Inter-Lighting in Greenhouses: Putting Photons Where the Canopy Actually Needs Them

For decades, greenhouse lighting design followed a relatively simple principle: place more light above the crop and production will increase. That approach works well up to a point. However, as greenhouse crops became taller, denser, and more productive, growers began to encounter a different limitation. The challenge was no longer simply how much light entered the greenhouse, but how effectively that light was distributed throughout the plant canopy.

Inter-lighting bars run between the rows, inside the canopy — the defining move that puts photons where overhead light can't reach.

This issue is particularly apparent in high-wire crops such as tomato, cucumber, and sweet pepper. A modern tomato crop may extend three or four metres vertically while maintaining a dense canopy of leaves and fruit clusters. Although the upper canopy receives substantial amounts of sunlight and supplemental lighting, much of that energy never reaches the middle and lower sections of the plant. The upper leaves intercept and absorb the majority of incoming radiation, leaving deeper canopy layers operating under significantly lower light levels.

From a plant physiology perspective, this creates a surprisingly inefficient situation. The leaves hidden within the canopy remain metabolically active and continue to consume water and nutrients, yet their contribution to photosynthesis is restricted simply because insufficient photons reach them. Inter-lighting was developed to solve this problem. Rather than delivering all photons from above, LED modules are installed directly within the crop canopy, allowing light to reach leaves and fruiting zones that would otherwise remain shaded for much of the production cycle.

With overhead lighting alone, the lower leaves of the canopy receive progressively less light, and photosynthesis in those leaves falls off accordingly. Inter-lighting — placing LED modules within the canopy, either as a sole source or as a supplement to sunlight and overhead electric light — improves light distribution through the full depth of the crop.

The concept may appear straightforward, but its implications extend well beyond simply adding more light. Inter-lighting changes how a crop captures, distributes, and utilizes photons, making it one of the most effective tools available for improving productivity in dense greenhouse crops.

Why light distribution matters more than total light

One of the most important concepts in horticultural lighting is that photosynthesis does not increase indefinitely with increasing light intensity. Every leaf has a light-response curve. At very low light levels, photosynthesis increases almost linearly as additional photons become available. Eventually, however, the leaf approaches saturation and further increases in light intensity produce progressively smaller gains.

This principle explains why inter-lighting can be remarkably effective even when total greenhouse DLI remains unchanged. A leaf near the top of the canopy may already be receiving 700 to 900 µmol·m⁻²·s⁻¹ and operating close to its maximum photosynthetic capacity. Adding another 100 µmol·m⁻²·s⁻¹ from overhead lighting may provide only a modest increase in carbon assimilation. By contrast, a leaf deeper in the canopy may be receiving only 50 to 100 µmol·m⁻²·s⁻¹ and still be operating on the steepest portion of its response curve. Every additional photon delivered to that leaf can generate a substantial increase in photosynthesis.

Under overhead light alone the inner canopy is light-starved; inter-lighting relights the mid and lower strata, putting idle leaves back to work.

Inter-lighting takes advantage of this biological reality. Instead of concentrating photons on leaves that are already approaching saturation, it redistributes them toward leaves where they generate the greatest return. The result is not necessarily a dramatic increase in total light input, but a much more efficient use of the light already being supplied.

This distinction is increasingly important as growers focus on maximizing yield per kilowatt-hour rather than simply increasing installed lighting power.

The relationship between Inter-Lighting and Daily Light Integral

Growers often use Daily Light Integral, or DLI, as the primary metric for evaluating crop light availability. DLI combines light intensity and photoperiod into a single value representing the total number of photosynthetically active photons delivered to a square metre of crop each day.

While DLI remains one of the most useful metrics in horticulture, it does not tell the entire story. Two greenhouses can have identical DLI values and produce different results if the spatial distribution of light differs significantly. In dense tomato or cucumber canopies, a large proportion of the available DLI may be intercepted by upper leaves before reaching lower foliage. From the plant’s perspective, the total number of photons entering the greenhouse matters less than how evenly those photons are distributed throughout the canopy.

Inter-lighting effectively increases the useful DLI experienced by shaded leaves without necessarily increasing the total greenhouse DLI. It transforms previously underutilized foliage into productive photosynthetic tissue and improves the overall efficiency with which the crop converts light into biomass and fruit.

