PAR is traditionally defined as light in the wavelength range:
400–700 nm
This range is used throughout horticulture, plant science, greenhouse lighting and PAR-meter specifications.
But why exactly 400 to 700 nm?
Do plants suddenly begin photosynthesizing at 400 nm and stop at 700 nm?
No.
The 400–700 nm range is a highly useful measurement convention, not a hard biological boundary. Research on far-red light has made that distinction especially important.
Understanding why this range was established—and where its limitations are—helps prevent one of the most common misunderstandings in plant-light measurement.
First: PAR Is a Wavelength Range, Not a Measurement Unit
PAR stands for:
Photosynthetically Active Radiation
Traditionally, it refers to optical radiation between:
400 and 700 nanometers
PAR itself does not mean:
µmol/m²/s
The quantity normally measured in µmol/m²/s is:
PPFD — Photosynthetic Photon Flux Density
PPFD tells us how many photons within the selected photosynthetic waveband reach one square meter each second.
So technically:
PAR describes the wavelength region.
PPFD describes the photon flux density within that region.
This distinction becomes particularly important when comparing traditional PAR with newer concepts such as extended PAR, or ePAR.
Why Was 400–700 nm Chosen?
The answer comes from the development of plant-light science and the need for a practical, standardized way to quantify radiation associated with photosynthesis.
Early researchers measured how effectively different wavelengths drove photosynthesis.
These measurements produced what is called a:
photosynthetic action spectrum
An action spectrum describes how biological activity—in this case photosynthesis—changes with wavelength.
Classic work by plant physiologist Keith J. McCree and others showed that photosynthesis occurs across a broad portion of the visible spectrum and that photon-based measurements across approximately 400–700 nm provided a useful way to characterize photosynthetically active light in crop plants.
By the early 1970s, 400–700 nm had become one of the widely used definitions of PAR.
McCree also compared different ways of measuring this radiation and concluded that photon flux within the 400–700 nm waveband provided a practical measurement that correlated reasonably well with photosynthetic activity across different light sources.
The result was not a claim that nature contains an exact photosynthetic wall at 400 or 700 nm.
It was a useful standard.
Why Count Photons Instead of Lux?
Light can be measured in many different ways.
For plants, photon measurements are especially useful because photosynthesis is fundamentally driven by photons.
Lux is very different.
Lux weights light according to the sensitivity of the human visual system.
Human vision is particularly sensitive to green-yellow light, so two light sources with the same photon output can have very different lux values depending on their spectra.
A PAR quantum sensor instead attempts to count photons across its defined wavelength region.
For traditional PPFD, that region is:
400–700 nm
This is why a PAR meter and a lux meter can report very different-looking results under the same grow light.
They are answering different questions.
Are All 400–700 nm Photons Equally Effective?
Not exactly.
Traditional PPFD counts each photon between 400 and 700 nm equally.
A photon at:
450 nm
and a photon at:
660 nm
each contribute one photon to the PPFD total.
But plant responses are not perfectly identical at every wavelength.
Photosynthetic efficiency varies with wavelength, plant species, leaf structure, light intensity and other environmental conditions.
Blue, green and red photons can all contribute to photosynthesis.
Green light should not be considered “wasted” simply because chlorophyll absorption spectra often show stronger absorption in blue and red regions.
Green photons can penetrate more deeply into leaves and plant canopies, which can affect whole-canopy photosynthesis.
Traditional PPFD therefore does not attempt to predict the exact biological effectiveness of every wavelength.
Instead, it provides a standardized photon count within a defined waveband.
That simplicity is one of the reasons PPFD became so useful.
So Why Does Traditional PAR Stop at 700 nm?
Historically, wavelengths above approximately 700 nm appeared much less effective at driving photosynthesis when they were tested alone.
This phenomenon is associated with the classic:
red drop
Researchers observed a sharp decrease in photosynthetic quantum efficiency toward the far-red region.
That made 700 nm a practical upper boundary for the traditional definition of PAR.
But another important discovery complicated the picture.
When far-red photons were supplied together with shorter-wavelength photons, photosynthetic efficiency could increase substantially.
This interaction became known through research associated with the:
Emerson enhancement effect
In other words:
Far-red light may perform poorly when considered by itself, but its contribution can change when other wavelengths are present.
That distinction is central to our modern understanding of PAR.
Far-Red Light Changed the Picture
Modern LEDs and narrow-band light sources have allowed researchers to study far-red photons more precisely than was possible when the traditional PAR definition was established.
Recent research has shown that photons between approximately:
700–750 nm
can contribute significantly to photosynthesis when combined with photons from shorter wavelengths.
In experiments across multiple crop species, researchers found that added far-red photons could increase canopy photosynthesis similarly to traditional 400–700 nm photons under the tested conditions.
Far-red photons used alone, however, were much less effective.
This means the statement:
“Photons above 700 nm do not drive photosynthesis.”
is too simplistic.
