YielixDLI, PPFD and Lighting Hours
Begin at the crop canopy
Daily light integral (DLI) is a day’s photosynthetic light in mol/m²/day; PPFD is instantaneous photon flux in μmol/m²/s. A lamp schedule can turn measured canopy PPFD into a daily contribution, but cannot repair a measurement taken above the crop or at one unusually bright spot.
Measure lamp PPFD at canopy height with daylight excluded at representative bench positions. Record fixture height, spacing, dimming and crop height: fixture PPF (μmol/s) is not PPFD received per square metre.
Keep daylight and lamps as separate measurements
Use DLI measured inside the greenhouse at crop canopy over the same day. Outdoor DLI is not automatically crop DLI because glazing, framing, dirt, curtains and shade change transmission. Leave natural DLI unknown rather than label an outdoor reading as crop light.
Measured natural photons and lamp photons can be added for planning, but do not prove uniformity, crop response or profitability. The DLI calculator keeps the portions visible.
Formula and units
This is a constant-light approximation. For dimmed, staged or daylight-linked fixtures, calculate each constant period or use a defensible time-weighted average.
lamp DLI = 0.0036 × PPFD × lamp hours [mol/m²/day]
total DLI = lamp DLI + natural canopy DLI [mol/m²/day]
0.0036 = 3600 s/h ÷ 1,000,000 μmol/mol
PPFD is μmol/m²/s and lamp hours are h/day. The formula adds photons; it does not model efficiency, ageing, temperature or electricity cost.
Worked example: check total light and the window
Use the verified calculator fixture: 300 μmol/m²/s canopy PPFD for 14 h/day, 10 mol/m²/day natural canopy light, a 22 mol/m²/day planning target and a 12-hour maximum window.
- Lamp DLI:
0.0036 × 300 × 14 = 15.12 mol/m²/day. - Total:
15.12 + 10 = 25.12 mol/m²/day. - Supplemental deficit:
max(0, 22 − 10) = 12 mol/m²/day. - Runtime:
12 ÷ (0.0036 × 300) = 11.1111 h/day, about 11 h 7 min.
The runtime fits the 12-hour photoperiod limit. Above that limit, do not call the target met: change measured PPFD, the sourced target, or the operating plan. A positive deficit with zero PPFD has no finite runtime.
Enter canopy PPFD, measured daylight and your own photoperiod limit
Daily Light Integral (DLI) Calculator
Calculate greenhouse daily light integral (DLI) in mol/m²/day from canopy PPFD, lamp hours and measured natural light, or solve the supplemental lighting hours needed to reach your target.
Use a target as a stated assumption
The example’s 22 mol/m²/day is arithmetic, not a recommendation. A target belongs to a crop, cultivar, stage, objective and sourced plan. Once feasible, the greenhouse ROI calculator and CO₂ enrichment ROI calculator can examine separate investment assumptions; neither predicts yield.
Assumptions and limitations
- Lamp PPFD remains constant during entered runtime.
- Natural DLI, lamp reading and target describe the same canopy area and day.
- Re-measure as fixtures age, lenses soil, crop height changes or screens deploy.
Common mistakes
- Using outdoor DLI as canopy daylight or fixture PPF as canopy PPFD.
- Using one bright spot rather than a representative canopy average.
- Ignoring the photoperiod limit or making a crop claim from a photon total.
Sources
The 0.0036 conversion and supplemental run-time method follow the University of New Hampshire Extension supplemental lighting run time worksheet. Use a crop- and stage-specific source for any target.