LIGHTING FUNDAMENTALS
Light spectrum describes the mix of wavelengths reaching a crop. Alongside light intensity and duration, it influences photosynthesis, plant development and crop quality.
Learn how blue, green, red and far-red light contribute to the growing environment—and how to evaluate spectrum as part of a complete commercial lighting strategy.
SPECTRUM BASICS
Light spectrum describes the combination of wavelengths present in light and their relative proportions. Wavelengths are measured in nanometers (nm) and are commonly grouped into bands such as blue, green, red and far-red.
In horticulture, spectrum is often called light quality. It works alongside light intensity and lighting duration to shape the conditions experienced by the crop.
Plants use light both as an energy source for photosynthesis and as a signal that influences growth and development. Different spectral combinations can affect plant shape, leaf expansion and flowering, with responses varying by species, growth stage and growing conditions. Research on plant responses to different LED spectra.
For commercial growing, spectrum selection should support the crop, production stage and desired characteristics within the facility’s complete lighting and climate strategy.
In a greenhouse, crops receive a combination of natural daylight and supplemental lighting. The contribution from each changes throughout the day and across seasons.
In an enclosed indoor farm, fixtures typically provide the entire growing-light spectrum.
Evaluate this combined light environment alongside canopy-level PPFD, photoperiod and DLI when comparing lighting options.
WAVELENGTH OVERVIEW
Different wavelength bands contribute to photosynthesis and provide signals that influence plant development. Their effects overlap and interact, so each band should be evaluated within the complete spectrum reaching the crop.
| Light band | Approximate wavelength range | Role in crop lighting |
|---|---|---|
| Blue | 400–500 nm | Supports photosynthesis and helps regulate plant form. Increasing the blue proportion often produces more compact growth, although responses vary with the crop and lighting conditions. |
| Green | 500–600 nm | Supports photosynthesis and penetrates deeper into leaves than strongly absorbed blue and red light. This helps distribute light to photosynthetic tissues below the leaf surface. |
| Red | 600–700 nm | Efficiently supports photosynthesis. The balance between red and far-red also provides signals that influence plant shape and flowering responses. |
| Far-red | 700–800 nm | Influences shade responses, including stem elongation and leaf expansion, and can affect flowering in responsive crops. Effects depend on its proportion, timing and interaction with other wavelengths. |
These are practical horticultural groupings with approximate boundaries. The green band used here includes yellow wavelengths; definitions of far-red can vary between references.
Blue-light research. Green-light research. Spectral-response research. Far-red and plant development.
Research on far-red’s contribution to photosynthesis commonly focuses on wavelengths between 700 and 750 nm.
When supplied together with light in the traditional 400–700 nm PAR range, these photons can contribute to canopy photosynthesis. Far-red supplied alone drives relatively little photosynthesis. Its contribution depends on the wavelength and the surrounding spectrum. Research on far-red and canopy photosynthesis.
MEASUREMENT BASICS
When comparing lighting data, check which wavelengths are included in each reported value. PAR and ePAR describe wavelength ranges, while PPFD and ePPFD quantify the photons reaching a surface within those ranges.
| Term | Wavelength range | Meaning |
|---|---|---|
| PAR | 400–700 nm | Photosynthetically active radiation: the traditional wavelength range used for reporting photosynthetic light. |
| ePAR | 400–750 nm | Extended photosynthetically active radiation: includes the traditional PAR range plus far-red wavelengths from 700 to 750 nm. |
| PPFD | 400–700 nm | Photosynthetic photon flux density: the photon arrival rate per unit area within the PAR range. |
| ePPFD | 400–750 nm | Extended photosynthetic photon flux density: the photon arrival rate per unit area within the ePAR range. |
PPFD and ePPFD are both expressed in µmol·m⁻²·s⁻¹. The wavelength range determines which photons are included in the total.
Horticultural measurement terminology, ePAR measurement documentation.
Consider an illustrative canopy receiving 300 µmol·m⁻²·s⁻¹ within 400 700 nm, plus 30 µmol·m⁻²·s⁻¹ within 700–750 nm.
