Photon scalar irradiance.
Photon scalar irradiance h is defined as the total number of photons per unit time and area arriving at a point from all directions about the point when all directions are weighted evenly. Reasons for considering photon scalar irradiance an optimum measure of radiant energy available for photosynthesis are briefly reviewed. A new technique for the measurement of h and considerations for calibrating an underwater photon scalar irradiance collector on an absolute basis are outlined.