Considering that natural light is always partially polarized (reflection, Rayleigh scattering, etc.) and the alteration of the spectral response of spectrometers due to the polarization, some concerns were raised about the accuracy and variability of spectrometer outdoor measurements in field campaigns. We demonstrated by simple experiments that, in some circumstances, spectral measurements can be affected by the polarization. The signal variability due to polarization sensitivity of the spectrometer for the measured sample was about 5-10%. We noted that, measuring surfaces at right angle (a frequently used measurement protocol) minimized the problems due to polarization, producing valid results. On the other hand, measurements acquired with a slant angle are more or less accurate; an important proportion of the signal variability is due to the polarization. Direct sun reflection and reflection from close objects must be avoided.
A new goniometric device for spectro-polarimetric BRDF measurements is under development. This goniometer will measure linear and circular polarization of materials. Different instrument quality tests were realized in order to identify factors affecting the measured spectra and to obtain functional relationship between the instrument output and the real reflective properties of the materials. The most limiting of all factors influencing measured spectra was the polarization sensitivity of the spectrometer. Moreover, optical fiber bundle connected to the spectrometer input has unpredictable behavior in polarized light.In this paper, we present the artefacts produced by sensitivity of the spectrometer to the polarized light and describe the adequate measurement procedure to maintain instrument response under control and to acquire reliable BRDF measurements.
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