 PPFD is an explicit measure of the quantity of photons hitting a surface per square meter per second, a more accurate way to measure how plants interact with photons.Another useful way to measure light quantity is through the daily light integral, or DLI.The DLI takes into account the PPFD and the total number of hours a plant is exposed to that PPFD to get the total quantity of photons per day, in units of mol m−2d−1.