Traditional PAR covers wavelengths from:
400 to 700 nm
But plants do not suddenly stop responding to photons at exactly 700 nm.
Research over the past several years has shown that far-red photons between approximately 700 and 750 nm can contribute to photosynthesis when they are supplied together with shorter-wavelength photons.
This has led researchers to propose a broader measurement range:
ePAR — Extended Photosynthetically Active Radiation
commonly defined as:
400–750 nm
The corresponding photon-flux-density measurement is often called:
ePPFD — Extended Photosynthetic Photon Flux Density
The idea is scientifically important.
But there is also an important distinction:
ePAR has not simply replaced conventional PAR.
Traditional 400–700 nm PPFD remains widely used in horticultural measurement and current lighting standards.
ePAR should therefore be understood as an expanded measurement concept that captures additional biologically relevant far-red photons.
Quick Answer: What Is ePAR?
ePAR stands for:
Extended Photosynthetically Active Radiation
It extends the traditional PAR wavelength range from:
400–700 nm
to:
400–750 nm
The additional wavelengths are primarily in the far-red region from:
700–750 nm
When photon flux density is measured across this extended range, the quantity is commonly referred to as:
ePPFD
and is expressed in:
µmol/m²/s
So the simplest distinction is:
PPFD = photons from 400–700 nm
ePPFD = photons from 400–750 nm
Why Was Traditional PAR Defined as 400–700 nm?
The traditional PAR range was developed from foundational research into the wavelength dependence of photosynthesis.
Photons between approximately 400 and 700 nm were found to drive photosynthesis effectively, and the range became a practical standard for measuring plant light.
Modern quantum sensors therefore typically measure photon flux across:
400–700 nm
and report:
PPFD — Photosynthetic Photon Flux Density
This system has been used successfully for decades.
The traditional definition was never intended to mean that every photon outside 400–700 nm has no biological effect.
It provides a standardized and practical measurement range.
What Changed Our Understanding of Far-Red Light?
Far-red light generally refers to wavelengths immediately beyond conventional red light.
For ePAR discussions, the most important region is:
700–750 nm
Historically, these wavelengths were excluded from PAR partly because far-red photons used alone have relatively little ability to drive photosynthesis.
But photosynthesis does not operate using isolated wavelengths.
Plants simultaneously absorb photons across a broad spectrum.
Experiments have demonstrated that far-red photons can work synergistically with photons at shorter wavelengths.
Under those conditions, far-red photons can make meaningful contributions to photosynthesis.
This is the scientific basis for ePAR.
Far-Red Alone and Far-Red With Other Light Are Different
This distinction is essential.
The statement:
“Far-red photons contribute to photosynthesis.”
does not mean:
“Far-red light alone works exactly like conventional PAR.”
Research has shown that far-red photons alone generally produce relatively little photosynthetic activity.
When far-red photons are combined with shorter wavelengths, however, photosynthetic efficiency can increase substantially.
This interaction is connected to how the two photosystems used in oxygenic photosynthesis work together.
So far-red should be considered in the context of the complete spectrum, not as an isolated replacement for visible plant light.
The Emerson Enhancement Effect
The interaction between far-red and shorter-wavelength light has deep historical roots in plant science.
Early experiments showed that photosynthesis under combined wavelengths could exceed what would be predicted from measuring those wavelengths separately.
This became known as the:
Emerson enhancement effect
The later discovery of two interacting photosystems helped explain the phenomenon.
Modern LED technology has made this subject easier to study because researchers can precisely control wavelength and photon flux.
That has allowed the contribution of 700–750 nm photons to be measured much more carefully.
What Modern Research Found
Research using multiple crop species has shown that far-red photons can contribute significantly to canopy photosynthesis when combined with traditional 400–700 nm photons.
One influential study evaluated:
14 diverse crop species
and found that adding far-red photons to shorter-wavelength background light increased canopy photosynthesis.
The effect of the far-red photons was comparable to adding photons within the traditional PAR range under the conditions studied.
This provided important evidence that excluding all photons above 700 nm can underestimate photosynthetically relevant radiation.
Evidence From Long-Term Lettuce Growth
Longer-term experiments have also examined how far-red affects entire crop canopies rather than only short-term leaf gas exchange.