Its PPFD is 300 µmol·m⁻²·s⁻¹, while its ePPFD is 330 µmol·m⁻²·s⁻¹. These values describe the same light environment using different wavelength ranges.
Match the measurement range to the crop target or research recommendation being used. There is no universal conversion factor between PPFD and ePPFD because the far-red contribution varies with the spectrum.
A conventional quantum sensor measures the total photon flux density within its measurement band. That total does not reveal the individual proportions of blue, green, red and far-red light.
A calibrated spectroradiometer measures how light is distributed across wavelengths. It can provide the spectral information needed to examine wavelength peaks and calculate band proportions within its measurement range. Quantum sensors and spectroradiometers explained.
READING SPECTRAL DATA
A spectrum graph shows how light is distributed across wavelengths. Reading the axes, peaks and wavelength bands helps explain a fixture’s spectral composition and makes comparisons more meaningful.
Illustrative photon spectrum, normalized to its highest point. This example explains graph interpretation and does not represent measured CultiLight fixture data.
The horizontal axis shows wavelength in nanometres (nm). Check both ends of the displayed range to understand which wavelengths are visible, particularly when assessing far-red output.
Identify whether the graph represents radiant power or photon output, and whether its values are absolute or relative. Energy-based and photon-based distributions require different interpretation when calculating spectral proportions.
Peaks identify wavelength regions with stronger emission. On a photon-based graph, a band’s contribution depends on the area beneath the curve across that band. A broad region can contribute more photons than a taller, narrower peak.
Check the wavelength boundaries and total range used for reported blue, green, red or far-red percentages. Photon fractions calculated over 400–700 nm will differ from those calculated over 400–750 or 400–800 nm when additional photons are present.
Relative graphs scale their values to a reference, often setting the highest point to 1.0 or 100%. Two fixtures with different total photon outputs can therefore have graphs with identical peak heights. CIE definition of relative spectral distribution.
Use the graph to examine spectral shape. Compare total photon output using verified PPF or ePPF data over the same wavelength range, and assess crop-level light using a lighting layout or canopy measurements.
READING SPECTRAL DATA
Commercial grow lights are often described as white, red-dominant or full spectrum. These descriptions can overlap: a fixture may combine white and red LEDs while also being marketed as full spectrum.
Understanding the measured spectral composition helps determine how a fixture fits the crop and growing environment.
Many white LEDs combine blue emission with phosphors that produce a broader range of wavelengths, including green and red.
Broad white light supports photosynthesis and can make crop colour easier to assess than strongly coloured red-and-blue lighting. The proportions of individual wavelength bands vary between LED designs.
A red-dominant spectrum places a large share of its photon output within the red wavelength band. Fixtures may combine red LEDs with blue LEDs, white LEDs or other selected wavelengths.
Review the actual photon proportions and their reporting range. Suitability depends on the crop, production stage and contribution from natural daylight.
For project evaluation, treat “full spectrum” as a description that requires supporting data.
The label alone does not quantify wavelength coverage, photon proportions or ultraviolet and far-red output. Request a measured spectrum and clearly defined band percentages to understand what the fixture delivers.
Correlated colour temperature, or CCT, is expressed in kelvin (K) and describes the warm or cool appearance of white light.
Two sources with the same CCT can have different spectral distributions. A rating such as 4000 K therefore cannot specify the blue, green, red or far-red photon proportions. U.S. Department of Energy guidance on colour and spectrum.
For horticultural selection, evaluate the measured spectrum alongside photon output, crop requirements and the complete growing environment.
CROP AND FACILITY REQUIREMENTS
Begin with the crop, cultivar, production stage and desired commercial outcome. Priorities may include compact growth, leaf colour, harvest weight, flowering time or product uniformity.
Define these objectives before selecting a spectral composition. Then evaluate how the proposed lighting will work within the facility’s daylight, climate and cultivation conditions.
Greenhouse crops receive a changing combination of sunlight and electric lighting. At a given fixture output, supplemental lighting contributes a larger share of the total light when daylight is weak.
