PAR
Photosynthetically active radiation – equal weighting of all wavelengths between 400 and 700 nm
DLI
Integral of daylight (of photosynthetically active radiation)
PPFD
Density of the flow of photosynthetic photons on a surface
What light plants need: Active Photosynthetic Radiation (APR)
How much light do plants need: Full-day light – DLI
The required intensity is derived from the total light needs of the plant during the day. An index for the different light requirements is the diurnal light integral, called DLI (Daily Light Integral). It describes how many photons reach a certain area in 24 hours.
For several species of plants, it is known how much light they need or can sensibly use over the course of a day.
Calculation of the DLI
PPFD in µmol/m²s [number of photons per second per m²]. * 3600 [Number of seconds in an hour] * t(h) [Number of light hours in a day] / 1,000,000 = DLI in mol per day
What actually reaches the plants – PPFD
These theoretical values are very good for determining what a light source emits in photons and how efficiently it works. What, in turn, reaches the plants cannot be determined using theoretical values, but depends on the individual use scenario.
The so-called photosynthetic photon flux density (PPFD) indicates how many photons from the PAR region impact one square meter. This should be based on the needs of the plants (see DLI) and the goal should be to achieve the most uniform distribution possible.
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