Photosynthetically active radiation

The photosynthetically active radiation (English: Photosynthetically Active Radiation, PAR short or PHAR ) is the range in the spectrum of solar radiation that can be used by photosynthetic organisms. The PAR is usually given of 400-700 nm in W / m². This area largely coincides with the area of ​​human visible radiation ( 380-780 nm), which is about 50% of the global radiation accounts.

The photosynthetically active radiation is composed of photons with very different energy together (blue: energetic, red: low energy ). Therefore, a tipped with filters radiation sensor must be used so that the short-wavelength, high-energy spectral regions are given the same weight as the long-wavelength, low-energy to determine the PAR. So the photons between 400 and 700 nm can be registered approximately equivalent. Because of the direct stoichiometric relationship between absorbed photons ( in the range of 400-700 nm) and photosynthetic CO2 fixation, the photon flux density (English Photosynthetically Active Photon Flux Density, PPFD or short PFD) in biology became standard. It is measured in contrast to PAR, in mol / ( m² s ).

Dividing the APAR, the absorbed PAR by the PAR itself, the so-called FPAR results ( fraction of PAR absorbed by canopy ) strongly correlated with the NDVI.