The conversion factors of illumination in different weather type are deduced and calculated according to the basic principle of chroma, the light penetrates through the canopy at different high level above ground are measured, and the measured radiation intensities are compared with converted ones. The results are: (1) the two illuminometers can be used to natural sunlight measured at 1 and 2 classes sunlight but not at 3 and 4 classes; (2) the deviation ratios of the quantum converted from measured illuminations are about 0 2 at the inner active surface level on farmland, meanwhile at outer active surface level the deviation ratios are higher because of the effect of light spots and drop shadows, so it naturally needs more repetitions.