DLI, or daily light integral which is a measurement of the total amount of usable light that reaches a plant every day. This is something that greenhouse growers are concerned with since the amount of light they get each day is weather dependent. For indoors, if you are using a grow light only, you will know the output of your light every day and PPFD is all you need to know. The DLI is a cumulative measurement of all the photons that reached the plants’ canopy over the course of the 24 hour day. This is expressed as moles per square meter per day (mol/m2/d). A simple analogy – Think of DLI as the total amount of snowfall after a blizzard as opposed to how fast the snow was accumulating (which would be PPFD in this analogy). Like PPFD, DLI is a spot measurement, so it is important to take multiple readings from several different locations within a coverage area.