A more accurate statement is:
Far-red photons above 700 nm can contribute to photosynthesis, particularly when they act together with shorter-wavelength photons.
This is one reason scientists have proposed expanding the useful photosynthetic photon range beyond traditional PAR.
What Is ePAR?
The term:
ePAR
means:
Extended Photosynthetically Active Radiation
It is commonly used for the wavelength range:
400–750 nm
The additional 50 nm includes part of the far-red region excluded from traditional PAR.
The proposal is based on evidence that photons between 700 and 750 nm can make meaningful contributions to canopy photosynthesis under appropriate spectral conditions.
So we now have two useful concepts:
Traditional PAR
400–700 nm
ePAR
400–750 nm
They should not be treated as interchangeable measurements.
A meter integrating 400–750 nm may report a higher photon flux than a traditional 400–700 nm PAR meter under a light source with significant far-red output.
Neither instrument is necessarily wrong.
They may simply be measuring different wavelength ranges.
Why Haven’t We Just Changed PAR to 400–750 nm?
Because measurement standards need consistency.
Traditional 400–700 nm PAR has been used for decades in:
- plant science
- greenhouse research
- grow-light specifications
- PPFD maps
- DLI calculations
- PAR sensors
- horticultural lighting design
Millions of historical measurements are based on that definition.
If the wavelength range changes without being clearly identified, comparisons become confusing.
For example, imagine a grow light producing:
800 µmol/m²/s from 400–700 nm
plus:
100 µmol/m²/s from 700–750 nm
A traditional PPFD measurement would report approximately:
800 µmol/m²/s
A 400–750 nm measurement could report approximately:
900 µmol/m²/s
The second number is not simply a “more accurate version” of the first.
It represents a different measurement definition.
That is why the term ePAR is useful: it preserves the distinction.
Does This Mean Traditional PAR Is Outdated?
No.
Traditional PAR remains extremely useful.
A 400–700 nm PPFD measurement provides a standardized way to compare:
- plant locations
- grow lights
- greenhouse zones
- canopy positions
- supplemental lighting
- daily light exposure
The important point is understanding what the number includes—and what it does not include.
A traditional PAR sensor cannot tell you how much 700–750 nm far-red photon flux is present unless that sensor is specifically designed to measure it.
For many conventional horticultural measurements, 400–700 nm PPFD remains a useful and widely understood reference.
When far-red is a significant part of the spectrum, however, additional spectral information or an ePAR measurement may provide useful context.
What About Wavelengths Below 400 nm?
The lower boundary should also not be interpreted as a biological switch.
Below 400 nm lies the ultraviolet region.
Ultraviolet radiation can influence plants in many ways, including:
- photomorphogenesis
- pigmentation
- secondary metabolites
- stress responses
- DNA damage and repair mechanisms
Some near-UV photons may also interact with photosynthetic processes.
But these effects do not mean that UV should simply be added to traditional PPFD.
PAR was designed as a standardized photosynthetic photon waveband, not as a measurement of every wavelength capable of influencing plant biology.
Plants respond to a much broader spectrum than PAR alone.
That is an important distinction.
PAR Does Not Mean “All Light That Matters to Plants”
This is perhaps the most important takeaway.
Plants respond to radiation outside traditional 400–700 nm PAR.
Examples include:
UV radiation below 400 nm
which can affect development, pigmentation and stress responses.
Far-red radiation above 700 nm
which influences photosynthesis under some spectral conditions and strongly affects phytochrome-mediated plant responses.
So the phrase:
“PAR is all the light plants use.”
is not scientifically precise.
A better description is:
PAR is the traditional standardized 400–700 nm waveband used to quantify photosynthetic photon radiation.
That definition leaves room for other wavelengths to have important biological effects.
Far-Red Also Affects Plant Shape
Far-red is important for another reason.
Plants contain photoreceptors called:
phytochromes
that respond strongly to the balance between red and far-red light.
Changes in this spectral balance can influence:
- stem elongation
- leaf expansion
- branching
- flowering
- shade-avoidance responses
For example, vegetation absorbs much of the red light passing through a canopy while reflecting and transmitting proportionally more far-red.
Plants beneath other plants can detect this altered red-to-far-red environment.
This can trigger morphological responses associated with competition for light.
Therefore, far-red can influence plant growth in at least two different ways:
- through its interaction with photosynthesis
- through photomorphogenic signaling
These functions should not be confused.
Why Spectrum Still Matters Even When PPFD Is the Same
Suppose two lights both produce:
500 µmol/m²/s PPFD
within 400–700 nm.
That does not guarantee identical plant responses.
They could have different proportions of:
- blue
- green
- red
- far-red
- ultraviolet
Their traditional PPFD values may be identical while their spectral distributions are very different.
PPFD therefore answers:
How many photons within the defined photosynthetic waveband reach this area each second?
It does not answer:
What is the complete spectrum of this light?
Nor does it predict every possible plant response.
