Abstract: An apparatus and method for determining the photosynthetic activity of a plant by determining the chlorophyll fluorescence of the plant is provided. The apparatus includes a light impermeable housing, a light to illuminate the housing, a light intensity in the housing and to adjust the light intensity controller. The light intensity may be between zero and 700 micromoles of photons per square meter per second. The monitor may be a photodiode protected by light filters which permit only light of wavelengths corresponding to plant fluorescence emission to pass to the photodiode. A computer analysis of the data obtained, corrects for Dark and Straylight signals in the housing and normalizes the data by correcting for the Fo fluorescence.