Plant growth under electric lighting is usually specified in red and blue photons, because chlorophyll a and b absorb most strongly near 450 nm and 660 nm. That specification gap leaves out the two far-red channels that decide stem elongation, leaf area, flowering time and canopy architecture. This white paper explains what 730 nm and 780 nm photons actually do inside a plant, how phytochrome photostationary state (PSS) converts a spectrum into a morphology prediction, and how QUEENDOM’s Z-06 L2 ZW (780 nm), Z-07 L3 CK (730 nm) and Z-08 Industrial Plant High-Bay (730 nm) can be combined with conventional R/B fixtures.
1. Why far-red is not optional
Growers who buy fixtures on PPF alone commonly end up with compact, dark-green, slow-elongating crops that fill the tray but never reach the buyer’s grade. The missing variable is the phytochrome system, which reads the ratio of red (660 nm) to far-red (730 nm) photons as a signal, not as an energy source. A fixture that emits 660 nm without complementary 730 nm pushes the phytochrome ratio permanently toward one end of its range and forces the plant into a shade-avoidance-like state that is inappropriate for the crop and the season.
Three commercial consequences follow from ignoring far-red:
- Morphology drift. Stem length, internode spacing and leaf angle shift away from the target grade for cut flowers, herbs and young transplants.
- Photoperiod control failure. Short-day and long-day crops are controlled by the duration of far-red-free darkness, so a poorly filtered far-red source leaks a photoperiod signal.
- Yield ceiling. Emerson enhancement, the increase in photosynthesis when 660 nm and 730 nm photons are delivered together, is worth 10 to 40 percent of assimilation on many leaf canopies and is simply forfeited.
The table below summarises what each band does, and what happens when it is missing.
| Photon band | Primary photoreceptor | Dominant plant response | Consequence if absent |
|---|---|---|---|
| 660 nm (red) | Phytochrome Pr | Converts Pr to Pfr; drives photosynthesis via chlorophyll | Low PPF, poor vegetative vigour |
| 730 nm (far-red) | Phytochrome Pfr | Converts Pfr back to Pr; controls PSS and photoperiod | Compact growth, delayed flowering, low Emerson gain |
| 780 nm (far-red, extended) | Phytochrome tail absorbance; thermal | Deep-canopy penetration; end-of-day extension | Weak canopy penetration in dense crops |
| 450 nm (blue) | Cryptochrome, phototropin | Stomatal opening, compactness, photomorphogenesis | Stretch, thin leaves, low chlorophyll |
| 660 + 730 combined | Both phytochrome states | Emerson enhancement of assimilation | Up to 40 % of potential assimilation lost |
2. Phytochrome, PSS and the Emerson enhancement window
Phytochrome is a biliprotein that switches between two spectrally distinct forms. Pr absorbs maximally in the red near 660 nm and, on absorbing that photon, converts to Pfr. Pfr absorbs maximally in the far-red near 730 nm and converts back to Pr. Because both forms also absorb weakly across the visible band, the equilibrium between them depends on the entire spectrum, not just on two wavelengths. That equilibrium is expressed as the phytochrome photostationary state:
PSS = Pfr / (Pr + Pfr)
Under unfiltered sunlight PSS sits near 0.72. Under a pure 660 nm LED it rises above 0.85, and under pure 730 nm it collapses below 0.15. Each crop has a range that produces the morphology it was bred for; extending or compressing internodes is largely a matter of dialling PSS with the far-red channel.
The second mechanism is Emerson enhancement. When a leaf receives 660 nm and 730 nm photons simultaneously, assimilation exceeds the sum of the two separate responses, because the two photosystems are excited in a more balanced way. The enhancement peaks when far-red is supplied at roughly 15 to 30 percent of the red photon flux, and it saturates rather than growing without limit.
3. Spectral parameters that matter, and how to select the channel
Four parameters carry almost all of the engineering decision for a far-red channel.
Peak wavelength and centring. A far-red die centred at 730 nm sits exactly on the Pfr absorption maximum. A die centred at 780 nm sits on the far tail, where conversion is slower but tissue penetration is deeper. Centring accuracy of ±5 nm is adequate for 730 nm channel work; drift beyond ±8 nm measurably reduces the PSS response per photon.
