Sometimes called Photosynthetically Active Radiation. Photon flux density (photons per second per square meter) within the visible wavelength range (usually 400 to 700 nm). It indicates the total energy available to plants for photosynthesis, and is thus a key parameter for biological and ecological studies.
Typically it is measured with a broadband detector that has nominally constant quantum response throughout the visible wavelength range, and nominal spherical angular response (to measure scalar irradiance). However the efficiency of photosynthesis is not constant for all wavelengths as implied by the definition of PAR. A much more thorough characterization can be obtained by measuring the spectral scalar irradiance with a spectroradiometer such as a HydroRad with scalar irradiance collectors.