Evaluate the combined light reaching the canopy during bright conditions, cloudy periods and any scheduled lighting hours without daylight. Glazing, screens and crop shading also influence the light environment.
Select the supplemental spectrum with this changing daylight contribution in mind.
In an enclosed indoor farm, fixtures typically provide the entire growing light spectrum. Their spectral composition therefore shapes the light environment throughout the scheduled lighting period.
Evaluate the spectrum alongside intensity, photoperiod, temperature, humidity and crop-management practices.
For vertical racks, also consider plant height, leaf expansion, spacing and clearance to the fixtures as the crop develops. The desired plant form should fit both the production objective and the available growing space.
Research in romaine lettuce shows that changing blue and far-red proportions can affect leaf expansion, dry weight and pigment concentrations in different ways. This illustrates why several crop characteristics should be evaluated together. Research on lettuce growth and pigment responses.
Before adopting a new spectral strategy across a facility, use relevant crop research and, where practical, a representative trial. Record the cultivar, growth stage, spectrum, light intensity, photoperiod and environmental conditions.
When far-red changes, record both PPFD and ePPFD so
differences in photon delivery are visible. Assess marketable yield, crop form,
colour, production time and electricity use against the project’s objectives.
LIGHTING FLEXIBILITY
A lighting system’s spectrum and control capabilities should be evaluated together. A fixture may offer a fixed spectral composition, adjustable light output or independent control of selected wavelength bands.
The available functions depend on the fixture’s LED channels, drivers and compatible control system.
A fixed-spectrum fixture uses a selected wavelength mixture that the grower cannot independently adjust during normal operation.
Its overall output may still be dimmable. A spectrum customised during manufacture can therefore remain fixed during use, while allowing the grower to adjust light intensity and operating schedules.
Dimming adjusts the overall light output. With compatible controls, it can support scheduled intensity changes or adjustments based on daylight availability and crop-light targets.
Dimming alone does not provide independent adjustment of blue, green, red or far-red proportions. Even when those proportions remain similar, reducing overall output reduces the absolute photon delivery within each band.
Spectrally tunable fixtures use independently controlled LED channels. Adjusting their relative outputs changes the combined spectrum.
Each channel may contain one wavelength band or a fixed mixture of LEDs. The available spectral adjustments therefore depend on the actual channel configuration.
Before specifying spectral control, confirm which LED channels are independently adjustable, their usable dimming ranges and the required drivers, controller and software.
Request spectral and photon-output data for the intended operating settings. Changing the channel mixture can also change total photonoutput, electrical consumption and efficacy.
Check any limits on operating multiple channels simultaneously. A fixture’s maximum-power setting may differ from an “all channels at full output” setting. DLC explanation of operating states and channel testing.
Choose control capabilities around a defined crop strategy, with settings that the growing team can measure, validate and manage consistently.
PROJECT EVALUATION
Use this checklist when reviewing a proposed lighting spectrum or planning a new installation. Bring the crop objectives, measured fixture data and intended operating conditions together before making a selection.
Identify the crop, cultivar and production stage. Establish which outcomes matter most, such as marketable yield, plant form, leaf colour, flowering time or production consistency.
Identify whether the fixtures will supplement daylight or provide the entire growing-light supply. Record the target PPFD, photoperiod and DLI, together with seasonal daylight conditions, growing dimensions and mounting height.
Obtain a spectrum graph and photon band proportions for the exact fixture configuration being proposed. Confirm the graph’s measurement basis, wavelength boundaries and total range used to calculate percentages.
Compare photon output and efficacy using matching wavelength ranges and operating settings. Include electrical input power, and verify that the proposed layout can achieve the required canopy-level PPFD and uniformity across the cultivated area.
Confirm whether the system offers a fixed spectrum, overall dimming or independently adjustable spectral channels. Check driver and controller compatibility, usable dimming ranges and any limits on channel combinations.
Use relevant crop research and, where practical, a representative trial. Document lighting and environmental conditions, then assess marketable yield, crop quality, plant form, production time and electricity use against the project objectives.
