PPFD (photosynthetic photon flux density) is the rate at which PAR photons strike a surface, in micromoles per square metre per second (µmol/m²/s). It is what a quantum (PAR) meter reads, and it is the number to use when setting light height and dimmer levels. Full summer sun at noon is roughly 2,000 µmol/m²/s.
In practice
Typical indoor targets without added CO2:
| Stage |
PPFD (µmol/m²/s) |
| Clones and seedlings |
100–300 |
| Veg |
400–700 |
| Flower |
700–1,100 |
| Flower with CO2 at 800–1,200 ppm |
up to about 1,500 |
Cannabis keeps converting extra light into yield further than most crops. Rodriguez-Morrison and colleagues (2021) saw flower yield rise roughly linearly up to about 1,800 µmol/m²/s, and Llewellyn and colleagues (2022) found yield proportional to intensity. Past about 1,000 µmol/m²/s, though, heat, VPD, CO2 and nutrition must keep up or the extra light shows up as bleaching instead of weight. Established.
Measure a grid of points (centre, corners, edges) at canopy height, not one reading under the middle of the fixture.
DLI (mol/m²/day) = PPFD × hours of light × 0.0036.
Don't confuse with
- PPF: total fixture output in µmol/s, not per area.
- DLI: the daily total; the same PPFD gives a higher DLI on longer days.
- Lux: an eye-weighted brightness unit that misleads under grow lights.
Photosynthetically active radiation: light in the 400–700 nm waveband that plants use for photosynthesis, counted in photons rather than brightness.CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Light stress is damage when leaves or buds absorb more light than they can use or dissipate, showing as bleached white or yellow tops nearest the fixture.Daily light integral: the total number of PAR photons a square metre of canopy receives in one day, in mol/m²/day; PPFD multiplied by hours of light.Photosynthetic photon flux: the total number of PAR photons a light fixture emits each second, in µmol/s, regardless of where they land.The light saturation point is the PPFD above which photosynthesis stops rising; for cannabis it is unusually high and moves upwards with CO2 and temperature.Light intensity is how much photosynthetic light reaches the plants per unit area and time, measured as PPFD in µmol/m²/s; it is the biggest light lever on yield.
