A new spectrogoniophotometer to measure leaf spectral and directional optical properties

In this paper we present a new spectrogoniophotometer (SGP) dedicated to the assessment of plant leaf bidirectional optical properties. It consists of a mechanical apparatus coupled with an imaging spectrometer using a bidimensional CCD photodetector. Unpolarized light fluxes are sampled at high spectral and directional resolution to provide biconical reflectance and transmittance factors, every nanometer from 500 nm to 880 nm and at 800 source-sensor configurations (four illumination directions by 200 viewing directions covering the whole sphere). From these calibrated measurements we derive the leaf Bidirectional Reflectance and Transmittance Distribution Functions (BRDF and BTDF). The angular-integrated quantities defined as the Directional Hemispherical Reflectance and Transmittance Function (DHRF and DHTF) are also calculated. The first three sections emphasize the instrumental and calibration issues, as well as the radiometric definitions. In the last section we present some experimental results acquired on various monocot and dicot leaves with special attention to surface reflection. The shape, position and magnitude of the specular lobe, which is a characteristic of many leaves in the forward direction, is investigated for beech (Fagus sylvatica L.) and laurel (Prunus laurocerasus L.) using a leaf BRDF model. The width of the specular peak is very variable according to the species and the illumination angle, as well as its contribution to the directional-hemispherical reflectance. Finally, implications in plant physiology or remote sensing are broached.