Top Lighting vs Interlighting: Greenhouse Design Guide

Key Takeaways

• Start with crop architecture and the production constraint, then choose where photons enter the canopy.
• Compare equal emitted photons or equal electricity explicitly; these answer different questions.
• Map top and side light separately, with the sensor orientation recorded.
• Approve interlighting through crop response, service access and total operating cost, not a universal yield promise.

For commercial growers, integrators and research facilities, the useful question is where extra light can improve marketable production. This guide focuses on the distribution decision; use the greenhouse supplemental-lighting guide for the wider system workflow.

Top Lighting vs Interlighting: What Is the Difference?

The difference is source location and the direction from which light reaches the crop. Top lighting sits above the canopy. Interlighting, also called intra-canopy lighting, places light sources among tall plants, often within or beside double rows. A combined arrangement uses both locations.

MarsEVOL design recommendation: treat overhead lighting as the first layout to evaluate for shallow crops. Consider interlighting when a maintained, active lower canopy remains light-limited. The distinction depends on training and leaf distribution, not crop name alone. A young tomato plant and a mature high-wire tomato crop need different geometry.

Before adding equipment, review row spacing, pruning, leaf health and existing top-light coverage. For fruiting crops, connect that review to the tomato lighting guide or cucumber canopy guide. An empty lower stem zone provides little useful foliage for an interlighting system to target.

Does Interlighting Always Improve Yield?

No: results depend on the treatment, crop and comparison. Separate evidence for better light distribution from evidence for extra marketable fruit.

Schipper and colleagues (2023) used a validated tomato light model. Their combined arrangement distributed absorbed light more evenly than either location alone under the modeled conditions. Interlighting alone also produced local variation close to the sources. This supports testing combined layouts; it does not establish a universal split or a yield guarantee. ^[1]

Paponov and colleagues (2020) reported higher tomato productivity after adding LED interlighting to an HPS-lit greenhouse. Because the treatment added lighting input, the outcome cannot isolate placement from the effect of additional photons. Its cultivar, spectrum and climate also bound the result. ^[2]

De Visser and colleagues (2014) modeled lamp direction, leaf orientation and crop architecture together, showing why geometry matters. Dueck and colleagues (2012) found that different lighting arrangements also changed heating requirements and crop management. Historical fixtures should not be used as current efficiency benchmarks, but the need to include climate remains relevant. ^{[3], [4]}

The practical inference is to evaluate top lighting vs interlighting as a complete installed treatment. Record whether a trial redistributes the same photon budget, adds photons, or holds electricity constant. Without that distinction, the resulting “improvement” may answer a different question from the investment decision.

How Should You Measure Top and Side Light?

Use repeatable surveys with an explicit position, sensor orientation, wavelength band and source state. A single reading above the crop cannot characterize a tall canopy illuminated from several directions.

Conventional PPFD concerns the 400–700 nm band. ANSI/ASABE S640 provides the plant-radiation terminology framework; quantum-sensor documentation explains spectral response and cosine correction. Cosine correction improves measurement of oblique light relative to the sensor face; it does not turn a planar detector into an all-direction light-absorption meter. ^{[5], [6]}

1. Record geometry. Label row, position along the row, height, distance from emitters and sensor-facing direction. Include crop height, density, leaf removal and screen state.
2. Measure each source separately. At night or under controlled daylight, survey top-only, interlighting-only and combined operation after output stabilizes.
3. Survey several planes. Include upward-facing readings above the crop and side-facing readings at upper, middle and lower foliage, on both sides of a row.
4. Keep the canopy representative. Map near-source positions, gaps, row ends and service aisles. Avoid shadows cast by the operator.
5. Repeat after crop changes. Plant lowering, pruning and canopy closure can change the optical result even when the dimmer setting stays fixed.

Schipper’s measurement method used upward and sideward sensor orientations and isolated lighting sources. That is a useful methodological precedent; its measurement spacing is not a specification for every greenhouse. ^[1] Adapt the PPFD mapping protocol to your row geometry.

Do not add readings from different planes as a single DLI. A top-facing daily integral and a side-facing daily integral describe different receiving surfaces. Contributions can be combined at the same fixed sensor position and orientation when the measurement conditions support it. Estimating total canopy absorption requires a defined photon balance or a validated canopy model. Keep that estimate separate from measured incident PPFD. For the underlying quantities, see PPFD and DLI definitions.

How Do You Compare Photon Output and Electricity Fairly?

