A technical overview comparing the key differences between PPFD and illuminance.
| Comparison | PPFD | Illuminance | Notes |
|---|---|---|---|
| Full Name | Photosynthetic Photon Flux Density | Illuminance | — |
| Definition | Number of photosynthetically active photons reaching a unit area per second | Luminous flux received per unit area | — |
| Unit | μmol/m²/s | lux | — |
| Wavelength Range | 400–700 nm | 380–780 nm | — |
| Primary Use | Evaluating plant-lighting environments and photosynthetic light availability | Evaluating lighting for human vision, visual comfort, and safety | — |
| Measurement Position | Plant canopy plane | Work surface or eye level | — |
| Measurement Instrument | PAR meter | Lux meter | — |
Conversion
When the spectral distribution of a light source is fixed, PPFD can be converted to lux, and lux can likewise be converted to μmol/m²/s.
For standard AM1.5 sunlight:
1 μmol/m²/s ≈ 52–54 lux
1 lux ≈ 0.0185–0.0195 μmol/m²/s
AM1.5 refers to the standard solar spectrum at an air mass of 1.5.
Spectral Weighting
PPFD:
Photons within the defined wavelength range are counted equally.
Illuminance:
Light is weighted according to the CIE photopic luminous efficiency function, which represents the sensitivity of human vision. The human eye is most sensitive at approximately 555 nm.
Evaluating Photosynthetic Light
PPFD can appropriately account for the cumulative contribution of photons at different wavelengths when evaluating photosynthetic light.
Lux, however, is based on human visual sensitivity. When a spectrum contains a high proportion of blue or red light, the lux reading may appear relatively low and therefore may not accurately represent the light’s contribution to photosynthesis.
Light Compensation & Light Saturation
PPFD values in μmol/m²/s can be used more directly when evaluating whether the light reaching a plant is around its light compensation point or light saturation point.
Lux is less suitable for this purpose. In spectra rich in blue and red wavelengths, lux readings can be relatively low, making them less reliable for evaluating whether photosynthetic light levels have reached these thresholds.
