Maximum Yield Industry News October/November 2020 - Page 32

DEFINING by Ian Ashdown , P . Eng ., FIES



Proper science , not marketing , should be the only priority when defining lighting metrics .

There has been some discussion online and in presentations recently about the issue of photosynthetic photon flux . The argument goes as follows :

1 . Photosynthetically Active Radiation ( PAR ) is somewhat arbitrarily defined as optical radiation within the spectral range of 400nm to 700nm .
2 . Exposing plants to far-red radiation ( defined as 700nm to 800nm ) results in an increase in the rate of photosynthesis — the “ Emerson effect ” that was first noted in 1957 and confirmed by recent research .
3 . Many horticultural luminaire manufacturers are now including far-red ( 725nm ) LEDs in their products .
4 . The photosynthetic photon efficacy ( PPE ) of these luminaires is penalized by the definition of PAR because the far-red radiation is not taken into consideration .
5 . The definition of PAR therefore needs to be changed to allow fair comparison of these products .
The one-word answer to this argument is … no .
32 Maximum Yield