DLI (daily light integral) is a method to understand how much light energy your plant is receiving each day.
DLI is measured in mols/m2d (moles per meter squared per day) and is calculated by multiplying the PPFD (photosynthetic photon flux density) of your light by the daily photo period and then divided by the number of micromoles in a mole.
Since PPFD is measured in umols/m2s (micromoles per meter squared per second) first you need to break down one hour into seconds. It comes out to 3600 seconds per hour.
Second, you need to measure your PPFD at multiple spots on your canopy and take the average. If you don’t have a fancy meter this is fine. Just use the manufacturer data sheet and measure the distance from the canopy to the light and find the ppfd measurement in the data that should have come with your light. So let’s just say your light has a ppfd of 1000 umols/m2s at 18".
Now we just look at the lighting schedule. If you are using an 18/6 lighting schedule for veg your photoperiod would be 18.
Now, we can figure out your veg DLI and compare it to what is needed for that growth stage to see if you need more or less light.
So to figure the DLI with the above information we would work it out like this: 1000 x (3600x18) ÷ 1,000,000.
That would come out to 64.8 mols/m2d.
Cannabis requires between 20 and 30 mols/m2d to grow and yield with decent results during veg and flower. The above measurement is over double what is needed.
You can use this method to adjust your light height and ppfd values through each lighting stage to ensure you are receiving at least 20 to 30 mols/m2d with the exception of seedlings, clones, and mothers.
Here are the minimum optimal DLI values for stages of growth.
Seedling/clone/mother-12.96 mols/m2d
Vegetative-25.92 mols/m2d
Bloom/flower-25.92 mols/m2d
The maximum values can easily exceed 80 mols/m2d but generally anything in excess of approximately 65 mols/m2d will require additional Co2 to take advantage of the light intensity for optimal growth.
Happy growing! 
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