Photon flux rather than radiometric power. Plant response is quantised in micromoles of photons, so the specification is PPF in µmol/s, not watts. A 730 nm emitter at 1 W of radiometric power delivers far fewer photons than a 660 nm emitter at the same wattage, because each 730 nm photon carries less energy. Sizing must therefore be done in photons.
Beam angle and canopy penetration. Far-red photons pass deeper into a canopy than red photons because fewer pigments intercept them. A wide beam is therefore useful for under-canopy and inter-lighting positions, while a narrow beam suits top lighting in a tall crop.
Duty and channel independence. Red and far-red must be independently dimmable, otherwise PSS cannot be steered. A fixture that ties both channels to one driver cannot deliver end-of-day far-red treatment or a red-to-far-red sweep.
| Parameter | Z-06 L2 ZW | Z-07 L3 CK | Z-08 Industrial Plant High-Bay |
|---|---|---|---|
| Far-red peak | 780 nm | 730 nm | 730 nm |
| Nominal PPF | 130 µmol/s | 160 µmol/s | 900 µmol/s |
| Photon efficacy (400–800 nm) | 2.2 µmol/J | 2.5 µmol/J | 2.6 µmol/J |
| Channel layout | 780 nm dedicated bar | 730 nm + 660 nm mixed | 730 nm + full spectrum |
| Beam angle | 120° × 110° | 120° symmetric | 60° / 90° / 120° selectable |
| Independent far-red dimming | Yes, 0–100 % | Yes, 0–100 % | Yes, 0–100 % |
| Rated power | 60 W | 65 W | 350 W |
| Рабочая температура / температурный диапазон | -20 to +45 °C | -20 to +45 °C | -30 to +50 °C |
3.1 PSS versus far-red fraction
The curve below shows how PSS falls as the far-red fraction of total red-band photons rises. The shaded engineering band is the region in which most greenhouse crops retain their target morphology.
4. Emerson enhancement and photon-budget calculation
Emerson enhancement is specified as an index: the increment in assimilation rate when far-red is added to a red baseline, divided by the baseline assimilation. The table below gives representative indices at three far-red fractions for three canopy types, and the resulting photon budget.
| Crop canopy | Far-red fraction | Enhancement index | Net PPF gain | Recommended channel |
|---|---|---|---|---|
| Tomato, mature canopy | 10 % | 1.12 | +12 % | Z-07 L3 CK |
| Tomato, mature canopy | 22 % | 1.28 | +28 % | Z-07 L3 CK |
| Tomatoes, far-red above 35 % | 35 % | 1.30 | +30 %, diminishing | Z-07 L3 CK, dimmed |
| Lettuce, young tray | 15 % | 1.18 | +18 % | Z-06 L2 ZW at low duty |
| Cannabis-type dense canopy | 20 % | 1.38 | +38 % | Z-08 High-Bay + Z-07 |
| Cucumber, tall trellis | 18 % | 1.22 | +22 % | Z-08 High-Bay |
Sizing a far-red retrofit follows five steps:
- Establish the red baseline. Record the existing fixture’s PPF in µmol/s and its installed density in µmol/m²/s at canopy height.
- Choose the far-red fraction. Select the target from the table above, guided by crop and growth stage. Do not exceed the saturation knee, which typically sits between 25 and 35 percent.
- Convert to far-red PPF. Multiply the red baseline PPF by the chosen fraction.
- Select the fixture count. Divide the required far-red PPF by the single-fixture far-red PPF of Z-06 or Z-07, then check that the mechanical layout fits the trellis geometry.
- Verify electrically. Confirm that the added load fits the existing circuit and that a separate control channel is available for the far-red driver.
5. Photoperiod control and end-of-day treatment
Photoperiod is controlled by the length of uninterrupted darkness, and the phytochrome system reads darkness as a period during which Pfr slowly reverts to Pr. Interrupting darkness with even a small amount of red light resets the clock, which is why a faulty far-red filter on a night-break installation can ruin a short-day programme.
Two techniques are in commercial use:
End-of-day far-red. A short pulse of far-red light at the end of the photoperiod accelerates the conversion of Pfr to Pr, shortening the effective day length perceived by the plant. In practice a 15 minute pulse of 730 nm at 20 to 40 µmol/m²/s is used to promote elongation in young plants and to advance flowering in some short-day crops.
