Reports of the Cambridge Expedition to British Guiana, 1933. Illumination in the rain forest at ground level.

The intensity of the light at the ground level in the rain forests of British Guiana was measured by means of a Bernheim photo-electric cell. It was found that the shade light in different types of undisturbed forest lies between 0.8 and 0.2 per cent. of sunlight; and that the illumination of the ground, which is lighted by sun flecks as well as by the shade light, lies between 2 and 0.4 per cent. of sunlight. A proportionate reduction of 20 per cent. in the light at the red end of the spectrum was found in the illumination of the forest floor.