When spectral composition matters, PPFD should be considered together with spectral information.
What Does This Mean for PAR Meters?
Before comparing measurements, check the wavelength definition used by the instrument.
A meter may measure:
traditional PPFD: 400–700 nm
while another system may integrate:
ePAR: 400–750 nm
If the light source contains little far-red, the difference may be relatively small.
If the fixture contains dedicated far-red LEDs, the difference may become much more important.
For meaningful comparisons, always compare:
the same measurement quantity
using:
the same wavelength range
under:
the same measurement conditions
Otherwise, apparently different readings may simply result from different definitions.
What Does This Mean for DLI?
DLI is the accumulated photon exposure over a day.
Traditional DLI is normally calculated from PPFD measured over the traditional PAR range.
For constant light:
DLI = PPFD × hours × 0.0036
If PPFD is based on 400–700 nm, the resulting DLI is also based on 400–700 nm photons.
If an instrument instead integrates 400–750 nm photons, its accumulated value represents an extended photosynthetic photon range.
Again, the wavelength definition matters.
A DLI number without knowing what spectral range was integrated can therefore be incomplete information.
Traditional PAR vs ePAR
| Measurement | Typical Wavelength Range | What It Represents |
|---|---|---|
| Traditional PAR | 400–700 nm | Conventional photosynthetically active waveband |
| PPFD | Usually 400–700 nm | Instantaneous photon flux density within traditional PAR |
| ePAR | 400–750 nm | Extended photosynthetically active waveband including far-red |
| ePPFD | 400–750 nm | Photon flux density within the extended range |
| DLI | Usually based on 400–700 nm | Daily accumulated traditional photosynthetic photons |
Always check the manufacturer’s definition rather than assuming that every instrument uses exactly the same spectral range.
Frequently Asked Questions
Why exactly 400–700 nm?
It became a practical and standardized wavelength range for quantifying photosynthetically active photons based on historical photosynthesis research and measurement practice.
The boundaries are conventions, not exact biological cutoffs.
Do plants use light above 700 nm?
Yes, under some conditions.
Research has shown that far-red photons from approximately 700–750 nm can contribute significantly to canopy photosynthesis when combined with shorter-wavelength photons.
Far-red also affects plant development through phytochrome signaling.
Is far-red as effective as PAR light?
There is no universal answer independent of spectral context.
Research has shown that far-red photons added to shorter-wavelength light can be highly effective at driving canopy photosynthesis under tested conditions.
Far-red used by itself is much less effective.
Its effect also varies across wavelength and spectral composition.
Does PAR include ultraviolet?
No.
Traditional PAR begins at 400 nm.
Ultraviolet radiation below 400 nm is excluded from the conventional PAR definition, even though UV can produce important biological responses in plants.
Is ePAR replacing PAR?
ePAR is increasingly useful for describing plant-light measurements that include 700–750 nm photons.
However, traditional 400–700 nm PAR remains widely used in horticulture, research and instrumentation.
The most important practice is to state clearly which wavelength range is being measured.
Is a higher ePAR reading always better?
No.
A higher photon reading alone does not automatically mean better plant growth.
Plant response depends on many factors, including:
- species
- growth stage
- spectrum
- intensity
- photoperiod
- temperature
- CO₂
- water availability
- nutrient status
Measurement describes the light environment; it does not by itself define the optimal environment for every plant.
The Bottom Line
PAR is traditionally limited to:
400–700 nm
because this range became a practical standardized way to quantify photons associated with photosynthesis.
But it should not be interpreted as a hard biological boundary.
Plants do not suddenly begin responding to light at exactly 400 nm and stop at exactly 700 nm.
Modern research has shown that far-red photons between approximately 700 and 750 nm can make meaningful contributions to photosynthesis when combined with shorter-wavelength photons.
That evidence led to the concept of:
ePAR — 400–750 nm
Traditional PAR is therefore not “wrong.”
It is a defined measurement convention.
The better question is not:
“Which wavelength range is the only one plants use?”
It is:
“Which wavelength range does this measurement include, and is that the right measurement for the question I am trying to answer?”
Once that distinction is clear, PAR, PPFD, ePAR and spectrum data become much easier to interpret.
References
McCree, K. J. (1972). Research on photosynthetic action spectra and definitions of photosynthetically active radiation.
McCree, K. J. (1973). The Measurement of Photosynthetically Active Radiation. Solar Energy, 15(1), 83–87.
Zhen, S. & Bugbee, B. (2020). Research on far-red photons and canopy photosynthesis across multiple crop species.
Zhen, S. & Bugbee, B. (2020). Research on far-red substitution, canopy quantum yield and photon capture.
Zhen, S., van Iersel, M. W. & Bugbee, B. (2021). Why Far-Red Photons Should Be Included in the Definition of Photosynthetic Photons and the Measurement of Horticultural Fixture Efficacy. Frontiers in Plant Science.
ANSI/ASABE S640. Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms).