Two methods have been developed concurrently for hyperspectral measurements of plant canopy reflectance in two narrow wavelength bands centred around 531 and 570 nm. A laboratory-built two-channel radiometer provided an easy and quick estimation of the Photochemical Reflectance Index PRI = (R531 - R570)/(R570 + R531) of a plot of alfalfa. A CCD digital camera provided multispectral imaging and the analysis of this index on the same target. The two devices are complementary. The results of measurements are complementary with those of chlorophyll fluorescence induction. and M. Méthy, R. Joffre, S. Rambal.