# Inverse square law

> The inverse square law says light from a point source falls with the square of distance; grow fixtures are not point sources, so it only roughly predicts PPFD.

- Canonical: https://genetixbay.com/knowledge/glossary/inverse-square-law.html
- Also written: inverse-square law, inverse square rule, 1/r² law, light distance rule
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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](https://genetixbay.com/knowledge/glossary/led.md) 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](https://genetixbay.com/knowledge/glossary/ppfd.md) falls much more slowly than the law predicts; far from it, the fixture starts to behave more like a point. A bare [HPS](https://genetixbay.com/knowledge/glossary/hps.md) 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](https://genetixbay.com/knowledge/glossary/even-canopy.md) 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.

## Related terms

[PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [Light intensity](https://genetixbay.com/knowledge/glossary/light-intensity.md), [LED](https://genetixbay.com/knowledge/glossary/led.md), [HPS](https://genetixbay.com/knowledge/glossary/hps.md)

## Read more

- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)