Night interruption avoidance. Where a short-day response is required, all fixtures must be fully extinguished and any far-red leakage quantified. A measurement of 0.1 µmol/m²/s of 730 nm during the dark period is enough to alter PSS in some species.
| Treatment | Dose | Timing | Typical morphological outcome |
|---|---|---|---|
| End-of-day far-red pulse | 20–40 µmol/m²/s for 15 min | Immediately after lights-out | Internode extension, earlier flowering in some cultivars |
| End-of-day extended | 15 µmol/m²/s for 60 min | After lights-out | Stronger extension, higher lodging risk |
| Continuous far-red supplement | 20 % of red PPF | Throughout photoperiod | Compact-to-normal habit, Emerson gain |
| Night interruption (to avoid) | Any measurable 730 nm | During dark period | Photoperiod signal lost, flowering delayed |
6. Common mistakes and how to avoid them
| Mistake | Consequence | Correction |
|---|---|---|
| Specifying far-red in watts rather than photons | 30–45 % underdose versus intent | Convert to µmol/s before selecting the fixture |
| Tying red and far-red to one driver | No PSS control, no end-of-day treatment | Use independent dimming channels |
| Exceeding the enhancement plateau | Wasted electrical energy, stretchy plants | Cap far-red fraction at the crop-specific knee |
| Ignoring dark-period leakage | Short-day programme fails | Measure 730 nm irradiance during the dark period |
| Placing far-red fixtures only at the canopy top | Lower canopy receives no far-red | Use wide beam Z-06 for inter-canopy positions |
| Assuming 780 nm behaves like 730 nm | Slower PSS response than predicted | Treat 780 nm as a penetration channel, not a switch channel |
7. Verification and test methods
- Spectral measurement — record the emitted spectrum at rated drive current and at operating case temperature on a calibrated spectroradiometer, and compute the 660 nm to 730 nm photon ratio directly from the measured curve.
- PPF verification — measure total PPF in an integrating sphere per IES LM-79, then verify the far-red channel independently by filtering the measurement band.
- PSS estimation — compute PSS from the measured spectrum using published phytochrome absorbance cross-sections, and compare against the crop target range.
- Morphology trial — run a paired trial with a far-red-supplemented bay and a control bay of at least 30 plants each, and record internode length, leaf area and days to flowering at fixed intervals.
- Dark-period leakage audit — measure irradiance at canopy height with all fixtures off, using a spectroradiometer sensitive at 730 nm.
8. Conclusion and selection guidance
For a photon-precise 730 nm channel that steers PSS and supports end-of-day treatment, Z-07 L3 CK is the correct first choice: it delivers 160 µmol/s with independent dimming and a 120° beam suited to both top and inter-canopy positions. Add Z-06 L2 ZW at 780 nm where deep canopy penetration or an extended-far-red tail is required, particularly in dense crops under a closed canopy. For tall crops in single-fixture installations, Z-08 Industrial Plant High-Bay integrates the 730 nm channel at 900 µmol/s with selectable 60°, 90° and 120° optics, which removes the mount-rail congestion that comes with adding many small bars. In all three cases, select the far-red fraction on the enhancement curve, then confirm PSS against the crop target before committing to a layout.
9. Referenced standards
- IES LM-79 — Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products
- IES LM-80 — Approved Method: Measuring Luminous Flux and Color Maintenance of LED Light Sources
- IES TM-21 — Projecting Long Term Lumen, Photon and Radiant Flux Maintenance of LED Light Sources
- ANSI/ASABE S640 — Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms)
- EN 12464-1 — Light and lighting of work places, including horticultural work areas
- IEC 62471 — Photobiological safety of lamps and lamp systems
- DLC Horticultural Lighting Technical Requirements (design-ready reference)
10. Contact us
QUEENDOM supplies Z-06, Z-07 and Z-08 horticultural luminaires from stock, together with measured spectral files, PPF data and layout proposals for mixed red and far-red installations. Contact our horticultural engineering group to request sample quantities, photometric files and a canopy-specific far-red sizing study.
Related products and applications
The far-red channels and the luminaires that deliver them are available in the following families.
- Far-red grow light (Z-06)
- Far-red grow light (Z-07)
- Far-red grow light (Z-08)
- Application overview: Lighting application solutions
- More technical papers: Lighting knowledge resources