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Getting a usable PPFD figure without lying to yourself

v1 — 2026-07-07.

The DLI calculator is exactly as trustworthy as the PPFD you type into it. PPFD — photosynthetic photon flux density — counts the photons between 400 and 700 nm landing on a square meter each second. It is the input plants actually respond to, and it is the number this site's calculator expects. Here are the honest ways to get one, best first.

A quantum PAR meter

A quantum meter measures PPFD directly and is the only measurement-grade option. Take readings at canopy height, at several points across the growing area — light falls off fast toward the edges — and use a representative value, not the hot spot under the fixture's center. If your canopy will sit under very different intensities, compute DLI for the dimmest and brightest spots and treat the result as a range.

The manufacturer's PPFD map

Reputable fixture makers publish PPFD maps: a grid of readings at a stated hanging height over a stated footprint. This is brochure-grade rather than measurement-grade — maps are produced under ideal conditions, without walls, dust or driver ageing — but a map at your actual hanging height is a fair starting estimate. Read the map's average for your footprint, not its center peak, and expect your real canopy to receive somewhat less.

What a lux meter cannot tell you

Lux weighs light by human eye sensitivity, not by photosynthetic response, so there is no universal lux-to-PPFD conversion: the correct factor depends on the exact light spectrum, and using a borrowed factor from a different spectrum can be badly wrong. If a lux reading is all you have, treat any converted figure as a rough guess — and prefer the manufacturer's map. The calculator will refuse obviously lux-scaled magnitudes (above 3000), but it cannot detect a wrong conversion that lands in range.

From PPFD to a plan

Once you have a defensible PPFD and your photoperiod, the calculator gives you DLI — moles of photons per square meter per day — which is the quantity worth planning against. The conversion itself is exact; the honesty of the result was decided when you chose the input.

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