The inverse square law states that intensity from a point source falls in proportion to 1 ÷ distance². Double the distance and you get a quarter of the light. It is exact for a true point source radiating into empty space, and that is the catch: grow rooms have neither.
In practice
A large LED board or bar fixture is closer to an extended source, and tent walls reflect much of the stray light back. Near a wide fixture, PPFD falls much more slowly than the law predicts; far from it, the fixture starts to behave more like a point. A bare HPS bulb with a reflector sits in between.
Example: the law predicts that raising a light from 30 cm to 60 cm (12 to 24 in) cuts PPFD by 75%. Under a 1.2 m wide LED in a reflective tent, a meter might show a drop of only 25–40% at canopy centre. Height also changes spread: a higher fixture gives a more even canopy at a lower peak.
The only reliable answer is to measure PPFD at canopy height in a grid and adjust height or dimmer.
Units and formula
I₂ = I₁ × (d₁ ÷ d₂)² (point source only).
Don't confuse with
- Light spread (beam angle): how widely a fixture distributes light; a separate property from fall-off.
