PAR (Photosynthetically Active Radiation)
The light wavelengths plants use for photosynthesis, and how growers measure them.
PAR is a wavelength range (400–700 nm), not a measurement. When growers say 'this lamp has 1000 PAR,' they mean PPFD—photons per square meter per second. What matters for cannabis is daily total (DLI in mol/m²/day), fixture efficiency (µmol/J), and spectrum balance. PAR alone doesn't tell you if buds will be better; that depends on uniformity, CO₂, and your light's actual red/blue ratio.
Definition
Photosynthetically Active Radiation (PAR) is the band of electromagnetic radiation from 400 to 700 nanometers (nm) that plants use to drive photosynthesis Strong evidence[1][2]. It is a spectral range, not a measurement—saying a lamp "puts out X PAR" without specifying how it is measured (PPFD, DLI, or PPE) is technically meaningless.
The definition was formalized by Keith McCree in 1972, whose action-spectrum research showed that photon absorption across 400–700 nm correlates well with the photosynthetic response in higher plants Strong evidence[1]. In recent years, some researchers have proposed an extended range—termed "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. Three measurement units matter for growers:
PPFD (Photosynthetic Photon Flux Density): micromoles of photons striking one square meter per second (µmol/m²/s). This is the intensity at leaf level.
DLI (Daily Light Integral): total photons delivered per square meter per day (mol/m²/day). For cannabis flowering, typical targets are 35–65 mol/m²/day, though absolute yield gains diminish above ~55 mol/m²/day, and CO₂ enrichment can extend the response curve Weak / limited[4][5].
PPE (Photosynthetic Photon Efficacy): how many PAR photons a fixture produces per joule of electricity (µmol/J). Modern horticultural LEDs typically deliver 2.5–3.1 µmol/J Strong evidence[6].
What PAR does and doesn't tell you
PAR reveals:
- Which wavelengths a light source contributes to photosynthesis
- (Via PPFD or DLI) the approximate quantity of photosynthetic activity your canopy receives
- A standardized basis for comparing light fixtures
PAR does not reveal:
- Spectrum quality. Two fixtures with identical PPFD can have very different ratios of red, blue, and green light. These ratios influence plant morphology (stretch, branching), and may affect secondary metabolism, though controlled evidence on cannabis-specific terpene and cannabinoid responses to spectrum remains limited Weak / limited[3].
- UV or far-red effects. UV-B (~280–315 nm) and far-red (~700–750 nm) lie outside the classic PAR band but influence plant morphology and cannabinoid responses Weak / limited[3][7].
- Heat load, spatial uniformity, fixture lifespan, or cost.
- Whether your plants are light-limited. Above the light-saturation point for a given cultivar, CO₂ concentration, temperature, or nutrient availability typically becomes the limiting factor.
- Color rendering or flicker, which may affect grower perception or worker safety.
Common misuses
'This lamp has 1000 PAR' Meaningless without a unit. The speaker likely means 1000 µmol/m²/s (PPFD) at an unspecified distance, or possibly 1000 mol/m²/day (DLI). Always ask: which metric, and at what distance or over what period?
'Higher PAR = better bud' False past the light-saturation point. Yield and potency depend on spectrum uniformity, CO₂, temperature, and nutrient balance—not PAR intensity alone.
Inaccurate meter readings Cheap consumer PAR meters calibrated for broadband sunlight can misread narrow-spectrum LEDs by 10–20% or more Weak / limited[6]. For accurate measurements with LEDs, use a spectroradiometer or a meter with LED-specific calibration curves.
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—An Introduction to Photosynthesis.
- 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
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Related
- Bushy Growth — Short internodal spacing and wide lateral branching creating a shrub-like canopy.
- Internode Spacing — Distance between leaf nodes on the stem; shaped by genetics and light.