A chemical light meter that uses chlorophyll is described. The quantity of radiant energy that is utilizable for photosynthesis can be estimated through use of crude chlorophyll extracts in conjuction with a thermopile and recorder. Two hundred or more determinations can be made per day at a negligible cost relative to the cost of using a comparable number of thermopiles and recorders. Preliminary tests with this apparatus revealed that about 75% of the forest floor under a dense hardwood stand received enough radiation to compensate for the respiration losses by the plants. The light in a 28—year—old, 6— by 8—ft Pinus virginiana Mill. plantation was considerably brighter than that under a dense hardwood stand. All of the sample points under the pine plantation received sufficient radiation to permit photosynthesis to compensate for respiration. The experimental results and theoretical analysis of several research workers are combined to develop the generalization that radiant energy is transmitted through a canopy of leaves in the same manner as radiant energy is transmitted through a solution–i.e. in accord with Beer's law: Optical density @8 —log (fraction of radiant energy transmitted) ° KF K is a constant of proportionality and F is the concentration of leaves per acre. Because of the exponential relationship between basal area per acre and quantity of leaves, basal area may be substituted for quantity of leaves in the equation.