PAR (Photosynthetically Active Radiation)
The slice of sunlight plants actually use for photosynthesis, measured in micromoles of photons per square meter per second.
PAR is one of the most misused terms in indoor growing. It's not a measure of brightness, not a measure of how 'good' a light is, and a high-PAR LED isn't automatically better than a cheaper one. PAR just defines which wavelengths plants use. To compare lights or dose your canopy, you want PPFD (intensity at a point) or DLI (daily total) — both built on PAR. Marketing copy that brags about 'high PAR output' without giving you PPFD maps or efficacy numbers is selling vibes.
Definition
Photosynthetically Active Radiation (PAR) is the band of electromagnetic radiation from 400 to 700 nanometers that plants can use to drive photosynthesis Strong evidence[1][2]. It is a spectral range, not a measurement — saying a lamp 'puts out X PAR' is technically meaningless without specifying how it's measured.
The definition was formalized by Keith McCree in 1972, whose action-spectrum work showed that photon absorption across 400–700 nm correlates well with photosynthetic response in higher plants Strong evidence[1]. Some researchers now argue for an extended range (e.g. 'ePAR,' 400–750 nm) to account for far-red light's role in photosystem function via the Emerson enhancement effect Weak / limited[3].
How PAR is measured
Because plants respond to photon count, not energy, PAR is quantified in micromoles of photons rather than watts. The three numbers growers actually care about:
- PPFD (Photosynthetic Photon Flux Density): photons hitting one square meter per second, in µmol/m²/s. This is the 'intensity at the leaf' number.
- DLI (Daily Light Integral): total PAR photons delivered per square meter per day, in mol/m²/day. Cannabis flower generally performs well at roughly 35–65 mol/m²/day, though returns diminish at the high end and CO₂ enrichment shifts the curve Weak / limited[4][5].
- PPE (Photosynthetic Photon Efficacy): how many PAR photons a fixture produces per joule of electricity, in µmol/J. Modern horticultural LEDs land around 2.5–3.1 µmol/J Strong evidence[6].
What PAR does and doesn't tell you
PAR tells you: which wavelengths a light source contributes toward photosynthesis, and (via PPFD/DLI) roughly how much photosynthetic 'food' your canopy is getting.
PAR doesn't tell you:
- Spectrum quality. Two fixtures with identical PPFD can have wildly different red/blue/green ratios, affecting morphology, terpene production, and trichome development Weak / limited.
- UV or far-red effects. UV-B (~280–315 nm) and far-red (~700–750 nm) sit outside the classic PAR band but influence cannabinoid and morphological responses Weak / limited[3][7].
- Heat load, par-to-canopy uniformity, or fixture longevity.
- Whether your plants are light-limited. Above a certain PPFD, CO₂, temperature, or nutrients become the bottleneck.
Common misuses
- 'This lamp has 1000 PAR' — meaningless. PAR is a range; the number is presumably PPFD at some unspecified distance.
- 'Higher PAR = better bud' — false past the light-saturation point, and irrelevant without spectrum and uniformity data.
- Consumer 'PAR meters' calibrated for sunlight can misread narrow-spectrum LEDs by 10–20% Weak / limited[6]. Use a spectroradiometer or a meter with LED-specific calibration for accurate readings.
Sources
- Peer-reviewed McCree, K.J. (1972). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191–216.
- Government USDA Natural Resources Conservation Service. Plants and Light (Photosynthetically Active Radiation definitions). ↗
- Peer-reviewed Zhen, S., & Bugbee, B. (2020). Far-red photons have equivalent efficiency to traditional photosynthetic photons: Implications for redefining photosynthetically active radiation. Plant, Cell & Environment, 43(5), 1259–1272.
- Peer-reviewed 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.
- Peer-reviewed Chandra, S., Lata, H., Khan, I.A., & ElSohly, M.A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiology and Molecular Biology of Plants, 14(4), 299–306.
- Peer-reviewed Kusuma, P., Pattison, P.M., & Bugbee, B. (2020). From physics to fixtures to food: current and potential LED efficacy. Horticulture Research, 7, 56.
- Peer-reviewed Lydon, J., Teramura, A.H., & Coffman, C.B. (1987). UV-B radiation effects on photosynthesis, growth and cannabinoid production of two Cannabis sativa chemotypes. Photochemistry and Photobiology, 46(2), 201–206.
How this page was made
Generation history
Drafting assistance and fact-check automation are used, with a human operator spot-checking on a weekly basis. See how articles are made.