mole per square meter per day
Photon integralexact by definition
The mole per square metre per day is the daily light integral, DLI — the total count of photosynthetically active photons a surface receives over a whole day. It is PPFD integrated over the photoperiod, and it is the single number that best predicts how a crop will grow.
Why it is written : Written mol/(m²·day), but dimensionally this is just moles per square metre: the "per day" names the accounting period rather than a dimension. Two lighting plans delivering the same DLI over 12 and 18 hours deliver the same total photons.
| 1 | 1 |
About the mole per square meter per day
DLI is where photon flux becomes agronomy. A grow light's PPFD tells you how bright it is at an instant; DLI tells you how much light the crop actually banked, and it is the quantity that correlates with yield.
The conversion is one line: DLI = PPFD × photoperiod in seconds ÷ 1,000,000, or more conveniently DLI = PPFD × hours × 0.0036. So 600 μmol/(m²·s) for 18 hours is 600 × 18 × 0.0036 = 38.9 mol/(m²·day).
Target ranges are well established. Lettuce and herbs finish well at 12–17. Tomatoes and peppers want 20–30. Cannabis in flower is usually run at 35–45, and above that the plant generally needs supplemental CO₂ before it can use the extra photons. Outdoors, a June day in southern Canada delivers about 45–55 mol/(m²·day) and a December day around 5 — which is precisely why greenhouse supplemental lighting exists and why it is a winter expense.
The useful consequence of DLI being a total is that intensity and duration trade against each other. The same 38.9 mol can be delivered at 900 μmol for 12 hours or 600 for 18. For a day-neutral crop that choice is an economic one about fixture count against electricity hours. For a photoperiod-sensitive crop it is not a choice at all, because the plant is reading the length of the dark period to decide whether to flower.
One mole per square meter per day in every photon integral unit
| 1 | mole per square meter per day (mol/(m²·day))this unit | |
| 1,000 | millimole per square meter per day (mmol/(m²·day)) |