Lighting: spectrum, PPFD and DLI
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Beginner · Light

Lighting: spectrum, PPFD and DLI

A from-zero guide to how grow light works: what the key numbers mean, what to aim for at each stage, and how to spot problems before they cost you a crop.

Beginner9 diagramsEvidence-linked · 9 sources~14 min read
In this guide
01 Purpose and scope02 Definitions03 Evidence and limitations04 What each wavelength does to the plant05 PPFD and DLI targets at each growth stage06 How day length triggers flowering07 Choosing a fixture: LED, HPS or CMH08 Setting height and intensity at each stage09 Diagnosing common light problems10 Expected results and limitations11 References
01 · Start here

Purpose and scope

Light is the raw fuel a plant converts into sugar — the single biggest variable you control indoors. This paper starts from first principles: it defines every term, gives concrete targets for each stage, and explains the schedule change that makes a plant flower.

The plant captures energy from light and uses it to build sugar from CO2 in the air and water from the roots — like a solar panel that makes its own fuel instead of storing electricity. This process is photosynthesis, and it is why more usable light means more growth, up to the point where CO2, water or temperature becomes the limiting factor[8]. The three numbers that matter most are PPFD (how bright, right now), DLI (how much total light per day), and spectrum (the color mix). Every term is defined the first time it appears.

From light to growth1Light + CO2 + waterphotons hit theleaf→2Photosynthesisenergy capturedin the leaf→3Sugarsthe plant's food→4Growth + budsleaves, stems,flowerIf CO2, water or nutrients run short, extra light stops helping. Light is one input among several.
Figure 1. Light is the input, but it only pays off when CO2, water and nutrients keep pace.
This pairs with the acclimation paper

Read this one for the targets. Read the light acclimation paper for how to ramp up to them safely, so young plants adapt instead of bleaching.

02 · The vocabulary

Definitions

Get the gist of these five terms and the rest of the paper falls into place. They all describe the same thing from different angles: how much usable light a plant is getting.

PAR (Photosynthetically Active Radiation)The slice of light from 400 to 700 nm that plants can use for photosynthesis[5]. Lumens and watts do not measure this, which is why a bright-looking bulb can be useless for plants.
PPFD (Photosynthetic Photon Flux Density)How many usable photons land on one square meter each second, in micromoles (umol/m2/s). This is ‘how bright’ at the canopy. Measure it with a quantum/PAR meter, not a phone lux app.
DLI (Daily Light Integral)The total photons delivered over a whole day, in moles per square meter (mol/m2/day). This is the number that actually drives yield. DLI = PPFD x seconds of light per day / 1,000,000[1].
Efficacy (umol/J)How many usable photons a fixture makes per joule of electricity. This is the headline efficiency number when shopping: higher means more light per dollar of power.
PhotoperiodHours of light per 24 hours. ‘18/6’ means 18 on, 6 off.
Same daily dose, different bright/hours mixDLI in mol/m2/day. 400 PPFD over 18h gives the same dose as 600 PPFD over 12h.01223344613.0 mol200x18h25.9 mol400x18h25.9 mol600x12h34.6 mol800x12h38.9 mol900x12h
Figure 2. Worked DLI examples: a lower PPFD over more hours can match a higher PPFD over fewer hours. The daily total is what counts.[1]
Where PAR sits on the spectrumPAR is the 400-700 nm window. Lumens/lux instead peak near green (~555 nm), which is why they mislead.UVBlueGreenRedFar-red280 nm515 nm750 nm
Figure 3. PAR is the 400 to 700 nm band plants use. Lux meters weight toward green, so they are the wrong tool for plant light.[5]
Evidence assessment

Evidence and limitations

How sure is this paper?

We've gone to great lengths to keep these guides honest. One of the main ways we do that is self-review: we actively look for claims that are subjective, only lightly backed by literature, or based on grower practice rather than a controlled study — and we call those out instead of dressing them up as settled science.

Often there simply is no paper for the decision you're making. In those cases we're drawing on what other growers report and what has worked in our own rooms. That can still be useful — but it is not a lab proof. Do what works for your plants, your room, and your meters. If a table disagrees with your crop, believe the crop and log the difference.

Solid
Well supported by plant science, standards, or broad multi-source consensus
  • PAR/PPFD/DLI definitions and photoperiod night-length control of flowering
  • More light raises yield only until another factor limits (Liebig)
  • LED efficacy bands for modern fixtures (era-dependent; check current DLC/maps)
Operational
What many growers and rooms actually run — start here, then tune
  • Stage PPFD tables for indoor cannabis without/with CO₂
  • 9-point map uniformity checks
Grain of salt
Subjective, thin literature, single studies, or “this works for us” practice
  • Inverse-square as exact hang-height math for multi-bar LED panels (use maps + meters)
  • Red light 'drives flowering' as the primary mechanism (photoperiod does; R:FR affects morphology)
  • UV-B as a reliable potency booster (evidence mixed; safety cost real)

See something glaringly wrong? Tell us and we'll fix it. Please open a GitHub issue with the paper name and what looks off (include a source if you have one): Report an accuracy issue. Local law, labels, and licences always override any recipe here. Inline notes labelled grain of salt flag the highest-risk over-trust points in the text.

03 · Core concept 1

What each wavelength does to the plant

Blue light, roughly 400 to 500 nm, keeps plants compact with tight internode spacing and is linked to denser growth and resin in flower[3]. Red light, 600 to 700 nm, is the most photosynthetically efficient band and drives flowering and stretch[7].

‘Full-spectrum white’ LEDs are rated by color temperature in Kelvin. Higher-K and bluer (around 4000 to 6500K) leans veg, lower-K and redder (around 3000 to 3500K) leans flower. A good broad white spectrum works fine across both stages for beginners. The practical takeaway: do not over-optimize spectrum early. Intensity matters far more for yield than chasing a perfect color recipe.

What each part of the spectrum does. Green is not wasted: it reaches lower leaves, but it is a minor lever.
Band (nm)NameMain effectWhen it matters
280-400UVStress response, possible resin; safety hazardOptional, end of flower
400-500BlueCompact growth, tight internodes, thicker leavesVeg
500-600GreenPenetrates deeper into the canopy than expectedMinor lever, all stages
600-700RedHighest photosynthetic efficiency; stretch is driven more by low blue and far-red, and flowering by photoperiodFlower
700-750Far-redSpeeds the dark response, adds stem stretchFine-tuning only
Beginner rule on spectrum

A quality full-spectrum white LED covers veg and flower. Intensity beats spectrum tuning, so spend your attention on PPFD and DLI before you chase color recipes.

Leaf vs canopy light response graphs
Example. Intensity targets are canopy-scale. A single leaf gas-exchange curve is not the whole room’s yield ceiling.Educational diagram
04 · Core concept 2

PPFD and DLI targets at each growth stage

Young tissue cannot process intense light, so targets climb as the plant matures. Clones and seedlings want about 100-250 PPFD (DLI roughly ~6-16 mol)[1], early-to-late veg about 300-600 PPFD (DLI ~20-35 mol), and flower about 700-900 PPFD without added CO2 (DLI ~30-45 mol)[2].

Pushing past about 900 PPFD only pays off if you also raise CO2 to 1000-1200 ppm and tighten temperature and humidity[8]. Otherwise extra light just causes stress and bleaching. Because DLI bundles intensity and hours together, you can hit the same daily dose with lower PPFD over more hours (veg at 18/6) or higher PPFD over fewer hours (flower at 12/12).

Flowering PPFD: where useful ends and risk beginsAbove ~900 PPFD without CO2 and tight climate control, you buy stress and heat, not yield.no-CO2 sweet spotmarginalCO2 + climate only07501500
Figure 4. Flower intensity zones. Most rooms top out around 700-900 PPFD. The 1000-1400 band needs CO2 and serious climate control.[9]
Yield response to DLI: rising, then flat, then stressedDLI (mol/m2/day) on the x-axis. Returns rise, flatten at saturation, then bend down into stress. CO2 shifts the flat point right.0204060805152535455565relative growth
Figure 5. Growth rises with DLI, levels off at light saturation, then falls as stress and bleaching set in. Adding CO2 moves the saturation point to the right.[8]
Stage targets. 600 PPFD x 18h (~39 mol) is closer to ~900 PPFD x 12h than to 800 PPFD x 12h.
StagePPFD (umol/m2/s)DLI (mol/m2/day)Photoperiod
Clone / seedling100-250~6-1618/6
Early veg300-450~20-2918/6
Late veg450-600~29-3918/6
Flower (no CO2)700-900~30-3912/12
Flower (CO2 1000-1200 ppm)1000-1400~40-6012/12
PPFD through the day adds up to DLIIntensity (PPFD) times hours is the day's total light: DLI in mol/m2/day. The area under this curve is what the plant actually gets.02505007501000offmiddayoffPPFD umol/m2/s
Diagram. Intensity over time is DLI: the area under the daily PPFD curve.
05 · Core concept 3

How day length triggers flowering

Photoperiod-type cannabis stays vegetative under long days (commonly 18/6) and is forced to flower by switching to 12 hours of light and 12 hours of uninterrupted dark[4]. This is ‘the flip.’

The plant tracks darkness, not light hours — think of it as a sand timer that runs only while the lights are off. If anything resets it mid-run, even a brief flash of light, it starts from zero. The mechanism is a pigment in the leaves called phytochrome, a light-sensitive molecule that shifts between two forms depending on light exposure; the balance between those forms tells the plant how long the night has been. When the dark period is long enough, phytochrome triggers the production of florigen — a mobile hormone that carries the signal from the leaves to the growing tips to start producing buds[5]. A light leak resets phytochrome mid-cycle: even a phone screen, an indicator LED, or a pinhole in a tent during lights-off can stall or revert flowering, cause re-vegging, or trigger hermaphrodites[4].

How the dark period triggers flowering1Lights onPr converts toactive Pfr: 'itis day'→2Lights offPfr slowlyreverts to Pr→3Long enough nightflorigen signalmade in leaves→4Flipgrowing tipsswitch to budproductionA red or far-red light leak re-converts Pr back to Pfr and breaks the cycle, so the plant 'thinks' it is still day.
Figure 6. Phytochrome tracks the dark period. A light leak during lights-off resets the clock and stalls the flip.[4]
Seal your dark period

Light leaks during the dark period are the number one beginner flowering failure: stalled bloom, re-veg, or hermaphrodites. Far-red and red are exactly what phytochrome senses. Seal pinholes, cover indicator LEDs, use light-proof ducting. If you can see in the dark, so can the plant.

Photoperiod: the light schedule flips the planthours of light vs dark over 24hVegetative 18/618h light6h darkFlower 12/1212h light12h dark12 hours of uninterrupted dark triggers and holds flowering. A light leak in the dark can revert or stress the plant.
Diagram. Photoperiod by stage: 18/6 in veg, 12/12 to flower.
06 · The hardware

Choosing a fixture: LED, HPS or CMH

Modern LED is the efficiency leader at roughly 2.7-3.0 umol/J for good fixtures (budget units 2.0-2.3), runs cooler, and lasts longer[6]. HPS (high-pressure sodium) sits around 1.7-1.9 umol/J and runs hot but is cheap to buy. CMH/LEC (ceramic metal halide) lands lower, around 1.3-1.9 umol/J, but has a pleasant broad spectrum.

Efficacy (umol/J) is the number to compare. A 3.0 umol/J LED makes about 60% more usable light than a ~1.85 umol/J double-ended HPS for the same power bill[6]. For beginners, a reputable full-spectrum LED with a published PPFD map and efficacy at or above ~2.5 umol/J is the safe default. Ignore inflated ‘equivalent watt’ marketing and look at actual PPF (total umol/s) and coverage.

Fixture efficacy (umol/J)Higher is more light per watt. The target line marks a sensible 'good buy' threshold.01233target2.1Budget LED2.85Good LED1.85HPS DE1.7HPS SE1.5CMH
Figure 7. Efficacy by fixture type. Good LED clears the ~2.5 umol/J threshold comfortably; HPS and CMH fall short.[6]
Compare on efficacy and total PPF plus a real PPFD map, never on lumens or 'equivalent watts.'
TypeEfficacy (umol/J)HeatUpfront costBest for
LED2.0-3.0LowHigherDefault choice, all stages
HPS1.7-1.9HighLowBudget builds, red-heavy flower
CMH / LEC1.3-1.9MediumMediumBroad natural spectrum incl. some UV
07 · Do this

Setting height and intensity at each stage

Intensity falls with distance, but the inverse-square rule (a quarter of the light at twice the distance) is a point-source ideal that LED bars only approximate, so map PPFD with a meter rather than trusting the arithmetic. Height is your coarse intensity dial, the dimmer is the fine one. Hang about 60 cm (24 in) for seedlings and clones, ~45 cm (18 in) for veg, and ~30-40 cm (12-16 in) for flower, then fine-tune with the dimmer and a PAR meter.

Verify coverage by taking PPFD readings at nine points: four corners, four edge-midpoints, and the center. Aim for a min-to-average ratio above 0.75 so edge plants are not starved while the center bleaches. Hanging higher trades peak intensity for more even spread, so use a manufacturer PPFD map as your starting point and confirm with real readings at canopy height.

A 9-point PPFD grid (example, umol/m2/s)Average ~640, lowest corner 560: min/avg = 0.875, above the 0.75 floor. A reading under ~480 here would flag a weak corner.0225450675900560Corner640Edge760Center650Edge590Corner
Figure 8. Sample of a 9-point grid. Compare the lowest reading to the average: a min-to-average ratio above 0.75 is acceptable uniformity.
Starting heights. Always confirm against your fixture's PPFD map and a meter at canopy height.
StageHang heightTarget PPFDPhotoperiod
Clone / seedling~60 cm (24 in)100-30018/6
Veg~45 cm (18 in)300-60018/6
Flower~30-40 cm (12-16 in)700-90012/12
Ramp, do not slam

Pair this with the light acclimation paper: raise the dimmer or lower the fixture over several days rather than jumping a fresh clone to full intensity.

08 · When it goes wrong

Diagnosing common light problems

Too much light shows as bleaching (white or yellow bud tips directly under the fixture), upward-cupping or ‘taco’ leaves, and faded color even when nutrients are fine. The fix is to dim or raise the light, not to feed more.

Far-red (~730 nm) can speed the transition to dark via the phytochrome system and slightly stretch plants[4]. UV-B in the final 1-2 weeks is a popular potency play, but the evidence that UV-B reliably raises cannabinoids or yield is mixed[2], and it carries real eye, skin and plant-stress risks. Treat it as optional and advanced.

The biggest avoidable errors: jumping a clone to flower-level PPFD, trusting lux/wattage over a PAR meter, and ignoring light leaks.
SymptomLikely causeWhat to do
Bleached / white tops under the fixtureToo much PPFDRaise or dim the light, do not feed
Taco / upward-cupping leavesLight plus heat stressRaise the light, check leaf-surface temp
Stretchy, pale growthToo little light or hung too farLower the fixture or boost intensity
Stalled flowering, re-vegLight leak during the dark periodSeal the room light-tight
Scorched tops, PPFD looks fineRadiant heat (esp. HPS)Raise the fixture, watch leaf temp
09 · Reality check

Expected results and limitations

More light only helps up to the point where something else (CO2, water, nutrients, temperature or genetics) becomes the limiting factor. Past saturation you pay for electricity and heat with no extra yield, and eventually with stress[1].

Yield is capped by the shortest inputYield is held to the level of the lowest stave. Here CO2 is the bottleneck, so adding more light is wasted.0255075100target90Light45CO280Water70Nutrients60Temp75Genetics
Figure 9. The limiting-factor idea: pushing light far above the other inputs wastes effort. Lift the shortest stave first.[8]
What to actually do
  1. Without CO2, ~700-900 PPFD / ~35-45 mol DLI in flower is a sensible ceiling. The 1000-1400 PPFD regime needs CO2, cooling and humidity control: a whole-room commitment, not just a brighter light.
  2. Nail intensity, dose and photoperiod first. Spectrum tweaks like far-red and UV are fine-tuning, not the main lever.
  3. Buy on efficacy and a real PPFD map. Hit the stage targets, seal your dark period, and lighting stops being your bottleneck.

Once lighting is handled, the rest is climate, feed and genetics. Read the light acclimation paper for how to ramp safely, and the flowering stages paper for what happens after the flip.

Related papers
Light acclimation
Ramp light without bleaching
Cloning
Cuttings that root every time
Flower week by week
The flip through to ripe

References

  1. 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. https://doi.org/10.3389/fpls.2021.646020
  2. Llewellyn, D., Golem, S., Foley, E., Dinka, S., Jones, A.M.P., & Zheng, Y. (2022). Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science, 13, 974018. https://doi.org/10.3389/fpls.2022.974018
  3. Magagnini, G., Grassi, G., & Kotiranta, S. (2018). The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19-27. https://doi.org/10.1159/000489030
  4. Kusuma, P., Westmoreland, F.M., Zhen, S., & Bugbee, B. (2021). Photons from NIR LEDs can delay flowering in short-day soybean and Cannabis: Implications for phytochrome activity. PLOS ONE, 16(7), e0255232. https://doi.org/10.1371/journal.pone.0255232
  5. Eichhorn Bilodeau, S., Wu, B.-S., Rufyikiri, A.-S., MacPherson, S., & Lefsrud, M. (2019). An Update on Plant Photobiology and Implications for Cannabis Production. Frontiers in Plant Science, 10, 296. https://doi.org/10.3389/fpls.2019.00296
  6. Nelson, J.A., & Bugbee, B. (2014). Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures. PLOS ONE, 9(6), e99010. https://doi.org/10.1371/journal.pone.0099010
  7. Westmoreland, F.M., Kusuma, P., & Bugbee, B. (2021). Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLOS ONE, 16(3), e0248988. https://doi.org/10.1371/journal.pone.0248988
  8. 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. https://doi.org/10.1007/s12298-008-0027-x
  9. Kotiranta, S., Pihlava, J.-M., Kotilainen, T., & Palonen, P. (2024). High light intensity improves yield of specialized metabolites in medicinal cannabis (Cannabis sativa L.), resulting from both higher inflorescence mass and concentrations of metabolites. Industrial Crops and Products, 211, 118210. https://doi.org/10.1016/j.indcrop.2024.118210

Citations marked in-text as [n] map to this list. Primary literature and official guidance except where noted. Cannabis tissue culture is strongly genotype-dependent, verify dilutions, hormone doses and local regulations against the primary sources before relying on them.