A suitable spectrum supports the crop’s requirements within the facility’s complete lighting and climate strategy.
Evaluate spectral composition together with photon delivery, uniformity, lighting duration, controls and operating cost. Base the final selection on verified data and the crop outcomes the project needs to achieve.
PROJECT EVALUATION
Spectrum selection should reflect the crop, cultivar, production stage and desired outcomes, alongside light intensity, duration and growing conditions.
For greenhouse projects, include the contribution from natural daylight. Evaluate the proposed spectrum using relevant crop research and, where practical, trials under representative production conditions.
Neither approach is automatically better for every application. White LED lighting can support crop growth while making plant colour easier to assess than strongly red-and-blue lighting. The resulting growth and development depend on the actual spectral composition and growing conditions. Research comparing white and blue-plus-red lighting.
Compare measured spectral data, photon efficacy and crop
performance when selecting a fixture.
Yes. Green light contributes to photosynthesis and can penetrate deeper into leaves than blue or red light.
Its contribution depends on light intensity and leaf characteristics. The green appearance of a leaf does not mean that all green light is reflected or unavailable for photosynthesis. Research on blue, green and red light.
No. Far-red light can change leaf expansion, growth and pigment concentration, with different implications for production and quality. Lettuce research illustrates why several crop characteristics should be assessed together when evaluating a spectral change. Research on blue and far-red light in lettuce.
Confirm that the resulting crop form, quality and marketable yield meet the project’s objectives.
Conventional PPFD covers photons within 400–700 nm. Extended PPFD, or ePPFD, covers 400–750 nm and therefore includes part of the far-red region. Apogee’s explanation of PAR and ePAR measurement ranges.
Always identify the wavelength range when comparing measurements or crop targets. The same units do not make PPFD and ePPFD interchangeable.
A Kelvin rating describes the correlated colour temperature of white light—its warm or cool appearance. It does not fully describe the spectral distribution, and different spectra can have the same colour temperature. U.S. Department of Energy guidance on colour and spectrum.
Request the measured spectrum and photon proportions within clearly defined wavelength bands.
Dimming capability alone does not establish independent spectral control. Whole-fixture dimming adjusts overall output, while spectral tuning requires independently controllable LED channels that can change the proportions of different spectral components. DLC guidance on horticultural spectral tuning.
Confirm the available channels, adjustment ranges and compatible controls for the exact fixture configuration.
A standard PAR meter reports total photon flux density within its measurement band. It does not show individual spectral peaks or the proportions of blue, green and red light.
A suitable calibrated spectroradiometer measures the distribution across wavelengths within its specified range. Apogee’s comparison of quantum sensors and spectroradiometers.
TECHNICAL REFERENCES
The following research papers and technical resources provide further detail on spectral effects, light measurement and lighting controls. Interpret research findings within the crops and growing conditions tested.
Park & Runkle (2018), PLOS ONE. An experimental comparison of ornamental seedling growth, visual colour quality and photon efficacy under different LED spectra. Read the study →
Liu & van Iersel (2021), Frontiers in Plant Science. Research examining how light intensity and wavelength affect photosynthesis in lettuce leaves, including the contribution of green light. Read the study →
Zhen & Bugbee (2020), Plant, Cell & Environment. Research investigating how far-red photons within 700–750 nm contribute to canopy photosynthesis when combined with shorter wavelengths. Read the study →
Apogee Instruments — Quantum Sensor FAQs. Manufacturer guidance on PAR and ePAR measurement ranges, sensor selection and the differences between quantum sensors and spectroradiometers. View the technical resource →
Design Lights Consortium. Technical guidance explaining spectral tuning, independently controlled LED channels and considerations when evaluating different operating settings. View the technical resource →
PROJECT SUPPORT
Your crop, growing environment and production objectives should guide the lighting specification.
CultiLight can help review your cultivated area, crop requirements, target PPFD and DLI, and project conditions to develop an appropriate commercial lighting approach. Spectrum selection forms part of that process, alongside fixture layout, photon delivery, controls and installation requirements.
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