PAR Charts

How Grow Light PPFD Changes at Different Heights

Hanging height is the single biggest lever you have over PPFD once a fixture is bought. Raise the light and the whole canopy dims but evens out; lower it and the center spikes while the edges lag. Here is how the fall-off actually behaves, with numbers from manufacturers’ own charts.

The inverse square rule, and where it breaks

The textbook rule for a point light source is the inverse square law: intensity falls with the square of the distance, so doubling the distance drops PPFD to about a quarter, not a half. Grow light guides state it plainly — going from 1 ft to 2 ft leaves you with roughly a quarter of the light (California LightWorks; Gorilla Grow Tent).

The catch is that a modern LED grow panel is not a point source. It is a wide board covered in diodes, and close to the canopy it behaves more like an area source. In that near field it loses intensity noticeably slower than the inverse square rule predicts. You can see this in the makers’ own multi-height maps.

What the real charts show

Take the Spider Farmer SF4000’s own PPFD map. The center reads 1821 µmol/m²/s at 12 in and 1371 at 18 in. The inverse square rule, applied to that 12→18 in change, would predict 1821 × (12/18)² ≈ 809 — but the chart shows 1371. The panel falls off far more gently than a point source would. (That 809 is our own arithmetic on Spider Farmer’s figure, shown to make the gap concrete; it is not a measurement.)

The Mars Hydro FC6500’s chart shows the same pattern across three heights. Its center peak runs about 1320 µmol/m²/s at 8 in, ~1265 at 12 in, and ~1150 at 16 in. Inverse square from 8 in to 16 in (a doubling of distance) would predict roughly a quarter of the 8 in peak — about 330 — yet the chart holds near 1150. Again, a wide board does not obey the point-source rule near the canopy.

The practical reading: the inverse square rule is a useful worst case for how much you will lose by raising a light, but a real panel usually keeps more than it predicts. Trust the maker’s map at your actual height over any formula. Both grids, with every value sourced, are on the SF4000 data page and the FC6500 data page.

Height buys uniformity, not just dimming

Raising a light does two things at once: it lowers the peak and it flattens the field. On the SF4000, going from 12 in to 18 in drops the center from 1821 to 1371, but the 4×4 ft corners rise from about 270 to about 360 as the beam spreads. The center-to-corner ratio tightens from roughly 7:1 toward 4:1. The FC6500 chart does the same: at 8 in the center peaks near 1320 while corners sit near 950; at 16 in the whole field collapses into a 750–1150 band — a lower but far more even canopy.

So height is a trade, not just a dimmer:

  • Lower — higher peak PPFD in the center, steeper fall-off, hotter middle and darker corners. Good for a small, central canopy or a demanding flowering stage.
  • Higher — lower peak, flatter field, corners and center closer together. Good for filling a full tent evenly or for seedlings that a hot center would stress.

How to use this

Set height from the map at the distance you actually hang, not from a formula. Confirm the reading at the canopy with a meter if you have one, or estimate it — see how to estimate PPFD without a quantum meter. Then match the result to your crop’s stage in is my grow light strong enough, and read the map correctly using how to read a PPFD chart.

Sources

Get notified when the app launches

We are building a phone app that measures PPFD and PAR with per-model calibration. Leave your email to hear when it is ready. No spam.