Light Intensity at a New Distance

Raising a lamp spreads its light over more area, and for a point source the spread follows the inverse square: double the distance and the intensity falls to a quarter. That is why moving a fixture 15 cm can undo a fortnight of careful feeding, and why PPFD must always be quoted with the distance it was measured at.

The honest caveat is that grow lights are frequently not point sources. A compact HID bulb or a small COB behaves close enough to one. A 1.2 metre LED bar array does not — near the canopy it behaves like an extended source, and intensity falls far more gently than the square law predicts. The rule of thumb is that the inverse square only holds once the distance exceeds roughly the largest dimension of the emitting surface, so for a wide panel, the equation over-predicts the loss from raising the fixture at typical hanging heights.

Manufacturers exploit exactly this ambiguity. A quoted "PPFD 1,200" is meaningless without both the distance and the measurement position, and the number is almost always taken directly beneath the centre of the fixture at close range, which is the most flattering point in the space. What matters agronomically is average canopy PPFD and its uniformity. Take a nine-point grid across the footprint: the ratio of minimum to average is the number that predicts whether the corner plants will be worth harvesting.