A measure, widely used in studies of photosynthesis in plants, of the quantity of light. It is defined as the quantity of radiant energy falling on a small sphere divided by the cross-sectional area of the sphere, and can be expresssed in two ways. Photon fluence (units: mol m⁻²) measures the number of photons falling on the sphere, whereas energy fluence (units: J m⁻²) measures the energy incident on the sphere. Hence the corresponding rates of input are the photon fluence rate (units: mol m⁻² s⁻¹) and the energy fluence rate (units: J m⁻² s⁻¹). Many radiation-measuring instruments now used in plant biology record only the part of the spectrum utilized by plants, i.e. radiation of wavelength in the range 400–700 nm; this is termed photosynthetically active radiation (PAR), and fluence rates used in practice are often qualified in this way.