DLI is the number I care about when comparing light plans: it combines intensity and time into one daily total. It does not make a short, intense day equivalent to a longer, gentler one in every respect.
| Measure | What it tells you | Units |
|---|---|---|
| PPFD: photosynthetic photon flux density | Photosynthetically active light arriving at the canopy right now. | µmol/m²/s |
| DLI: daily light integral | The total delivered to that surface over a day. | mol/m²/day |
The calculation
If light output is steady:
DLI = PPFD × hours × 3600 / 1,000,000
The 3600 converts hours to seconds; the 1,000,000 converts micromoles to moles. The shorter version is DLI = PPFD × hours × 0.0036.
For an illustrative 700 µmol/m²/s canopy reading on an 18-hour schedule:
700 × 18 × 0.0036 = 45.4 mol/m²/day
What can make the number misleading
A center-only reading. The center is often the brightest spot. Readings from several canopy positions give a more useful average than that single peak.
Treating the daily total as an intensity limit. The same DLI can come from lower PPFD over a longer day or higher PPFD over a shorter day. High instantaneous intensity can still cause stress, and plant stage, heat, and water status still matter.
DLI makes the light plan easier to compare. It is not permission to ignore how the plant is responding.
Use the DLI calculator and chart to convert canopy PPFD and photoperiod or view the existing stage-by-stage targets.