What Is DLI and How Is It Calculated From PPFD?
A practical introduction to what is dli and how is it calculated from ppfd for growers planning or operating commercial controlled-environment farms.
Daily light integral (DLI) measures the total number of photosynthetically active photons received by a square metre of crop canopy during one day. It is commonly expressed in mol/m²/day. Photosynthetic photon flux density (PPFD) describes the photon delivery rate at the crop surface in µmol/m²/s. These quantities are related but are not interchangeable.
When PPFD is approximately constant during a lighting period, DLI = PPFD × hours of light × 3,600 ÷ 1,000,000. For example, a crop receiving 200 µmol/m²/s for 16 hours gets about 11.52 mol/m²/day. If light intensity is not uniform throughout the day, calculate or integrate the actual readings instead of relying on one instantaneous measurement.
DLI is useful when comparing different schedules, lamp layouts and crops. However, a target from one lettuce study is not automatically appropriate for basil, strawberries, seedlings or medicinal plants. Spectrum, temperature, carbon dioxide, plant density and crop stage influence the amount of light that can be used productively.
A commercial lighting plan should include a measured PPFD map, fixture spacing, daily operating hours and electricity use. Compare average and minimum canopy-level values, not only the brightest measurement directly below a luminaire. Cornell's hydroponic lettuce material explains how cumulative photon delivery can be used in practical growing programs.
Technical reference: Cornell University CEA Program — https://cea.cals.cornell.edu/files/2019/06/Cornell-CEA-Lettuce-Handbook-.pdf