This concept helps explain why inter-lighting often produces measurable yield increases even in greenhouses where average DLI values already appear adequate on paper.

What research reveals about Inter-Lighting performance

One of the reasons inter-lighting has gained widespread acceptance in commercial greenhouse production is that its benefits have been demonstrated repeatedly across different crops, climates, and production systems. Unlike many emerging horticultural technologies that rely primarily on theoretical advantages, inter-lighting has accumulated a substantial body of evidence showing measurable improvements in yield, fruit quality, and crop efficiency.

Tomato production provides perhaps the strongest example. Multiple studies have shown that introducing red-blue or full-spectrum white LED inter-lighting into mature tomato canopies can increase marketable yield by approximately 16%, while also accelerating fruit ripening and improving fruit size. These gains are not simply the result of providing more photons. Rather, they stem from improving light distribution and activating photosynthesis in leaves that would otherwise remain partially shaded throughout the production cycle.

Sweet pepper crops have shown similar responses. Researchers have reported yield increases approaching 16%, largely driven by higher fruit set. In cucumber production, inter-lighting has produced commercial yield improvements of approximately 13%, including in tropical growing environments where annual solar radiation is already relatively high. These findings are particularly important because they demonstrate that inter-lighting is not exclusively a solution for northern European winters. Even in regions with abundant sunlight, crop architecture can create significant internal shading, making intra-canopy lighting beneficial.

Reported inter-lighting yield gains across selected trials. The crop and timing groups come from different studies and are not directly comparable.

Several studies have also observed changes in biomass partitioning. Rather than directing additional resources into vegetative growth, crops receiving inter-lighting frequently allocate more assimilates toward fruit production. For commercial growers, this may be one of the most valuable outcomes of all. Increased fruit yield without proportional increases in vegetative biomass improves both productivity and profitability.

Timing can be as important as Spectrum

When growers first evaluate inter-lighting systems, they often focus on fixture efficacy, spectrum, or installation cost. However, operating schedule can have an equally important influence on performance.

Because inter-lighting fixtures are positioned within the canopy, their waste heat is released directly around leaves and fruit clusters. Although modern LEDs generate significantly less heat than HPS fixtures, they still convert a portion of electrical energy into thermal energy. During winter, this can be beneficial. During hot summer conditions, however, additional canopy heating may become undesirable.

Research has shown that nighttime inter-lighting can improve yield while simultaneously reducing thermal stress during the hottest hours of the day. In some studies, nighttime operation proved more economically attractive than daytime operation because it combined productivity benefits with lower energy costs and improved greenhouse climate management.

For Mediterranean and warm-climate greenhouses, the question is often not whether to use inter-lighting, but when to use it. Intelligent scheduling increasingly represents one of the most effective ways to maximize the return on a lighting investment.

Spectrum selection for Inter-Lighting

Most commercial inter-lighting systems are based on combinations of red and blue LEDs, and for good reason. Decades of research have demonstrated that tomato and pepper crops respond strongly to these wavelengths. Red photons drive photosynthesis efficiently, while blue photons influence morphology, stomatal function, and photomorphogenic responses.

However, the industry has gradually moved toward more sophisticated spectral strategies. Broad-spectrum white LEDs are becoming increasingly common because they create a more uniform spectral environment throughout the canopy and allow greenhouse staff to accurately assess plant health, nutrient deficiencies, disease symptoms, and fruit colour.

Far-red supplementation has also attracted considerable interest. Research indicates that far-red light can improve light interception, stimulate canopy development, enhance dry matter allocation to fruit, and increase productivity under low-light conditions. However, far-red should not be treated as a universal solution. Under conditions of abundant natural sunlight, additional far-red may provide little benefit. As with every aspect of greenhouse lighting, spectrum should be adapted to the crop, season, and production environment rather than selected from a generic catalogue.

The engineering challenge behind successful Inter-Lighting

The biological principles behind inter-lighting are relatively straightforward. The engineering required to implement them successfully is not.

Unlike toplighting fixtures that direct light downward onto a horizontal crop surface, inter-lighting fixtures must deliver photons sideways into a dense three-dimensional canopy. This changes everything about the optical design. The objective is no longer simply generating PPFD but distributing it uniformly through multiple layers of leaves and fruit.

