PAR, PPFD & DLI for Cannabis

Reading plant light correctly: What PAR, PPF, PPFD, DLI, ePAR, and PPE mean, why lux and watts alone are not enough, and what cannabis studies actually show regarding light intensity, photoperiod, far-red, and UV.

A 600-watt lamp says little about what a cannabis plant actually receives.

Also:

"very bright"

is not a particularly botanical unit.

Plants care neither about the number on the power supply nor how dazzling a grow room looks to human eyes.

What is crucial are photons.

Which wavelengths hit the leaves?

How many of them arrive at the canopy per second?

How evenly are they distributed?

And how much light does the plant collect over the entire day?

This is precisely why plant lighting needs terms like:

PAR.

PPF.

PPFD.

DLI.

And, increasingly nowadays:

ePAR.

Anyone who can distinguish between these quantities needs significantly fewer marketing terms when it comes to grow lights.

What does PAR mean?

PAR stands for Photosynthetically Active Radiation.

The classic horticulture definition covers the wavelength range of:

400 to 700 nanometers.

This range roughly extends from violet or blue to deep red.

It is important, however, to note:

PAR itself is not a measured value like temperature or EC.

PAR primarily describes a spectral range.

How many photons within this range a lamp generates or how many of them reach the plant is only described by quantities like PPF and PPFD.

Modern plant lighting standards also continue to use the 400–700 nm range for classic PPF. The DesignLights Consortium requirements refer to the ANSI/ASABE-S640 terminology.

But does photosynthesis really end at exactly 700 nm?

No.

And this is precisely where the classic PAR definition has become somewhat historical.

For a long time, 400–700 nm was treated as the practically relevant limit. Recent plant physiology shows, however, that photons above 700 nm – especially in the far-red range of approximately 701 to 750 nm – can effectively contribute to photosynthesis in combination with shorter-wavelength photons.

From this, the term was born:

ePAR – extended Photosynthetically Active Radiation.

It expands the considered range to:

400 to 750 nm.

Work by Zhen and colleagues shows that far-red photons, in combination with shorter wavelengths, are significantly more photosynthetically active than the classic 700-nm limit would suggest. The authors therefore argue for ePAR as a better description of the actually usable radiation under appropriate spectra.

This does not mean, however:

PAR is wrong.

Rather:

400–700 nm remains the established standard range for PPFD and PPF. However, plant physiology does not end abruptly at 700 nm.

This is particularly relevant for modern LED lights that contain specific far-red components.

Why lux and lumens only partially describe plant light

Lumens and lux originate from lighting for humans.

Lumens describe the photometrically weighted luminous flux of a source.

Lux describes how much of that reaches a surface:

1 Lux = 1 lumen per square meter.

The crucial point is the weighting.

The human eye does not respond equally sensitively to all visible wavelengths. It perceives the green-yellowish range particularly strongly.

A plant does not possess this human brightness system.

Therefore, two light sources with similar lux values can provide different amounts of photosynthetically relevant photons.

For indoor plants, Iowa State therefore identifies PPFD as one of the significantly more useful metrics for measuring the plant lighting that actually reaches the leaf.

However, that does not make lux completely worthless.

With the same or a very similar white light source, a lux meter can, for example, show perfectly:

Middle brighter than edge.

Lamp higher or lower.

Dimming before and after.

For absolute plant light comparisons between significantly different spectra, however, lux is not enough.

The Cannaseuse article Lux in Cannabis Cultivation explains more about this.

PPF: How much plant light does the lamp generate in total?

PPF stands for Photosynthetic Photon Flux.

The value describes the total number of photosynthetically relevant photons that a light source emits per second within the defined PAR range.

The unit is:

µmol/s.

PPF is therefore primarily a property of the luminaire.

A lamp with a high PPF generates many PAR photons.

However, that does not yet say how many of these actually reach the plants.

Because that also depends on:

Distance,

Optics,

Luminaire geometry,

Reflection,

Surface area

and light distribution.

PPF is therefore comparable to the total output of an irrigation system.

Interesting.

But still not the answer to the question of how much reaches a specific point.

PPFD: What actually reaches the canopy

For growers, PPFD becomes much more concrete.

PPFD means:

Photosynthetic Photon Flux Density.

The unit is:

µmol·m⁻²·s⁻¹.

It describes how many PAR photons hit one square meter per second.

