Daily Light Integral (DLI)

Light in a growing space gets measured three different ways and only one of them is about plants. Watts measure what the fixture costs to run. Lux and foot-candles measure brightness weighted for the human eye, which peaks in the green that leaves reflect. PPFD counts photons between 400 and 700 nm — the band photosynthesis actually consumes — and DLI is that count totalled over a day.

The conversion is one multiplication, but the constant hides a unit change: $DLI = PPFD \times t$, and because PPFD is in micromoles per second while DLI is in moles per day, the working shortcut is $DLI = PPFD \times \text{hours} \times 0.0036$. The 0.0036 is 3600 seconds per hour divided by a million micromoles per mole. So 600 μmol/(m²·s) for 18 hours gives 38.9 mol/(m²·day).

Targets are well established by crop. Lettuce and herbs finish at 12–17. Tomatoes and peppers want 20–30. Cannabis in flower is generally run at 35–45, above which most growers find CO₂ enrichment is needed before the plant can use any more. For scale: a clear June day in southern Canada delivers 45–55 mol/(m²·day) outdoors, and a December day about 5. That thirty-fold seasonal swing is the entire economic case for greenhouse supplemental lighting.

Because DLI is a total, intensity and duration trade against each other freely — 900 μmol for 12 hours and 600 μmol for 18 both deliver the same 38.9 mol. For a day-neutral crop, choosing between them is purely an economic question about fixture count against electricity hours. For a photoperiod-sensitive crop it is not a free choice at all, because the plant is measuring the length of the dark period to decide whether to flower, and no amount of light integral overrides that signal.