I think we might want to get into the weeds (pardon the pun) on Daily light integral (DLI) that
@FelipeBlu mentioned, and there is a good article on the PL Lighting website that discusses both the DLI and the mole requirements for cannabis. I can't link it here because they're not a sponsor, but if you search "Daily Light Integral for cannabis" in Google, it's the first link (after the ad) to pllight.
Felipe and his brother BTzGrow discussed this in (I think) Felipe's auto journal earlier this year so I'm sure he will have more to contribute to the conversation.
PL Lighting posted a clear explanation without a lot of jargon, so I'll copy and paste some of it here:
QUOTE:
Understanding Micromoles
Most growers are aware of the term micromoles (µmol) and even have a number in mind of how many micromoles they want to grow. This number is usually between 850-1000 for indoor crops. Let’s take a minute to discuss where this number comes from. First, what is a micromole? A micromole is simply one millionth of a mole (micro). A micromole measurement of the PAR for your crop is based on the amount of photons being delivered per square meter per second. It is with this measurement of PPF (Photosynthetic Photon Flux) that we can then calculate total moles delivered per day. It is actually this number – total moles – that is important for your crop to produce.
If ‘Moles’ are the important number for determining how much light to put on my crop why does everyone refer to micromoles” Because a micromole is a per second, instantaneous picture of your light level at that moment in time we can measure this with a par sensor. This is why you see so many requests for 900 micromoles on a cannabis crop. Let’s break this down to understand the importance of this micromole number.
Any fruit bearing crop – tomatoes, peppers, etc. will want between 20-40 moles per day to produce. Let’s use tomatoes as an example. If we have a grower who wants to achieve superior growth from his crop, he may want to target our light levels to about 30-32 moles per day. The production curve from a crop in relation to the amount of moles delivered per day sharply increases between 20-30 moles, begins to really level off between 30-40 moles and over 40 begins to hit a point of diminishing returns for your crop. If our tomato grower wants to hit 32 moles per day to maximize his production then we will see how long he plans to light his crop. If he is going to run lights for 16 hours each day we can then do the math to figure out his light levels in micromoles we want to target:
32 moles / 16 hr photoperiod / 60 minutes / 60 seconds = 0.000556 moles
(This gives us the amount of moles delivered per second per square meter)
0.000556 moles x 1,000,000 =
556 µmols per second per meter squared
We will need to design a light layout of 556 µmols
The reason cannabis is a much larger number than the example above is due to the photoperiod requirement of this crop. To initiate flowering of the cannabis bud we need to allow for 12 hours of darkness. Also, currently cannabis is a higher profit margin crop than most produce, so even though the production curve in relation to amount of light starts to level off in the 30 – 40 mole range, most growers will push it out as much as possible. Let’s work our same math above, but start with the requirement of 900 µmols for our indoor cannabis grow with only a 12 hour photoperiod:
900 µmols x 60 seconds x 60 minutes x 12 hours = 38,880,000 µmols
(This gives us the amount of µmoles delivered in a 12 hr period)
38,880,000 / 1,000,000 =
38.88 moles per 12 hr photoperiod
900 µmols will deliver 38.88 moles over 12 hours
Not all growers will be successful growing at 38 moles per day and with that much light in an indoor grow—some will have to battle the heat if growing with HPS, or just the sheer size of the electric bill can be a detriment. Many growers produce very well at 30 moles per 12 hr period (about 700 micromoles).
/QUOTE
Something interesting here is that the 900umol number is based on the 12/12 time period, which is to say that if you are running your lights for 18 hours, they don't need 900umols, allowing you to run at lower wattages during veg. Based on the equation above, you could run at 600umols for 18 hours and also get to 38.88 moles during the 24 hour period.
DLI describes the sum of the per second PPFD measurements during a 24-hour period and is useful for outdoor growers, but mostly for those who grow in greenhouses with supplemental lighting. The DLI for any given place in the USA can be seen on this map, and of course it varies based on time of year:
Let's get back to the PL Lighting information:
QUOTE:
Greenhouse Application
“What does this mean if I am growing in a greenhouse? Should I target 900 µmols in my greenhouse?” This is a question we hear a lot! You are already receiving a good amount of light from sun in your greenhouse but depending on where your house is in the world and what time of year it is, you could receive as little as only 5 moles per day (As is the case in northern climates during winter) or even more light than you may need (as is Arizona in the summer). Let’s use our tomato grower above as a simple example. If our tomato grower is growing in a greenhouse which is located in central Oregon and wants to maintain 32 moles per day on his crop year-round we then have to refer to the above Daily Light Integral (DLI) map for this area.
You can see that through the three darkest months of winter this grower can expect to receive an average of about 10 moles per day from natural sunlight. When designing his light levels for his grow we would recommend supplementing this natural light with 22 moles of lighting per 16 hour photoperiod to achieve his 32 mole target:
22 moles /
16 hrs / 60 mins / 60 secs x 1,000,000 =
382 µmols
382 µmols of supplemental lighting needed
Now let’s do the same for our cannabis grower who is growing in the same location and also would like to achieve light levels of 32 moles per day all year for production. Again, we will allot for the 10 moles of natural light he will receive and target 22 moles with our lighting:
22 moles /
12 hrs / 60 mins / 60 secs x 1,000,000 =
509 µmols
509 µmols of supplemental lighting needed
/QUOTE
So if I am getting 15 moles a day of sunlight in December in L. A. (which I'm not because I'm not growing in an open field, but let's say I am), then I will need an additional 23 moles/day to get to 38 moles. I have about 12 hours to get that additional light (between 8pm and 8am) So I would have:
23 moles / 12 hours / 60 mins / 60 seconds x 1,000,000 = 532 umols of lighting in the shed at night.
However, if I am in flower and I have to get all my lighting into the remaining hours between sunset and the beginning of my 12 hour dark period (8pm-8am), then I would really only have about 3 hours to get the additional umols in. I would need over 2,000 umols to accomplish that, something I haven't got anything close to. Oh well! This is the life of an outside grower in December 
.
