectance is measured from the test sample in the vertical direction, as well as photometric ones, in which the transmittance of the direct radiation passed through a cuvette with a pigment solution is measured. The spectral reflectance measurements of spruce needles (Picea abies) in the spectral range 400-700 nm showed differences up to 15 % by employing extended or quasi-parallel sources. The vegetation index Vog2 for healthy needles is varied from -0.45 to -0.34 for different types of the sample location and from -0.42 to -0.39 for different orientations of the polarizer for the selected way of placing the sample. A relative deviation of the measured spectral reflectance in the spectral range 470-770 nm up to 5 % for 2 hours of measurements confirms the influence of the lab radiation source on the spectral reflectance of the needles. Quantitative measurements show that the sample of healthy needles contains 30 % more of the studied pigments than the sample of needles from a tree in a state of stress.