PPFD is the amount of light photons that hit a surface (your plant canopy) in one second. It’s measured in units called micromoles (μmols). Simply speaking – it’s the strength of the light that reaches your plants.
The light loses strength as it travels from source to endpoint (from grow light to plant). You’ll notice on most grow light intensity charts, the PPFD numbers decrease with increased distance from the light.
That’s why paying attention to your grow light hanging