Daily light integral (DLI) refers to the number of photosynthetically active photons (individual particles of light in the 400-700 nm range) that are delivered to a specific area over a 24-hour period. In other words, it is the amount of light intensity the plant receives each day.
The equation for converting Photosynthetic Photon Flux Density (PPFD) to DLI, assuming constant PPFD, is below.
DLI (mol/m²/d) = PPFD*Light-hours/day* 3600s
Light-hours is the number of hours in a day active photons are delivered to the target area, measured in hours.
3600s is one hour being converted to seconds.
For example, the PPFD that a plant needs is 600μmol/m²/s. If the plant can get a total 15 hours of lighting a day, then the DLI is 600μmol/m²/s*15h*3600s=32400000μmol/m²/d=32.40mol/m²/d.
If the plant needs a total 1000mol DLI throughout its growing stages and we give it 10mol each day, then it will get 100 days to harvest. But if we give it more mol, it will grow faster and can be harvested in a shorter time.
However, it is far more complicated due to the changeable weather and seasons. So, it is very essential to make reasonable supplemental lighting plans according to the species that you grow in different seasons.
Figure A is the DLI in December in the United States. It is 3mol per day on cloudy days and 9mol per day on sunny days. For most species, this DLI is not enough to reach their fastest growth, so it is necessary to use LED grow lights to fill up the gaps.
Figure B is the DLI of the US in June. The DLI is 12mol per day on cloudy days and that is enough. Of course, it is OK to provide supplementing light. On sunny days, it is 26mol per day, which is great for plants that prefer much light, but some plants that can't be placed directly under the light will need shading.