This turns lamp output into an area measurement.

That is exactly why PPFD is much more informative for an indoor grow than watts or PPF alone.

A luminaire can, for example, have a high PPF.

But if this photon output is distributed poorly, the following occur:

an extremely bright center point,

weak edges,

hotspots,

strong vertical differences.

This is why a PPFD map is often more meaningful than a single peak value.

The highest PPFD value of a lamp is not necessarily the best

Marketing loves maximum values.

“Up to 1,400 PPFD.”

“Peak PPFD 1,600.”

“Over 2,000 µmol.”

Without context, that says surprisingly little.

Where was it measured?

At what distance?

On what area?

With or without grow tent walls?

Only in the center?

What does the average look like?

A grow area with 1,400 µmol·m⁻²·s⁻¹ in the center and 450 at the edge can be less interesting for plant cultivation than an area that is illuminated much more evenly.

This is precisely why canopy management plays such a large role.

A flatter plant architecture keeps more leaves and shoot tips at a similar distance from the light source. The Cannaseuse article on Low Stress Training and Canopy Management explains exactly this correlation.

DLI: The amount of light for a whole day

PPFD is a snapshot.

However, a plant does not live for one second.

That is why there is the:

Daily Light Integral – DLI.

DLI describes the total number of PAR photons that hit one square meter over the course of a day.

The unit:

mol·m⁻²·day⁻¹.

In simplified terms, it can be calculated with constant lighting as:

DLI = PPFD × lighting hours × 0.0036

This makes it immediately visible why PPFD alone is not enough.

600 µmol·m⁻²·s⁻¹ for 12 hours results in approximately:

25.9 mol·m⁻²·day⁻¹.

The same 600 PPFD for 18 hours results in:

38.9 mol·m⁻²·day⁻¹.

The light intensity is identical.

The daily photon intake is not.

DLI explains a major difference between vegetation and flowering

With photoperiodic cannabis, it is not necessarily only the plant phase that changes when switching to flowering.

The daily amount of light can also drop drastically.

Example:

A plant grows vegetatively under 18 hours of light.

Then it is switched to 12/12.

If the PPFD remains identical, the DLI drops by a third simply due to the shortened lighting time.

This is why intensity and photoperiod are sensibly considered together.

The biological fundamentals of this reaction are explained in Photoperiod in Cannabis.

How much PPFD can cannabis actually use?

Under controlled conditions, cannabis can utilize surprisingly high light intensities.

A much-cited study from the University of Guelph examined a cannabis cultivar during flowering under average canopy PPFDs between approximately:

120 and 1,800 µmol·m⁻²·s⁻¹.

The dry flower yield increased linearly across this entire studied range.

From approximately 116 to 519 g of dry flowers per square meter.

That corresponds to about 4.5 times the amount.

Remarkable was:

The photosynthesis of individual leaves showed a saturation much earlier.

The total yield of the plant, however, did not yet.

This is an excellent example of why a single leaf measurement does not automatically predict how a complete canopy will react to more light.

Does this mean: Cannabis should be illuminated with 1,800 PPFD?

No.

That would be exactly the wrong conclusion.

The study shows:

Under these experimental conditions, this cultivar could translate additional photons up to the highest studied value into additional yield.

It does not show:

1,800 PPFD is the optimal value for every grow.

With a 12-hour photoperiod, 1,800 PPFD corresponded to a DLI of about:

77.8 mol·m⁻²·day⁻¹.

For indoor production, that is an enormous daily amount of light.

The authors themselves therefore discuss not only biological yield maximization, but expressly the economic question:

How much additional lighting is worthwhile in view of electricity, cooling, hardware, and additional yield?

A second cannabis study confirms the strong light reaction

Another study with the THC-dominant cultivar “Meridian” compared:

600,

800

and

1,000 µmol·m⁻²·s⁻¹.

At the highest compared to the lowest PPFD, the dry weight of the inflorescences increased by approximately 1.6 times.

Interesting:

The cannabinoid concentrations of the flowers did not increase significantly.

More light, therefore, primarily generated:

more flower biomass.

Not automatically:

more potent flowers per gram.

This distinction is important.

Yield and cannabinoid concentration are different variables.

Therefore, there is no scientific universal PPFD for cannabis

Grow charts like to convey:

Seedling = X.

Veg = Y.

Flowering = Z.

