PPFD

PPFD stands for photosynthetic photon flux density. It is a measure of the amount of light in the photosynthetically active radiation (PAR) range that reaches a surface each second, expressed in micromoles of photons per square meter per second (µmol/m^2/s).

PPFD specifically quantifies photons in the 400 to 700 nanometer range that can drive photosynthesis. It is a photon-based metric, not an energy flux, and it represents the density of PAR photons arriving at the surface rather than the total energy.

PPFD is distinct from PPF, the total photosynthetic photon flux emitted by a light source per second (µmol/s). The relationship between PPF and PPFD depends on geometry and losses; for a uniformly illuminated surface, PPFD is approximately PPF divided by the illuminated area, but real-world factors such as distance, beam angle, shading, and reflectivity affect the value.

Measurement is typically performed with a quantum sensor or PAR sensor that converts incident PAR photons into a PPFD reading. Readings are distance- and angle-dependent and should be taken at or near canopy level for horticultural purposes.

In horticulture and controlled environment agriculture, PPFD guides light selection and positioning. Typical indoor ranges span roughly 100 to 900 µmol/m^2/s, with higher values commonly used for flowering stages and light-demanding crops. Plant species and acclimation influence the optimal PPFD for growth and development.

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