PPFD vs Lux vs Foot-Candles: What Each One Actually Measures
These three units get used as if they were interchangeable. They are not, and the difference matters as soon as you are growing something.
Lux and foot-candles measure brightness to a human eye
Both are built on the lumen, which is already weighted by the photopic luminosity function — a curve describing how sensitive human vision is at each wavelength. That curve peaks in green, around 555 nm, and falls off steeply toward blue and red.
- Lux = lumens per square metre.
- Foot-candles = lumens per square foot. 1 fc ≈ 10.76 lux.
That weighting is exactly what you want when you are lighting a desk. It is the wrong weighting for a plant: deep red light around 660 nm drives photosynthesis strongly but contributes very little to a lux reading, because human eyes barely register it.
PPFD counts photons a plant can use
PPFD (photosynthetic photon flux density) is measured in µmol/m²/s — micromoles of photons in the 400–700 nm range hitting a square metre each second. Every photon in that band counts once, regardless of colour. That is the assumption behind photosynthesis being driven by photon count rather than perceived brightness.
Two related terms show up on spec sheets:
- PPF (µmol/s, no area) is the total photon output of a fixture. Manufacturers sometimes label it “average PPFD” by mistake — check the unit, not the label.
- DLI (mol/m²/day) is PPFD accumulated over a photoperiod. See the PPFD to DLI converter.
Why converting lux to PPFD is always approximate
There is no fixed conversion, because the answer depends on the spectrum of the light you are standing under. The commonly used divisors are:
| Light source | Approximate lux ÷ factor = PPFD | Source |
|---|---|---|
| Daylight | 54 | Apogee Instruments |
| HPS (dual-ended 1000 W) | 77 | Apogee Instruments |
| Metal halide | 71 | Apogee Instruments |
| White LED | 67 | commonly cited — not in Apogee’s table |
Note the last row. Apogee publishes factors for sunlight, HPS, metal halide and fluorescent, but not for white LED — white LED spectra vary too much between products for a single number to hold. The 67 is a widely repeated approximation, and it is the least trustworthy entry in this table.
A red-heavy bloom spectrum will read low in lux relative to its real PPFD, so the same divisor will underestimate it. A blue-white veg spectrum can go the other way. Treat a converted number as a rough bracket, not a measurement — and if you are dialling in hanging height or checking a coral, get a PPFD reading instead of converting one.
Which unit to use when
| Question | Unit |
|---|---|
| Is this room bright enough to work in? | lux or foot-candles |
| Is this windowsill bright enough for the plant? | PPFD, or DLI over the day |
| How far should the grow light hang? | PPFD at canopy height |
| Is the coral getting the light its species wants? | PPFD (often called PAR in reef contexts) |
| How much light did the spot get all day? | DLI |
A note on reef vocabulary: aquarists usually say “PAR” where growers say “PPFD”. PAR strictly refers to the 400–700 nm waveband; a “PAR meter” reports PPFD. When a coral care sheet gives a PAR target of 150, it means 150 µmol/m²/s.