PPFD is how much light lands on the canopy each second, in µmol/m²/s. DLI is the day's total, in mol/m²/day. To get DLI: DLI = PPFD × seconds of light ÷ 1,000,000, which is the same as PPFD × hours × 0.0036. So 500 PPFD for 18 hours is 32.4, and 800 PPFD for 12 hours is about 34.5.
Fixture wattage and dimmer percentage don't tell you either number. Measure at the canopy.
Our light targets
From our LED and HPS environmental guides. Veg runs 18 hours of light, flower 12.
| Stage | LED PPFD | LED DLI | HPS PPFD | HPS DLI |
|---|---|---|---|---|
| Veg week 1 | 250 | 16.2 | 200 | 12.9 |
| Veg week 2 | 400 | 25.9 | 375 | 24.3 |
| Veg weeks 3–4 | 500 | 32.4 | 500 | 32.4 |
| Flower week 1 | 800 | 34.5 | 800 | 34.5 |
| Flower week 2 | 875 | 37.8 | 850 | 36.7 |
| Flower week 3 | 950 | 41.0 | 900 | 38.8 |
| Flower week 4 | 1,050 | 45.3 | 1,000 | 43.2 |
| Flower weeks 5–6 | 1,100 | 47.5 | 1,000 | 43.2 |
| Flower week 7 | 1,000 | 43.2 | 925 | 39.9 |
| Flower week 8 | 800 | 34.5 | 800 | 34.5 |
| Flower week 9 | 600 | 25.9 | 600 | 25.9 |
Clones run under 24-hour light at 50–80 PPFD, which works out to about 4–7 DLI. Mothers get 300–500 PPFD on 18/6. You can follow the chart without supplemental CO₂.
At the flip
Going from 18 to 12 hours cuts the light period by a third. Don't raise intensity just to replace the lost hours. Set the flower week 1 level and let DLI land where it lands. If you were running brighter than the chart in veg, DLI will drop at the flip; that's expected.
More isn't automatically better. Other growers will tell you to run 1,500–1,800 PPFD in peak flower; we use the chart targets and check leaf temperature and VPD before pushing a room harder. If young plants are stalled, don't turn the lights all the way up. Check the light level and VPD first.
Measure it right
- Our meter: we use the Apogee MQ-610 at the canopy. It reads ePAR, 400–750 nm. A standard 400–700 nm PAR meter like the MQ-500 can read differently under the same fixture. Those ranges are wavelengths, not the meter's maximum reading.
- Center and edges at plant height. Measure again after you raise lights, dim them or train the canopy.
- Light movers: the peak reading as the lamp passes overstates the day. Measure through the full cycle or log the daily total.
- Steady light only: the DLI formula works from a single reading only when the light stays constant all day.
After any real light change, check leaf temperature and VPD too; a light change can shift leaf temperature and water use. See what to check after a light change.
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