Grow science · 12
Lux to PPFD
A lux meter sees light like a human. Your plant doesn’t.
Lux to PPFD calculator
Rough estimate only. The real factor depends on your lamp’s spectrum. Purple “blurple” LEDs can’t be converted at all.
Why lux isn’t PPFD
Lux weights light by how bright it looks to the human eye, which peaks in green and barely sees deep red and blue. PPFD counts every photon from 400 to 700 nm equally. That’s what drives photosynthesis.
So there is no single conversion: the factor depends on the spectrum of your lamp.
Conversion factors
| Light source | PPFD per lux | 30,000 lux ≈ |
|---|---|---|
| Sunlight | 0.0185 | 555 |
| White / full-spectrum LED | ~0.014–0.018 | 430–540 |
| HPS | ~0.012–0.013 | 370–390 |
µmol/m²/s per lux. Sunlight is a well-established value; lamp values vary with color temperature and model. Red/blue “blurple” LEDs can’t be converted reliably.
How to use a lux meter anyway
- Use it to compare: center vs. edges, before vs. after raising the lamp. Relative changes are more reliable than the absolute number.
- Measure at canopy height, sensor pointing straight up.
- If your lamp maker publishes a PPFD map, compare it with your own lux readings once. Then you have a factor for your lamp.
When to buy a PAR meter
If you dial in flower intensity, run CO₂ or use a lamp with a lot of red, a quantum (PAR) sensor is worth it. For checking even coverage, a lux meter is fine.
Targets per stage: PPFD guide · daily dose: DLI calculator.
Questions growers ask
How do I convert lux to PPFD?
Multiply lux by a factor for your light source: about 0.0185 for sunlight, roughly 0.014–0.018 for white LEDs and 0.012–0.013 for HPS. The result is an estimate. The exact factor depends on the spectrum.
How many lux do plants need in flower?
There is no reliable lux target because the conversion depends on the lamp. As a rough guide, 700–1000 µmol/m²/s PPFD in flower is about 40,000–65,000 lux under white LEDs.
Can I use my phone to measure PPFD?
Only roughly. Phone light sensors measure lux-like values and aren’t calibrated for grow lights. They’re fine for comparing spots in your tent, not for exact numbers.
Why can’t I convert purple LED light to PPFD?
Red and blue LEDs emit mostly light the human eye barely sees, so a lux meter reads very low while the plants get a lot of usable light. The lux value says almost nothing about PPFD there.