In one lettuce study, plants were grown under spectra with the same total photon flux from:
400–750 nm
Some treatments substituted part of the conventional 400–700 nm photon flux with far-red photons.
The treatments containing far-red produced:
- greater leaf expansion
- greater canopy photon capture
- greater daily carbon gain
- greater final biomass
The researchers reported approximately:
29–31% greater biomass
in the far-red substitution treatments.
Importantly, this result did not mean that far-red photons simply caused 30% more photosynthesis per photon.
Part of the growth increase occurred because far-red changed plant morphology, particularly leaf expansion, allowing the canopy to intercept more light.
This distinction matters.
Far-Red Affects More Than Photosynthesis
Far-red light has a second major role:
plant signaling
Plants contain photoreceptors called:
phytochromes
that are highly sensitive to red and far-red light.
Changes in the balance between red and far-red photons can signal information about the plant’s surroundings.
For example, leaves absorb much of the visible red light passing through a canopy while transmitting and reflecting proportionally more far-red light.
Plants underneath neighboring vegetation therefore experience a spectrum enriched in far-red.
This can trigger responses commonly associated with:
shade avoidance
including changes in:
- stem elongation
- petiole elongation
- leaf expansion
- leaf orientation
- biomass allocation
This is why far-red should not be treated merely as “extra PAR.”
It can simultaneously affect photosynthesis and plant architecture.
Does More Far-Red Always Improve Growth?
No.
The biological effect depends on:
- crop species
- far-red fraction
- total photon flux
- background spectrum
- growth stage
- photoperiod
- canopy structure
- environmental conditions
At some light levels, increasing far-red may promote leaf expansion.
Under other conditions, it can favor stem elongation.
A plant grown with a high proportion of far-red may therefore become larger without necessarily developing the morphology desired by the grower.
More far-red is not automatically better.
Newer Research Adds an Important Nuance
The science of far-red photosynthesis is still developing.
Earlier canopy studies provided strong evidence that 700–750 nm photons can make substantial photosynthetic contributions when combined with shorter wavelengths.
More recent research has added nuance to that conclusion.
A 2025 study examining incident far-red photons reported that far-red photons were less efficient than conventional PAR photons for instantaneous leaf photosynthesis under the tested conditions, while still being highly effective at promoting plant growth.
This illustrates an important point:
Photosynthetic efficiency measured at one leaf and final whole-plant growth are not the same outcome.
Far-red can alter:
- leaf area
- plant architecture
- photon interception
- canopy development
Those changes can influence final biomass independently of the instantaneous photosynthetic efficiency of an individual incident photon.
This is one reason ePAR should not be presented as a perfect biological predictor.
What Is ePPFD?
Traditional PPFD counts photons between:
400 and 700 nm
ePPFD extends the photon-counting range to:
400–750 nm
Conceptually:
ePPFD = 400–700 nm photon flux + 700–750 nm photon flux
The unit remains:
µmol/m²/s
For example, imagine a spectrum containing:
PPFD from 400–700 nm = 500 µmol/m²/s
and:
Far-red photon flux from 700–750 nm = 60 µmol/m²/s
The corresponding ePPFD would be approximately:
560 µmol/m²/s
This is a mathematical description of photon quantity.
It is not a claim that every one of those photons always has an identical biological effect under every condition.
PPFD vs ePPFD
The difference is straightforward.
PPFD
Wavelength range:
400–700 nm
Unit:
µmol/m²/s
Used widely for horticultural PAR measurement.
ePPFD
Wavelength range:
400–750 nm
Unit:
µmol/m²/s
Includes the additional 700–750 nm far-red photons.
Neither number tells you the full spectral distribution.
Two lights with identical ePPFD can still contain very different proportions of blue, green, red and far-red photons.
Why Can Two Lights Have the Same PPFD but Different ePPFD?
Suppose two grow lights both produce:
500 µmol/m²/s PPFD
between 400 and 700 nm.
Light A
Far-red 700–750 nm:
10 µmol/m²/s
ePPFD:
510 µmol/m²/s
Light B
Far-red 700–750 nm:
80 µmol/m²/s
ePPFD:
580 µmol/m²/s
Traditional PPFD would show both lights as:
500 µmol/m²/s
But ePPFD reveals the difference in extended photon quantity.
This is one of the main reasons researchers proposed ePAR.
Does ePPFD Tell You the Far-Red Percentage?
