Dr. Bruce Bugbee, president of Apogee Instruments and professor at Utah State University, explains the differences and evolution of the terms PAR (photosynthetically active radiation), PPF (Photosynthetic Photon Flux), PPFD (Photosynthetic Photon Flux Density), and PFD (Photon Flux Density). The meaning of these terms are often misunderstood and sometimes used interchangeably. Dr. Bugbee, one of the world's leading experts on crop physiology and photobiology explains how these terms came to be, how they are used now, and how their definitions are changing as research goes forward.
VIDEO CONTENT
00:40 1960s: Researchers measured radiation only with a pyranometer, which included all shortwave radiation from the sun (280 to 2800 nm), but only a small portion of that (400-700 nm) is used by plants.
01:59 We begin with the first term that evolved - photosynthetically active radiation (PAR). PAR is the light from 400 to 700 nm that is used in photosynthesis. It's not a measurement or metric, it defines a type of light.
03:05 1970s: A researcher clarifies that 400 to 700 nm is not referring to energy, it's the number of photons that cause photosynthesis. The next term to evolve is photosynthetic photon flux (PPF).
04:28 Language continuously evolves. As a result, PAR and PPF were used interchangeably when referring to the number of photons that cause photosynthesis.
05:19 PPFD and PPF have been used interchangeably for 50 years; however, these terms continue to evolve. PPF refers to an AMOUNT per TIME. Photosynthetic photon flux density (PPFD) refers to an AMOUNT per AREA per TIME.
07:57 Dr. Bugbee, about three years ago, started using PPFD in his publications after discovering different disciplines use the acronyms differently.
10:20 PAR is still used in common speech, and is used as a more generic term to refer to energy flux or photon flux. PPFD is used for an amount per area per time. PPF is used for the the amount per time.
11:00 The difference of PPF and PPFD further explained. PPFD helps define the intensity of the light over a given area.
13:46 When reading literature, make sure to always check the units. It should always be micromoles per meter squared per second. The older literature used to call that PPF.
CONCLUSION
14:30 There's a new term emerging that refers to photons beyond PAR - Photon flux density (PFD). Far-red and ultraviolet radiation have been discovered to also cause photosynthesis and effect plant growth. With PFD, we are no longer constrained to 400 to 700 nm. E.g., PFD can mean 300 to 800 nm.
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