PPFD / lux / DLI

Formula — PPFD / lux / DLI

A lux meter weights light by human eye sensitivity, which peaks in green — exactly where horticultural lamps emit least. Turning lux into photosynthetic photon flux density therefore needs a conversion factor specific to the spectrum, plus a correction for the meter itself.

• PPFD [µmol/m²/s] = lux × spectrum factor × sensor factor
• DLI [mol/m²/day] = PPFD × hours × 3600 ÷ 1 000 000
• lux = PPFD ÷ (spectrum factor × sensor factor)

• Spectrum factors: white LED 0.0166–0.0182, blurple LED 0.0195, HPS DE 0.0135, HPS SE 0.0125, CMH 0.0145, T5 0.013, sunlight 0.0185 µmol per lux.
• Sensor corrections: dedicated lux meter ×1.0, Photone with diffuser ×1.05, Android phone with diffuser ×0.9.

Worked example — PPFD / lux / DLI

A lux meter reads 45 000 lux at canopy height under a white LED bar with a factor of 0.017. PPFD is 45 000 × 0.017 = 765 µmol/m²/s. Over an 18-hour photoperiod the daily light integral is 765 × 18 × 0.0036 = 49.6 mol/m²/day.

For photoperiod plants in veg the DLI target is 15–30 mol, so that fixture is far too intense for vegetative growth: dropping the photoperiod to 18 h at ~350 PPFD (≈20 500 lux) lands at 22.7 mol, right in the band. In flower at 12/12 the same 765 PPFD gives 765 × 12 × 0.0036 = 33 mol, which sits comfortably inside the 30–45 mol flowering window. The same 45 000 lux under single-ended HPS would only be 45 000 × 0.0125 = 563 PPFD — a 26% difference from the spectrum factor alone, which is exactly why a lux figure with no lamp attached is meaningless.

Typical ranges — PPFD / lux / DLI

Take readings at several points — the centre of a tent is often 30–50% brighter than the corners, and an average that hides that spread will overstate the usable canopy. Lux-derived PPFD is an estimate with a realistic error of ±10–15%; a quantum sensor is the reference when precision matters.