What they are:
Daily Light Integral (DLI) is the total amount of photosynthetically active light a crop receives in one day, expressed in mol/m²/day.
It’s based on PAR (Photosynthetically Active Radiation) — the portion of sunlight that plants actually use for photosynthesis.
- PAR is the light in the range 400–700 nanometers, which corresponds to the visible spectrum most useful to plants.
- PPFD (Photosynthetic Photon Flux Density) measures the intensity of PAR at a given moment (µmol/m²/s).
- DLI is the daily integral of PAR.
Weekly Light Integral (WLI): the 7-day moving sum of DLI values, expressed in mol/m²/week.
- Instead of looking at light day by day, WLI captures the cumulative light environment over 7 days, making it useful for weekly crop management decisions.
Why they matter:
- DLI → ensures plants receive enough daily light, explains seasonal growth differences, and guides lighting/shading decisions.
- WLI → smooths out daily fluctuations and supports timing of crop management based on cumulative light (e.g., leaf pruning).
How they are calculated:
1. DLI (mol/m²/day) = Σ(PPFD x Δt)/106
Where:
- PPFD = Photosynthetic Photon Flux Density (µmol/m²/s), measured with a PAR sensor
- Δt = measurement interval in seconds, summed throughout the day
- Division by 1,000,000 converts µmol → mol
2. WLI (mol/m²/week) = Σ (DLI over 7 days)
⭐ Case in practice — OSU and WLI ⭐
Researchers at Ohio State University (OSU) used a smart alarm in Koidra’s DataPilot to assist their leaf pruning experiments based on WLI.
- A Smart Alarm is triggered when the WLI at the lowest leaf drops below a threshold.
- No more than three lowest leaves are pruned a week.
→ Result: WLI-based pruning labor reduced by 35–42% compared to the conventional method, while maintaining a healthier crop balance (Kim, C. and Kubota, C., 2025).
In case you missed it, check out our first episode on Vapor Pressure Deficit (VPD).