Daily light integral (DLI)
The total photosynthetically active photons reaching one square meter over a day, expressed in mol·m⁻²·d⁻¹. DLI combines light intensity with duration.
Daily light integral accumulates PPFD PPFD Photosynthetic photon flux density: the number of photons from 400 to 700 nanometers reaching one square meter each second, expressed in µmol·m⁻²·s⁻¹. across the photoperiod Photoperiod The duration and timing of light and darkness in a repeating daily cycle. Photoperiod-sensitive cannabis uses night length as a flowering signal. . A constant 900 µmol·m⁻²·s⁻¹ for 12 hours delivers about 38.9 mol·m⁻²·d⁻¹ because the photon flux is summed across 43,200 seconds.
The unit mol·m⁻²·d⁻¹ means moles of photons per square meter per day. It reports the daily total, while PPFD reports the rate at a particular moment.
Canopies with the same peak PPFD can receive different DLIs when their photoperiods or light variation differ. Plant response also depends on spectrum, canopy distribution, temperature, carbon dioxide, cultivar, and developmental stage, so DLI is not a yield prediction by itself.