It can help, but the total number alone does not.
For example:
ePPFD = 600 µmol/m²/s
does not tell you how many of those photons are far-red.
A useful additional calculation is:
Far-red fraction = 700–750 nm photon flux ÷ 400–750 nm photon flux
For example:
Far-red:
60 µmol/m²/s
ePPFD:
600 µmol/m²/s
Far-red fraction:
10%
The far-red fraction can provide useful spectral context that a total ePPFD number cannot.
Is ePAR an Official Replacement for PAR?
No — not currently.
This is an important correction to many simplified explanations of ePAR.
Research has provided substantial evidence supporting the biological importance of photons from 700–750 nm.
But traditional PAR and PPFD remain established measurement quantities.
ANSI/ASABE S640 defines horticultural photosynthetic photon metrics using the conventional:
400–700 nm
range.
Current horticultural-lighting qualification frameworks also continue to report traditional PPF over 400–700 nm.
Far-red output can be reported separately.
So it is more accurate to say:
ePAR is an important extended measurement concept under active scientific use.
It is not accurate to say:
ePAR has already replaced PAR.
Will ePAR Replace PAR in the Future?
Possibly, but predicting complete replacement is unnecessary.
There are several possible outcomes.
Horticultural lighting may continue using traditional PPFD while adding:
- ePPFD
- far-red photon flux
- far-red fraction
- complete spectral information
for applications where those measurements are useful.
This could be more informative than forcing every horticultural measurement into a single number.
Traditional PPFD also has a major practical advantage:
decades of existing crop data are expressed using 400–700 nm PPFD and DLI.
Changing the definition affects comparisons with historical recommendations, research and product specifications.
So conventional PPFD is unlikely to become useless simply because extended metrics provide additional information.
Why Historical PPFD Data Still Matters
Growers have decades of crop-light research using:
400–700 nm PPFD
and:
400–700 nm DLI
Those data include relationships involving:
- crop yield
- flowering
- propagation
- greenhouse production
- indoor farming
- supplemental lighting
If a crop recommendation states:
500 µmol/m²/s PPFD
you should not automatically substitute:
500 µmol/m²/s ePPFD
and assume the treatments are equivalent.
The second measurement can contain photons between 700 and 750 nm that were not counted in the original recommendation.
Always check which spectral range a value refers to.
What Is eDLI?
The same concept can be extended from instantaneous photon flux to daily accumulated light.
Traditional DLI accumulates conventional PPFD over time and is expressed in:
mol/m²/day
If ePPFD from 400–750 nm is integrated over the photoperiod, the result can be described as:
extended Daily Light Integral
or:
eDLI
also expressed in:
mol/m²/day
As with ePPFD, the wavelength definition should always be stated.
Simply writing “DLI” without identifying the measurement range can become ambiguous when far-red-rich lighting is involved.
Can a Conventional PAR Meter Measure ePAR?
Not necessarily.
A conventional quantum sensor is generally designed around the traditional:
400–700 nm
PAR range.
Its response above 700 nm may intentionally decline.
An ePAR-capable measurement system must be designed and characterized to measure photons through approximately:
750 nm
Therefore, a meter displaying PPFD should not automatically be assumed to measure ePPFD.
Check the specified spectral response of the instrument.
Can You Add a Far-Red Measurement to PPFD?
Conceptually, yes, if the measurements are compatible.
If you accurately measure:
400–700 nm PPFD
and separately measure photon flux from:
700–750 nm
the two photon quantities can be added to obtain:
400–750 nm ePPFD
However, this requires both measurements to be photon-based and correctly defined over their spectral ranges.
A far-red irradiance measurement in:
W/m²
cannot simply be added numerically to:
µmol/m²/s PPFD
because they are different physical quantities.
The far-red energy measurement must first be converted appropriately into photon flux.
Is 700–800 nm the Same as ePAR Far-Red?
No.
This distinction matters.
Some horticultural standards describe a broader far-red band extending approximately:
700–800 nm
But ePAR is commonly proposed as:
400–750 nm
So the far-red portion included in ePAR is generally:
700–750 nm
A measurement labeled “far-red 700–800 nm” therefore contains wavelengths that are not included in a 400–750 nm ePPFD calculation.
Always check wavelength boundaries instead of relying only on the name of the band.
Why Stop at 750 nm?
