Daily light integral (DLI)
The total number of usable photons delivered to a square meter of canopy over a full day, in mol·m⁻²·d⁻¹. Intensity multiplied by time, so it captures what PPFD alone cannot.
DLI is PPFD accumulated over the photoperiod. A room running 900 µmol·m⁻²·s⁻¹ for 12 hours delivers about 38.9 mol·m⁻²·d⁻¹, since 900 micromoles per second across 43,200 seconds works out to roughly 38.9 moles.
The unit is read the same way as PPFD, with two changes. The d⁻¹ means per day rather than per second, and the count is in whole moles rather than micromoles, because a full day of accumulated light would otherwise run to an unwieldy number of micromoles.
The distinction matters because two rooms can share a PPFD figure and deliver very different daily totals if their photoperiods differ. DLI is the honest basis for comparing light between facilities, and it is the number that tracks yield most closely.