An experimental analysis is presented concerning the effect on relative light absorption by the two photosystems caused by (a) a highly light scattering environment (the "detour effect") and (b) light filtration across successive chloroplast layers (the "light attenuation effect"). Both suspensions of isolated chloroplasts and leaves were employed.It is concluded that within a single spinach leaf these phenomena are likely to lead to only rather small increases in relative photosystem I absorption and activity with respect to photosystem II and will thus not exert a significant effect on non cyclic electron transport. On the contrary when light is filtrated across successive vegetation layers (shade light) significant increases in the relative PSI absorption and activity may be encountered.It is determined that the "detour effect" in mature leaves from a variety of plants increases overall photosynthetically useful light absorption by 35-40%.