The choice of 750 nm is based on experimental evidence showing that photosynthetic effectiveness decreases as wavelengths move farther into the far-red.
The boundary should not be interpreted as another absolute biological cliff.
Plants can respond to radiation outside 400–750 nm.
Instead, 750 nm provides a practical boundary for an extended photon metric supported by the observed photosynthetic contribution of far-red photons near the conventional PAR boundary.
As with 700 nm, biological responses do not suddenly become exactly zero at the cutoff.
Measurement ranges are practical definitions.
ePAR Does Not Describe the Complete Spectrum
Suppose two fixtures both provide:
600 µmol/m²/s ePPFD
One could contain:
- high blue
- moderate green
- moderate red
- little far-red
Another could contain:
- low blue
- moderate green
- high red
- substantial far-red
Their ePPFD values could be identical.
Their spectra would not be.
Plants could therefore respond differently.
So:
ePPFD describes extended photon quantity.
It does not replace:
spectral distribution.
Does Higher ePPFD Mean a Better Grow Light?
No.
Just as higher PPFD does not automatically mean a better spectrum, higher ePPFD does not automatically mean a better fixture.
A useful grow-light evaluation may consider:
- PPFD
- ePPFD
- far-red fraction
- spectral distribution
- PPFD uniformity
- photoperiod
- DLI
- fixture efficacy
- crop requirements
Increasing ePPFD by simply adding far-red may also change plant morphology.
The biological outcome therefore depends on more than the total photon number.
Far-Red and Shade-Avoidance Responses
One reason far-red requires careful interpretation is its strong relationship with phytochrome signaling.
Plants can use changes in the red-to-far-red environment to detect neighboring vegetation.
A far-red-rich spectrum may cause responses such as:
- longer stems
- longer petioles
- greater leaf expansion
- altered leaf orientation
These responses can sometimes be desirable.
For example, increased leaf expansion can increase canopy photon capture.
In other production systems, excessive elongation may be undesirable.
So far-red should not be evaluated only by asking:
Does it increase photosynthesis?
Growers should also ask:
How does it change plant architecture?
Far-Red Under Natural Sunlight
Far-red photons are not unusual artificial additions to plant environments.
They are naturally present in sunlight.
They become particularly important under vegetation canopies because leaves strongly absorb some visible wavelengths while transmitting and reflecting relatively more far-red.
Research under natural conditions has shown that photons from:
701–750 nm
can make substantial contributions to photosynthesis, especially in shaded leaves.
In deeply filtered vegetation light, the far-red fraction can become much greater than it is in open sunlight.
This reinforces the idea that conventional 400–700 nm PPFD does not capture every photon relevant to photosynthesis under all natural conditions.
When Is ePPFD Particularly Useful?
ePPFD can provide useful additional information when:
Comparing Far-Red-Containing Grow Lights
Two fixtures with similar conventional PPFD may have substantially different photon output above 700 nm.
Studying Canopy Photosynthesis
Far-red can penetrate deeper into plant canopies and interact with shorter wavelengths.
Working With Full-Spectrum Fixtures
Some broad-spectrum grow lights emit meaningful photon flux beyond 700 nm.
Investigating Plant Morphology
Far-red fraction can influence phytochrome-mediated development.
Performing Research
When the objective specifically involves the contribution of far-red photons, ePPFD can describe photon quantity more completely than conventional PPFD alone.
When Is Traditional PPFD Still Useful?
Traditional PPFD remains extremely useful for:
- comparing with established crop recommendations
- greenhouse measurements
- grow-light mapping
- calculating traditional DLI
- comparing historical experiments
- evaluating fixtures under existing standards
There is no need to discard conventional PAR measurement.
Instead, ePAR adds another layer of information when photons between 700 and 750 nm matter.
PPFD, ePPFD and Spectrum Should Work Together
A useful measurement strategy separates three questions.
How Much Conventional PAR Reaches the Plant?
Measure:
PPFD — 400–700 nm
How Much Extended Photosynthetic Photon Flux Reaches the Plant?
Measure:
ePPFD — 400–750 nm
Where Are Those Photons in the Spectrum?
Examine:
spectral distribution
These are related measurements, but they answer different questions.