Vertical mounting covers the full canopy height and adapts as plants grow; horizontal mounting uses less hardware and stays clear of irrigation. Both throw light sideways into the foliage, never down.

Thermal management becomes equally important. Every watt of wasted heat is released only centimetres from living plant tissue. Mechanical durability also becomes critical because inter-lighting modules operate in humid environments and are regularly exposed to crop handling, irrigation, and greenhouse maintenance activities.

For these reasons, successful inter-lighting systems require careful integration of LEDs, optics, substrates, thermal paths, mounting geometry, and environmental protection. It is fundamentally a system-engineering challenge rather than a simple lighting purchase.

Why Flexible LED Strip Technology Is emerging as an Ideal Platform for Inter-Lighting in greenhouses

While many commercial inter-lighting systems are based on rigid luminaires, the unique requirements of greenhouse crops increasingly favour a different approach: long, flexible LED strips specifically engineered for intra-canopy applications.

Inter-lighting operates in one of the most demanding environments found in horticulture. Fixtures must function reliably in high humidity, tolerate regular crop handling, withstand irrigation and cleaning procedures, and adapt to changing canopy geometry throughout the growing season. Traditional rigid fixtures can accomplish these tasks, but they often introduce compromises in installation flexibility, light distribution, and maintenance.

This is where Reel-to-Reel (R2R) manufactured flexible LED technology offers significant advantages. Unlike conventional lighting products assembled from multiple interconnected boards, an R2R-manufactured LED strip is produced as a continuous lighting system with highly consistent LED placement, electrical characteristics, and optical performance across its entire length. For greenhouse inter-lighting, this translates directly into improved PPFD uniformity and fewer potential failure points.

Continuous runs, minimal interconnects. A single R2R strip can be produced in one continuous length. For inter-lighting that matters more than it might first appear: a full row of high-wire tomato or cucumber can be lit by one uninterrupted strip rather than a chain of short modules joined by connectors.

One particularly valuable characteristic is the ability to manufacture continuous inter-lighting strips up to 50 metres in length. Long uninterrupted runs simplify installation along greenhouse rows and reduce the number of connectors, cables, and junction points exposed to moisture. Since connectors are among the most common causes of lighting failures in greenhouse environments, reducing their number improves both reliability and long-term maintenance costs.

Environmental protection is equally important. Inter-lighting modules operate directly within dense crop canopies where they are routinely exposed to humidity, condensation, irrigation water, fertilizers, and crop protection treatments. Lumistrips addresses this challenge with its LumProtect encapsulation technology, which allows flexible LED strips to achieve IP67 protection while maintaining excellent optical performance. The result is a lighting platform specifically designed for long-term operation in harsh greenhouse environments.

Our Lumprotect encapsulation makes the R2R strip IP67 waterproof, so the electronics tolerate the humidity, dripping, and cleaning that come with operating inside the canopy rather than above it. For inter-lighting this is not a nice-to-have; it is the difference between a fixture that lasts the crop cycle and one that corrodes partway through it.

The flexibility of the platform extends beyond mechanics. Different crops, seasons, and greenhouse locations require different spectral strategies. R2R-manufactured inter-lighting strips can be configured with high-performance horticultural LEDs such as Nichia Hortisolis™, providing a sunlight-like full-spectrum output optimized for greenhouse production. Alternatively, growers can select red-blue spectra focused on photosynthetic efficiency, or custom spectral combinations incorporating far-red, deep-red, blue, or white wavelengths tailored to specific crop objectives.

This level of customization is particularly valuable because inter-lighting is rarely a one-size-fits-all application. A winter tomato crop in Northern Europe, a cucumber greenhouse in the Mediterranean, and a pepper facility in North America may all benefit from inter-lighting, but each requires a different balance of spectrum, intensity, thermal management, and installation geometry. Flexible LED strip technology enables the lighting system to adapt to the crop rather than forcing the crop to adapt to the fixture.

Where a grower needs one or several rows of LEDs, with the proven red/blue recipe, or a tuned ratio with supplemental far-red for the low-light months, the same R2R platform accommodates it. The principle holds throughout: the design, manufacturing method, the sealing, and the LED choice are decided together, against the specific demands of the inter-lighting job — not pulled from a catalogue and hoped to fit.

As greenhouse production continues to move toward precision lighting strategies, long-life waterproof flexible LED systems offer a compelling combination of uniformity, reliability, scalability, and spectral flexibility. For many inter-lighting applications, they represent a natural evolution from conventional luminaires toward lighting solutions that integrate more seamlessly with the biology and structure of the crop itself.

The Future of Greenhouse Lighting

The evolution of greenhouse lighting is increasingly moving away from the idea of simply producing more photons. Instead, the focus is shifting toward delivering photons where they create the greatest biological value.

Inter-lighting is a clear case where a custom module beats a catalogue fixture, because almost every decision — beam pattern, spectrum, orientation, length, substrate, thermal path — depends on the specific crop, greenhouse, and climate.

Inter-lighting represents one of the clearest examples of this transition. By illuminating leaves that would otherwise contribute relatively little to photosynthesis, growers can improve the efficiency of the entire crop canopy and generate higher yields without necessarily increasing total light input proportionally.

As greenhouse production continues to intensify and energy costs remain a central concern, technologies that improve photon utilization rather than simply photon generation will become increasingly important. For many high-wire crops, inter-lighting is already proving that the future of horticultural lighting is not above the canopy—it is inside it.

What is inter-lighting in greenhouses?

Inter-lighting is a greenhouse lighting strategy where LED modules are installed inside the crop canopy instead of only above it. The goal is to deliver photons directly to shaded middle and lower leaves, improving photosynthesis and yield in dense crops such as tomato, cucumber and sweet pepper.

Which crops benefit most from inter-lighting?

Inter-lighting is most useful for tall, dense, high-wire crops such as greenhouse tomatoes, cucumbers, sweet peppers and eggplants. These crops develop deep canopies where upper leaves intercept much of the available sunlight and overhead supplemental light.

How does inter-lighting improve yield?

Inter-lighting improves yield by increasing the amount of useful light reaching lower and inner leaves. These shaded leaves often operate far below their photosynthetic capacity. By illuminating them directly, the crop can use more of its total leaf area for carbon assimilation and fruit production.

Is inter-lighting better than toplighting?

Inter-lighting is not usually a replacement for toplighting. It is a complementary strategy. Toplighting provides broad illumination from above, while inter-lighting improves light distribution inside the canopy. The best greenhouse lighting systems often combine both approaches.

What spectrum is best for greenhouse inter-lighting?

Red-blue LED spectra are widely used and well supported by research, especially for tomato and pepper crops. However, white or broad-spectrum LEDs can improve visual crop inspection and provide a more uniform spectral field inside the canopy. Far-red can also be useful under low natural light, especially in winter, but should be applied carefully.

Why are optics important in inter-lighting?

Inter-lighting fixtures must direct light sideways into the canopy rather than downward. Without proper optics, photons may hit only the nearest leaves, create hotspots, or spill onto walkways. Side-throwing and asymmetric optics help distribute PPFD more evenly through the crop.

Does inter-lighting increase greenhouse heat?

Yes, all LED fixtures produce some heat. Because inter-lighting modules sit close to leaves and fruit, thermal design is especially important. Efficient substrates, good heat spreading and intelligent operating schedules help avoid unwanted canopy heating.

Should inter-lighting run during the day or at night?

The best timing depends on climate, crop and energy cost. In cool winter conditions, daytime or nighttime use may be beneficial. In warm climates, nighttime or early-morning operation can reduce heat stress and may improve economic performance if electricity tariffs are lower.

Is inter-lighting suitable for Mediterranean greenhouses?

Yes, inter-lighting can be useful in Mediterranean greenhouses, but the design and operating strategy should differ from northern climates. Since total sunlight is higher, the main goal is usually to reduce inner-canopy shading and manage heat carefully rather than simply increase total light.

Can Lumistrips design custom LED modules for greenhouse inter-lighting?

Yes. Lumistrips designs custom horticultural LED modules for greenhouse inter-lighting, including rigid, FR4 and Reel-to-Reel flexible PCB solutions. We combine high-performance LEDs, suitable optics and application-specific thermal design to match the crop, canopy structure and greenhouse environment.