Why don't lumens matter for grow lights?

Q: Why don't lumens matter for grow lights?

Lumens don't matter for grow lights because lumens, lux and foot-candles measure light as the human eye sees it, weighted heavily to green-yellow, while plants use photons from 400–700 nm about equally. The plant-relevant numbers are PPF (µmol/s from the fixture), PPFD (µmol/m²/s at the canopy), DLI (mol/m²/day) and efficacy (µmol/J). Lux converts to PPFD only roughly, and only for a known spectrum.

Short answer

Lumens, lux and foot-candles measure light as the human eye sees it, weighted heavily to green-yellow. Plants use photons from 400–700 nm about equally. So the numbers that matter are PPFD (µmol/m²/s at the canopy), DLI (mol/m²/day) and fixture efficacy (µmol/J). Established.

Why

Photometric units (lumens, lux) and quantum units (µmol) describe different things. Converting between them needs a factor that depends on the light source's spectrum; published factors differ from lamp to lamp ^[1].

A red-heavy grow light looks dim to us and scores low lumens, yet delivers plenty of photosynthetic photons. A cool white household lamp can score high lumens and deliver fewer photons per watt than a horticultural fixture. Lumens rank these lights in the wrong order.

Grow lights are therefore rated in PPF (µmol/s) and efficacy (µmol/J). Good horticultural LEDs exceed 2.5–3 µmol/J ^[2].

PPFD at the canopy is the number that predicts yield: dry flower yield rose linearly from 120 to 1,800 µmol/m²/s in one controlled cannabis trial ^[3].

The numbers

Unit	Measures	Use for plants?
Lumen (lm)	total visible light, eye-weighted	no
Lux (lm/m²)	eye-weighted light per m²	rough only, known spectrum
Foot-candle	lux per ft² (1 fc ≈ 10.76 lux)	rough only
PPF (µmol/s)	fixture's 400–700 nm photon output	yes
PPFD (µmol/m²/s)	photons reaching the canopy	yes: the setpoint
DLI (mol/m²/day)	PPFD × hours × 0.0036	yes
Efficacy (µmol/J)	PPF ÷ watts	yes: running cost

Rough sunlight conversion: about 54 lux per µmol/m²/s ^[1]. Other spectra differ.

What to do

Ignore lumen and "equivalent watt" claims on grow lights.
Ask for PPF, µmol/J and a PPFD map at stated heights.
Set the light by PPFD: 150–300 seedlings, 300–700 veg, 800–1,200 flower.
Use lux meters only to compare spots under the same light.

Common mistakes

Choosing a grow light by lumens per square foot.
Applying the sunlight lux factor to LEDs.
Assuming a brighter-looking light grows better.

Full article: PPFD and DLI by stage
How can I measure PPFD without a PAR meter?
Can I grow with household LED or CFL bulbs?
Terms: lux · PPFD · DLI · foot-candle

Follow-up questions

What is the difference between lux and PPFD?

Lux measures how bright light looks to a human per square metre, weighted to the eye's peak sensitivity in green-yellow. PPFD counts the photosynthetic photons between 400 and 700 nm landing on a square metre each second, in µmol/m²/s, which is what drives photosynthesis. A red-heavy lamp can have low lux but high PPFD; the two don't convert without knowing the spectrum.

How many lumens does cannabis need?

Cannabis needs are not set in lumens: lumens measure brightness to the human eye, not plant-usable light. Use PPFD instead: about 150–300 µmol/m²/s for seedlings, 300–700 in veg and 800–1,200 in flower at ambient CO₂. As a rough sunlight-only guide, 1,000 µmol/m²/s is about 54,000 lux, but lamps with other spectra convert differently.

What do PPF, PPFD and DLI mean?

PPF (photosynthetic photon flux) is a fixture's total output of 400–700 nm photons, in µmol/s. PPFD (photosynthetic photon flux density) is how many of those land on a square metre of canopy each second, in µmol/m²/s. DLI (daily light integral) is the day's total, in mol/m²/day: PPFD × hours × 0.0036. Efficacy, in µmol/J, is PPF divided by watts.

Sources

Thimijan RW, Heins RD, 1983. Photometric, radiometric, and quantum light units of measure: a review of procedures for interconversion. HortScience 18(6):818–822. doi.org/10.21273/HORTSCI.18.6.818

Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. doi.org/10.1038/s41438-020-0283-7

Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. doi.org/10.3389/fpls.2021.646020