Light is the input growers most often get wrong, because it is measured in several confusing ways. Two numbers do most of the work: PPFD and DLI.
PPFD: light right now
Photosynthetic photon flux density (PPFD) is the number of photons in the 400 to 700 nm range that land on a square metre each second, measured in µmol m⁻² s⁻¹. It is what a quantum sensor at canopy height reads. Lux is not the right unit for plants because it weights light by how the human eye sees it.
DLI: light across the day
Daily light integral (DLI) is the total light received in a day, in mol m⁻² d⁻¹:
DLI = PPFD × photoperiod (hours) × 3,600 ÷ 1,000,000
For example, 200 µmol m⁻² s⁻¹ for 16 hours gives 11.5 mol m⁻² d⁻¹. 400 µmol for 12 hours gives 17.3.
Typical targets
- Hardening and low-light foliage: PPFD about 100 to 200, DLI roughly 6 to 12.
- Many ornamentals and leafy crops: PPFD about 200 to 500, DLI roughly 10 to 20.
- Fruiting crops: PPFD about 500 to 1,000 in the light period, DLI roughly 20 to 30.
Ranges vary by species and stage, so treat them as the start of a protocol.
Measuring and managing it
- Measure at canopy height, at several points, because fixtures are brighter below the centre than at the edges.
- Use supplemental lighting when natural DLI falls below target, and shade screens when it is too high.
- Dim or schedule fixtures by stage: low for establishment, higher for growth, tuned for flowering.
- Check fixture heat and moisture rating in humid greenhouses.
Spectrum
Blue light supports compact growth, red drives photosynthesis and stem extension, and far-red influences shape and flowering in some species. Tunable fixtures let the recipe change with the crop stage.
We design and build horticultural lighting as part of our greenhouse service. See also VPD in a greenhouse.