The daily light integral (DLI) is the number of photosynthetically active photons (photons in the PAR range) accumulated in a square meter over the course of a day. It is a function of photosynthetic light intensity and duration (day length) and is usually expressed as moles of light (mol photons) per square meter (m) per day (d), or: mol·m·d.
DLI is usually calculated by measuring the photosynthetic photon flux density (PPFD) in μmol·m·s (number of photons in the PAR range received in a square meter per second) as it changes throughout the day, and then using that to calculate total estimated number of photons in the PAR range received over a 24-hour period for a specific area. In other words, DLI describes the sum of the per second PPFD measurements during a 24-hour period.
If the photosynthetic light intensity stays the same for the entire 24-hour period, DLI in mol m d can be estimated from the instantaneous PPFD from the following equation: μmol m s multiplied by 86,400 (number of seconds in a day) and divided by 10 (number of μmol in a mol). Thus, 1 μmol m s = 0.0864 mol m d if light intensity stays the same for the entire 24 hour period.