Common ePAR Mistakes
Mistake 1: Saying Traditional PAR Is Wrong
Traditional PPFD remains a useful standardized horticultural metric.
ePAR expands the measurement range; it does not make decades of conventional PPFD research invalid.
Mistake 2: Saying All Far-Red Photons Behave Exactly Like PAR Photons
Far-red effects depend strongly on background light, wavelength and biological scale.
Mistake 3: Assuming More Far-Red Is Always Better
Far-red can strongly alter plant morphology.
Mistake 4: Comparing PPFD and ePPFD Without Checking the Wavelength Range
A 400–750 nm reading will generally be higher than a 400–700 nm reading when meaningful far-red is present.
Mistake 5: Calling 700–800 nm Far-Red Entirely ePAR
The commonly proposed ePAR range stops around 750 nm.
Mistake 6: Assuming a Conventional PAR Meter Measures ePAR
The sensor must have the appropriate spectral response through the extended range.
Frequently Asked Questions
What does ePAR mean?
ePAR means Extended Photosynthetically Active Radiation, commonly referring to photons from 400 to 750 nm.
What is ePPFD?
ePPFD is Extended Photosynthetic Photon Flux Density — the photon flux density across approximately 400–750 nm, expressed in µmol/m²/s.
What is the difference between PPFD and ePPFD?
Traditional PPFD measures photons from 400–700 nm.
ePPFD extends the measurement through 750 nm and therefore includes part of the far-red region.
Is far-red light photosynthetically active?
Far-red photons from roughly 700–750 nm can contribute significantly to photosynthesis when combined with shorter-wavelength photons.
Their effectiveness depends on wavelength, background spectrum and biological conditions.
Can far-red light work by itself?
Far-red alone generally drives photosynthesis much less effectively than when it is combined with shorter-wavelength photons.
The interaction between spectral regions is important.
Has ePAR officially replaced PAR?
No.
Traditional 400–700 nm PAR and PPFD remain widely used and are still incorporated into horticultural-lighting standards.
Should growers stop using conventional PPFD?
No.
PPFD remains useful, particularly when comparing measurements with established crop recommendations and existing research.
ePPFD provides additional information when far-red photon flux matters.
Is higher ePPFD always better?
No.
The appropriate photon intensity and far-red fraction depend on crop species, growth stage, spectrum and production goal.
Can two lights have the same PPFD but different ePPFD?
Yes.
If one light produces more photons between 700 and 750 nm, its ePPFD can be higher even though conventional PPFD is the same.
Does ePPFD tell me the spectrum?
No.
It is still a single integrated photon-flux value.
Spectral measurements are needed to determine the distribution of photons by wavelength.
The Key Principle
Traditional PAR asks:
How many photons from 400–700 nm reach the plant?
ePAR asks a broader question:
How many photons from 400–750 nm reach the plant?
Modern research shows that photons immediately beyond 700 nm can contribute to photosynthesis, particularly when they interact with shorter wavelengths.
That makes ePAR scientifically useful.
But the strongest conclusion is not:
“ePAR has replaced PAR.”
It is:
“Conventional PPFD remains useful, while ePPFD can provide additional information about far-red-rich plant-light environments.”
For growers and lighting professionals, the most informative approach is often not to choose between them.
It is to understand:
PPFD + ePPFD + far-red fraction + spectrum
and use the measurement that answers the actual growing question.
References and Further Reading
Zhen, S. & Bugbee, B. — Far-Red Photons Have Equivalent Efficiency to Traditional Photosynthetic Photons: Implications for Redefining Photosynthetically Active Radiation. Plant, Cell & Environment, 2020.
Zhen, S. & Bugbee, B. — Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO₂ Fixation and Increased Photon Capture During Long-Term Studies. Frontiers in Plant Science, 2020.
Zhen, S., van Iersel, M. W. & Bugbee, B. — Why Far-Red Photons Should Be Included in the Definition of Photosynthetic Photons and the Measurement of Horticultural Fixture Efficacy. Frontiers in Plant Science, 2021.
Zhen, S. et al. — Photosynthesis in Sun and Shade: The Surprising Importance of Far-Red Photons. New Phytologist, 2022.
Jin, W. et al. — Incident Far-Red Photons Drive Leaf Photosynthesis Less Efficiently Than PAR Light, but Are More Effective in Promoting Growth. Plant, Cell & Environment, 2025.
ANSI/ASABE S640 — Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms).