PPFD and DLI
Plants use light in the 400 to 700 nm range, called photosynthetically active radiation. Two measurements describe it:
- PPFD (photosynthetic photon flux density): the number of photons reaching a square meter each second, in µmol/m²/s. It is an instant reading, like a speedometer.
- DLI (daily light integral): the total number of photons received in a day, in mol/m²/day. It is like the distance driven in a day.
DLI = PPFD × hours of light × 3,600 ÷ 1,000,000
Example
A grow light delivers 400 µmol/m²/s at canopy level for 16 hours:
DLI = 400 × 16 × 3,600 ÷ 1,000,000 ≈ 23 mol/m²/day
To reach a target of 20 mol/m²/day over the same 16 hours, you would need about 347 µmol/m²/s.
Using a lux meter
Lux meters measure brightness as the human eye sees it, not photons, so the conversion depends on the light source. Approximate factors, in µmol/m²/s per lux:
| Source | Factor |
|---|---|
| Sunlight | about 0.0185 |
| White LED | about 0.015 |
| Fluorescent | about 0.0135 |
| High-pressure sodium | about 0.0122 |
These are rough values; a quantum (PAR) meter gives a much more reliable reading.
How much light do plants need?
| DLI (mol/m²/day) | Plants |
|---|---|
| 3 to 6 | low-light houseplants: ferns, pothos, snake plants |
| 6 to 12 | most houseplants, herbs, lettuce seedlings |
| 12 to 20 | leafy greens, lettuce, many bedding plants |
| 20 to 30 or more | tomatoes, peppers, cucumbers, cannabis, roses |
Natural light for comparison
Outdoors in full summer sun, DLI can reach 50 to 60 mol/m²/day; in winter in the northern United States it may drop to 5 to 10. A greenhouse typically passes 50% to 70% of outdoor light, and a sunny windowsill often provides only 2 to 8, which is why seedlings indoors often grow leggy.
Tips
- Measure PPFD at the top of the canopy, at several points, and average them: light is weaker at the edges.
- Adding hours of light raises DLI without increasing intensity, but some plants need dark periods to flower.
- Raise light levels gradually to avoid stressing plants used to low light.