What Is ePAR? Why 400–700 nm Under-Reports Far-Red LED Light

Published 14 November 2026 · XineLam technical notes · Reading time about 8 minutes

ePAR, or extended photosynthetically active radiation, is the 400–750 nm waveband used when a light source emits meaningful energy beyond the conventional 700 nm ceiling of PAR. It does not change how plants work. It changes how much of a fixture's output is counted, and therefore whether two products can be compared on the same number.

The PAR window is a convention, not a cliff edge

Photosynthetically active radiation is defined as photon flux in the 400–700 nm band. That definition is why a fixture is described by photosynthetic photon flux (PPF, micromoles per second), photosynthetic photon flux density (PPFD, micromoles per square metre per second) and daily light integral (DLI, moles per square metre per day). Every one of those numbers inherits the same boundary.

The boundary was a practical decision made when measurement hardware and action spectra were far cruder than they are now. It follows the broad shape of the McCree action spectrum, whose relative quantum efficiency for photosynthesis peaks in the blue near 440 nm and again in the red near 620 nm, with a trough in the green. But an action spectrum is a curve, and a curve does not stop at an arbitrary integer wavelength. Photons at 705 nm are still absorbed by photosynthetic pigments; they simply stop being counted.

That matters in practice because LED spectra are engineered rather than inherited from a discharge lamp. A broad white LED with a phosphor tail, or a fixture with a dedicated 730 nm channel, can place a measurable share of its output in the 700–750 nm region, where a PAR-metered datasheet will report nothing at all.

What actually happens between 700 and 750 nm

Two mechanisms sit behind the interest in this band, and they pull in different directions.

The first is energy transfer inside the photosynthetic apparatus. The Emerson enhancement effect, described in the late 1950s, showed that far-red and shorter-wavelength light supplied together drive more photosynthesis than the sum of the two supplied separately, because the two photosystems are fed in parallel. Work published in 2020 by Zhen and Bugbee concluded that far-red photons, when supplied together with photons in the traditional 400–700 nm range, have a quantum yield of photosynthesis equivalent to those shorter-wavelength photons, with the useful range extending to roughly 750 nm.

The second is photomorphogenesis. Far-red is read by phytochrome, whose two forms interconvert: the red-absorbing form peaks near 660 nm and the far-red-absorbing form near 730 nm. Adding far-red shifts the photostationary state and, with it, stem extension, internode length, leaf angle and the classic shade-avoidance response. A crop can therefore grow taller and capture more light while converting slightly less of it per photon, because the same signal that opens the canopy also reallocates assimilate toward stem tissue.

A 2023 study in the Journal of the American Society for Horticultural Science reported exactly this split: far-red photons increased light capture in a canopy while the photosynthetic capacity per unit of light was lower. Neither finding is wrong, and both are relevant to a grower deciding whether to buy a far-red channel.

How much does the number move?

The size of the change is set by two factors: the share of output that sits above 700 nm, and the energy carried by each photon at those wavelengths. Photon energy is inversely proportional to wavelength, so a far-red photon is cheaper in energy terms than a red one but counts exactly the same in an unweighted photon metric.

Read the last two rows together. A 730 nm photon carries about 9.6 percent less energy than a 660 nm photon, so a fixture that shifts output from red into far-red can post a higher micromole-per-joule figure while drawing the same wall-plug power and delivering no more photosynthetic photon flux in the conventional sense. That is arithmetic, not deception, but it is the reason two efficacy numbers are only comparable inside the same waveband.

The conversion itself is simple. If a measured spectrum puts a fraction f of its 400–750 nm photon flux above 700 nm, then the conventional and extended figures differ by that fraction:

• PPF_ePAR = PPF_PAR ÷ (1 − f)
• DLI_ePAR = DLI_PAR ÷ (1 − f)

For a fixture with a modest phosphor tail, f might be a few percent and the difference is inside measurement noise. For a fixture with a dedicated 730 nm channel running at a meaningful drive level, f can reach the low teens as a share of the extended band, and the gap becomes impossible to ignore. The only defensible way to obtain f is to integrate the fixture's own spectral power distribution rather than to assume a typical value.

Which metric answers which question

Four metrics get quoted interchangeably in datasheets and grow-room conversations. They are not interchangeable, and mixing them is the most common source of bad comparisons.

The useful discipline is to pick the metric that matches the decision. Procurement comparisons should use one waveband, one measurement standard and one power basis. Crop trials should use a metric that reflects the response being measured, which for far-red means watching morphology and dry mass rather than a single photon count.

What ePAR does not fix

Widening the window solves a counting problem and leaves several other problems untouched.

• It is not an action spectrum. ePAR counts a 730 nm photon exactly like a 660 nm photon. The evidence for equivalent quantum yield applies to far-red supplied alongside shorter wavelengths, and the morphology penalty for the same photons is real. An unweighted count cannot express that trade.
• It does not change what your sensor reads. Quantum sensors are filtered to 400–700 nm. If you meter a far-red-capable fixture with a PAR sensor and compare it to its ePAR datasheet, you will conclude the fixture is underperforming when the instrument is the limiting factor.
• It does not re-base your DLI targets. Published DLI recommendations were derived under the 400–700 nm definition. Holding DLI constant on an extended band means removing PAR photons to make room for far-red ones, which is a different treatment from adding far-red on top.
• It does not align qualification programmes. Horticultural qualification listings are built on 400–700 nm measurement. A vendor reporting ePAR outside that framework is describing a real quantity in a way no listing will confirm.

How to specify and verify a far-red-capable fixture

Everything above reduces to a short list of things worth asking for before a purchase order, and a short list of things worth measuring after delivery.

1. Ask for the spectral power distribution, not just the totals. A file covering 380–800 nm at 1–2 nm resolution, in absolute units, lets you integrate both bands yourself and compute f without trusting anyone's summary.
2. Pin the measurement standard and the ambient condition. Photometric reports for luminaires are produced at a controlled ambient temperature; the photon total you see is a laboratory figure, not a greenhouse summer figure. Ask which standard was used and at what ambient the fixture was stabilised.
3. Ask whether far-red is separately controllable. A dedicated channel with independent dimming converts a fixed spectral decision into a trial parameter you can switch off when a trial goes wrong. A fixed phosphor tail cannot be switched off.
4. Confirm the reporting basis in writing. State whether quoted micromoles per joule refer to 400–700 nm or 400–750 nm, and require the same basis from every vendor in the comparison.
5. Verify with your own meter, then interpret carefully. A PAR sensor reading below the datasheet is expected on a far-red fixture. Compare sensors against each other on the same fixture rather than against an extended-band datasheet.

About the publisher

XineLam is the horticultural lighting brand of Zhongshan Koray Opto-Electronic Co., Ltd., a manufacturer with 17 years of experience in LED lighting and 300+ patents in China and internationally. The company builds LED grow lights and horticultural LED modules for greenhouses, vertical farms, tissue culture and research installations, and publishes technical notes on light measurement and specification so that buyers can compare fixtures on the same basis. Product information is available at xinelam.com.

Frequently asked questions

Does ePAR replace PAR as a standard?

No. PAR, defined as 400–700 nm photon flux, remains the basis of PPF, PPFD and DLI figures in test reports, sensor specifications and horticultural lighting qualification programmes. ePAR is a wider reporting window of 400–750 nm used to account for far-red photons; it is complementary, and the two numbers are not interchangeable.

Will a standard PAR sensor measure far-red photons?

Generally not. Most quantum sensors are built with an optical filter that passes 400–700 nm and rejects longer wavelengths, so photons between 700 and 750 nm are excluded by design. A fixture that carries a far-red channel will therefore read lower on a PAR sensor than its ePAR output, and the gap grows with the far-red share of the spectrum.

Does counting far-red photons inflate efficacy claims?

It can, if the bands are mixed. Photon energy falls as wavelength rises, so a 730 nm photon carries about 9.6 percent less energy than a 660 nm photon, which mechanically raises micromoles per joule. Comparing a 400–750 nm efficacy figure against a 400–700 nm figure on the same fixtures is not a like-for-like comparison; insist on the same waveband and the same measurement standard for both.

Should I add far-red to a grow light spectrum?

Treat it as a morphology and canopy tool rather than a free efficiency gain. Far-red shifts the phytochrome photostationary state, which affects stem extension, leaf angle and the balance between light capture and photosynthetic capacity, and the response differs by species and by how much far-red is already present. A separately controllable far-red channel lets you test a treatment on your own crop before committing the whole canopy to it.

Is ePAR the same as yield photon flux?

No. ePAR simply widens the wavelength window and counts every photon in it equally. Yield photon flux and similar weighted metrics apply a species- or action-spectrum weighting to each wavelength, so a far-red photon does not score the same as a red one. ePAR answers how many photons are present; a weighted metric answers how useful a mix of them is expected to be.

References

1. Plant responses to extended photosynthetically active radiation (ePAR). Advances in Plants & Agriculture Research, 2017. doi.org/10.15406/apar.2017.07.00260
2. Zhen S., Bugbee B. Far-red photons have equivalent efficiency to traditional photosynthetic photons: implications for redefining photosynthetically active radiation. Plant, Cell & Environment, 2020. doi.org/10.1111/pce.13730
3. Liu et al. Far-red photons increase light capture but have lower photosynthetic capacity than traditional photosynthetic photons. Journal of the American Society for Horticultural Science, 2023. doi.org/10.21273/jashs05306-23
4. DesignLights Consortium. Horticultural lighting technical requirements and qualified products list. designlights.org/horticultural-lighting