Such ranges can be useful as a practical orientation.

However, they are not botanical constants of nature.

The usable light intensity depends, among other things, on genotype, developmental phase, canopy structure, leaf acclimatization, temperature, CO₂ availability, water status, and nutrient supply.

Light distribution also counts.

A high average value with an even canopy can be something different than the same nominal PPFD as a small hotspot directly on a single tip.

PPFD is a measurement – not a fertilization recommendation for photons.

Light stress: More is not endlessly better

That cannabis can use high amounts of light does not mean that light has no upper limit.

Under excessive load, photosystems can receive more energy than can be sensibly processed photochemically.

Possible consequences include:

Photoinhibition,

chlorophyll loss or bleaching,

altered leaf orientation,

reduced photosynthetic performance

and general stress.

Where this limit lies is not identical for every cultivar.

Climate also plays a role.

With high radiation, for example, the thermal load of the leaf increases. Water transport, stomatal regulation, and CO₂ uptake thus become increasingly relevant.

The correlation between leaf climate, transpiration, and photosynthesis is explained in more detail in the Cannaseuse article on VPD in Cannabis Cultivation.

12/12 and DLI: A grow dogma is cracking

Photoperiodic cannabis genetics are classically brought into flowering indoors with:

12 hours of light / 12 hours of darkness.

For good reason, this remains the established standard.

It works reliably for very many genotypes.

However, newer research shows:

For many modern cannabis genetics, 12 hours of light is not the maximum possible day length during stable flowering.

A study published in 2024 compared the "Incredible Milk" and "Gorilla Glue" cultivars under 12- and 13-hour days.

The PPFD remained at 540 µmol·m⁻²·s⁻¹.

As a result, the DLI only increased from:

23.8

to:

25.7 mol·m⁻²·day⁻¹.

Despite this relatively small difference in DLI, flower yields under 13 hours were approximately 35 and 50 percent higher for the two cultivars, respectively. However, flowering was slightly delayed for the Incredible Milk cultivar.

This is remarkable.

But here, too, the following applies:

Two cultivars do not make a universal 13/11 rule.

Rather, the study shows how significantly photoperiod sensitivity can vary genetically.

DLI and photoperiod are not the same biologically

This reveals something else.

Mathematically, DLI treats light as a daily quantity.

Plants, however, also interpret light as time information.

600 PPFD for 12 hours and 400 PPFD for 18 hours have roughly the same DLI.

For a photoperiodic cannabis plant, these two days are nevertheless not biologically identical.

The dark phase influences the flowering program.

Phytochromes respond to the spectrum and the day length.

Photoperiod controls development.

DLI describes photon delivery.

Both must be considered separately.

The spectrum: Plants use more than just red and blue

Grow lights were sold for a long time with a simple narrative:

Blue for growth.

Red for flowering.

There is a kernel of truth in that.

But plant lighting is significantly more complex.

Photons across the entire classic PAR range can drive photosynthesis.

This includes:

Blue,

green,

yellow,

orange,

and red.

Green photons are therefore not "useless light" just because chlorophyll leaves appear green.

Furthermore, spectra influence not only photosynthesis but also photomorphogenesis—the way a plant adapts its form and development to its light environment.

Blue influences morphology – but "more blue = better veg" is oversimplified

Blue light is perceived by cryptochromes and phototropins, among others.

It can:

Influence elongation growth,

alter stomatal responses,

modulate leaf development,

and affect plant architecture.

In many crops, a higher blue content is often associated with more compact growth.

However, how strongly this effect manifests in cannabis depends on the genotype and the rest of the spectrum.

Therefore, there is no universal "vegetative blue value."

Red and far-red are also more than just a flowering switch

Red and far-red strongly affect the phytochrome system.

This system provides plants with information about:

Day length,

the dark phase,

neighboring shade,

and the ratio of red to far-red.

More far-red, or a lower red:far-red ratio, can promote shade-avoidance responses in many plants—for example, increased elongation.

Cannabis studies confirm that the red:far-red ratio can change plant height, flower biomass, and secondary metabolites.

Interestingly, however, different experiments do not simply deliver the same "far-red = higher yield" narrative.

One study with FINOLA found taller plants under a low red:far-red ratio but lower inflorescence yield. Another, more recent study found more flower biomass with stronger far-red, while various cannabinoid and terpene concentrations decreased.

This is exactly why spectral claims without genotype and experimental conditions are of little value.

ePAR also changes light measurement

When a grow light provides virtually no far-red, classic PPFD and ePPFD barely differ.

If, however, a light has a significant portion between 700 and 750 nm, the measurement question becomes more interesting.

A classic PAR sensor essentially counts:

400–700 nm.

An ePAR-capable sensor takes into account:

400–750 nm.

Two measuring devices can therefore display different photon densities under the same far-red-rich light without either one necessarily being "broken."

They are simply measuring different spectral ranges.

For light specifications, one should therefore not just read the number in the future.

But also:

What was actually counted?

UV is not part of PAR

UV is below the classic PAR range.

UVA is approximately:

315–400 nm.

UVB is approximately:

280–315 nm.

For years, cannabis marketing has associated UV with:

more THC,

more resin,

more terpenes,

more "solar stress."

Research is much more cautious.

A controlled cannabis study using short-wavelength UVB exposure found no commercially relevant benefit in cannabinoids or flower yield; with stronger treatments, growth and quality disadvantages even occurred.

Another 2022 investigation also found no effect of additional UVA or UVA+UVB on the cannabinoid concentration of the flowers and no relevant yield advantage.

Does that mean UV does nothing for cannabis?

That, too, would be too absolute.

A study published in 2024 by Humboldt University of Berlin tested several UVA/UVB spectra and intensities.

None of the UV regimes changed the cannabinoid profile significantly.

However, with one specific, relatively low-dose and strongly UVA-dominated treatment, individual measured terpenes increased:

Linalool by about 29 percent,

limonene by about 25 percent,

myrcene by about 22 percent.

Other UV regimes did not show this reaction.

Thus, the scientifically more accurate statement is:

UV can influence cannabis. However, a universal UV boost for THC, resin, or aroma cannot be derived from this.

Light distribution can be more important than even more peak PPFD

A large light over a canopy inevitably creates a light gradient.

Upper leaves intercept photons.

Deeper in the canopy, photon density decreases.

This is precisely why modern controlled-environment horticulture is increasingly looking not only at:

How much light?

But:

Where does it land?

In cannabis, sub-canopy and inter-canopy lighting is now also being studied to better supply poorly lit areas within dense plant populations.

More on this is explained in Sub-canopy and Inter-canopy Lighting in Cannabis.

The principle behind this is important:

An additional kilowatt-hour of light is only worthwhile if the largest possible portion of it lands effectively on photosynthetically active plant surface.

Watts are still not meaningless

"Watts are irrelevant to plants" is now heard almost as often as "more watts = better light" used to be.

Both are wrong.

Watts describe electrical power consumption.

It does not directly say how many usable photons are generated.

However, for electricity costs, heat load, and efficiency, watts are, of course, decisive.

That is why there is:

PPE – Photosynthetic Photon Efficacy.

Measured in:

µmol/J.

PPE describes how many PAR photons a light generates per joule of electrical energy used.

A 500-watt light with higher PPE can therefore generate more PPF than a less efficient 600-watt light.

The DesignLights Consortium, for example, sets a minimum value of 2.5 µmol/J for qualified products in its current Horticultural Technical Requirements V4.0. This is an efficiency standard for that product list—not a biological cannabis target value.

What a useful grow light specification should contain

Anyone wanting to evaluate an LED should therefore not look for a single super-number.

Together, the following are relevant:

• PPF in µmol/s – total PAR photon output
• PPE in µmol/J – electrical efficiency
• PPFD map – distribution on the intended area
• Measurement height and area size – without them, a PPFD map is hardly interpretable
• Spectrum – including a relevant far-red portion
• Dimming – to adapt intensity to the phase and canopy
• Real power consumption – for heat and electricity usage
• Uniformity – not just the highest value in the center

A light is therefore not just a single PPFD value.

It is a light system.

The leaf itself is also part of the equation

PPFD is often measured horizontally at canopy height.

However, leaves are not all horizontal.

They turn.

Overlap.

Shade each other.

Change their position.

And together form a three-dimensional crown.

The leaf blade—botanically the lamina—is the actual photosynthetic work surface. More about its structure and function is explained in the lamina in cannabis.

Therefore, a perfect PPFD map on an empty tent floor does not automatically describe the real light absorption of a large plant.

Typical myths about PAR when growing cannabis

Myth: PAR is a light intensity

Not directly.

PAR primarily refers to the wavelength range. PPFD describes the photon density on a surface.

Myth: Plants can only use light between 400 and 700 nm

Too absolute.

400–700 nm is the classic PAR definition. Far-red up to approximately 750 nm can also contribute to photosynthesis along with shorter-wavelength photons.

Myth: Green light is useless for plants

No.

Green photons can drive photosynthesis and penetrate deeper into leaf and canopy tissue than strongly absorbed wavelengths.

Myth: Cannabis needs 1,800 PPFD during flowering

No.

One study found a linear yield increase up to 1,800 PPFD for the cultivar tested. That is an experimental finding, not a general light fertilization recommendation.

Myth: 1,000 PPFD automatically produces more potent flowers than 600 PPFD

Not necessarily.

In a controlled cannabis study, flower biomass increased significantly, but cannabinoid concentrations did not.

Myth: 12/12 is the only biologically possible flowering photoperiod

No.

12/12 remains the robust indoor standard. However, some modern cultivars can also flower reliably under longer photoperiods.

Myth: UV automatically increases THC

There is no reliable general evidence for this. Several controlled cannabis studies found no relevant THC benefit.

FAQ on PAR, PPFD, and DLI for Cannabis

What does PAR mean in cannabis?

PAR stands for Photosynthetically Active Radiation and classically encompasses the wavelength range from 400 to 700 nm.

What is ePAR?

ePAR stands for extended PAR and expands the range to 400–750 nm. The background is recent research showing that far-red photons above 700 nm can contribute to photosynthesis when combined with shorter-wavelength photons.

What is the difference between PAR and PPFD?

PAR describes a spectral range. PPFD describes how many photons within that range hit a square meter per second.

What is PPF?

PPF refers to the total photosynthetic photon output of a light source and is measured in µmol/s.

What is DLI?

DLI is the total daily photon intake per square meter. The unit is mol·m⁻²·d⁻¹.

How do you calculate DLI?

With constant PPFD, the approximate calculation is:

PPFD × hours of light × 0.0036.

What PPFD does cannabis need?

There is no universal scientific value for all cultivars and stages. Studies show a strong positive yield response to increasing PPFD, but the optimal intensity depends on the overall setup.

Is 1,800 PPFD too much?

Not necessarily: One cannabis trial found that yield was still increasing at 1,800 PPFD. However, this corresponded to about 78 mol·m⁻²·d⁻¹ with twelve hours of light and should not be understood as a general target value.

Is lux useless for cannabis?

No. With similar spectra, lux is suitable for relative comparisons. For spectrally different grow lights, PPFD is significantly more meaningful.

Is more light always better?

No. Additional photons can increase yield as long as the plant and the rest of the system can utilize them. Eventually, other limiting factors or light stress diminish the additional benefit.

Does cannabis need UV light?

For normal growth, additional UV is not necessary. A reliably positive UV effect on cannabinoids or yield is currently not proven.

Is a full-spectrum LED better?

"Full spectrum" is not a sufficiently precise quality term. What matters is the actual spectral distribution, PPF, PPE, PPFD uniformity, and the requirements of the specific setup.

What is more important: PPFD or DLI?

Both answer different questions.

PPFD describes the instantaneous photon density.

DLI describes the total daily amount.

For light management, one should consider both together.

Conclusion: PAR is the beginning – not the entire light strategy

PAR has changed plant lighting.

The term shifts the focus away from:

Watts.

Lumens.

Human brightness.

Toward:

Photons.

This makes grow light biologically measurable.

But even PAR is just the beginning.

PPF shows how many photons a light produces.

PPFD shows how many arrive on a surface.

DLI shows how many accumulate over a day.

PPE shows how efficiently electrical energy is converted into plant light.

The PPFD map shows how uniformly this light is distributed.

And ePAR reminds us that even the classic 400–700 nm limit was not the last word in plant physiology.

Cannabis makes this interplay particularly clear.

The plant can convert exceptionally high amounts of light into additional biomass. But this does not imply a universal 1,800 PPFD recommendation, nor the simple rule "more light = more THC".

Genotype.

Photoperiod.

Canopy.

Climate.

CO₂.

Water.

Nutrient supply.

Spectrum.

And daily light quantity

collectively decide what becomes of the photons.

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