Declare the comparison boundary before calculating cost. Equal electrical power, equal emitted PPF and equal estimated absorbed photons are different experiments. Matching one does not automatically match the others.

For top lighting vs interlighting, emitted photosynthetic photon flux (PPF, μmol·s⁻¹) provides a transparent starting budget. It describes source output, not crop interception. Use measured system efficacy at the proposed operating state, including driver losses, and preserve the same wavelength definition.

Illustrative assumptions: a 1,000 m² comparison area, total emitted PPF of 200,000 μmol·s⁻¹, top-light efficacy of 3.0 μmol·J⁻¹ and interlight efficacy of 2.8 μmol·J⁻¹. These are invented inputs for arithmetic, not MarsEVOL specifications, crop targets or measured installation results.

Both options emit the same PPF. Option B draws about 1.43 kW more because its assumed interlight efficacy is lower. At 3,000 equivalent full-output hours per year, annual lighting energy is approximately 200,000 versus 204,286 kWh. At an illustrative flat US$0.12/kWh, the difference is about US$514 annually. Calculations use unrounded values.

The 70/30 photon allocation is an example, not a recommended crop recipe. Equal output does not prove equal absorbed light, leaf photosynthesis or yield. A top-light plane reading must not be relabeled as the interlighting contribution. If installed output varies with dimming, use logged power over time instead of scheduled hours.

The commercial comparison must also include equipment, installation, cleaning, repairs, labor interference, heating and dehumidification. Obtain seasonal tariff and demand-charge data for the actual site. Use the lighting energy and heat-balance guide for the broader retrofit boundary.

Here, added crop contribution means additional sales less the variable costs of producing, harvesting and selling that output. Define an evaluation period matching the crop cycle. Test conservative yield and price cases, including zero extra yield; do not justify equipment with a borrowed percentage from another grower.

What Are the 6 Essential Design Checks?

Approve the layout through crop geometry, input budget, optical verification, controls, commissioning and a replicated crop trial. These are MarsEVOL’s recommended project checks, not a certification scheme.

1. Identify the productive leaf zone

Document cultivar, training system, row spacing, crop height, retained leaves and the pruning schedule. Name the constraint to solve: shaded active foliage, uneven fruit development or a verified distribution problem. Establish how that constraint will be measured.

2. Freeze the comparison basis

Set the total PPF or electricity boundary, spectrum, operating window and daylight conditions. Require source-output and power data for the exact proposed models. Record whether interlighting replaces part of overhead output or adds to it.

3. Check the actual geometry

Review source direction, mounting elevations, row ends, aisle spill and daylight obstruction. Use a crop-aware model where absorption is claimed. A clear-room horizontal PPFD map is insufficient evidence for a mature canopy. Confirm the layout through measurements before accepting model outputs.

4. Specify separate control zones

Give top and interlighting channels independent schedules, limits and logs where the design requires them. Define daylight response, crop-work overrides, communications failure and recovery behavior. A 0–10 V interface by itself does not demonstrate independent channels or a calibrated photon-output response.

5. Commission the installation

Confirm delivered spectrum, output states, channel isolation and electrical demand. Check leaf clearance, surface temperatures, cable routing, cleaning compatibility and access for plant handling. Set acceptance limits with the grower and responsible specialists before measuring; retain dated maps and photographs.

6. Run a crop and operations trial

Use independent treatment zones and sufficient replication to support the decision. Plants sharing one lighting zone may be subsamples. Match cultivar, planting date, climate, irrigation, nutrition, fruit load and crop work; track marketable mass, grade, losses, energy and labor. Compare across a relevant harvest period and report uncertainty before expanding.

Common Top Lighting vs Interlighting Mistakes

• Assuming all lower leaves need more light. Check retained leaf area, health and production role first.
• Calling added-light results a placement advantage. State the extra photons and electricity explicitly.
• Using one PPFD number for the whole canopy. Preserve measurement planes and source states.
• Copying a fixed top/interlight ratio. Optimize against the actual crop and optics.
• Ignoring plant handling. Include lowering, harvesting and cleaning in the layout review.
• Promising savings from fixture efficacy alone. Calculate installed power, annual operation and climate effects.

FAQ: Top Lighting vs Interlighting

Which is better for greenhouse tomatoes?

Top lighting can meet the overhead requirement; interlighting is worth testing where a mature crop retains shaded, productive foliage. Select the arrangement through mapped delivery and a controlled crop comparison.

Can interlighting replace all top lighting?

Do not assume it can. Verify light at the crop head and through the canopy under seasonal daylight. A source within the row serves a different geometry from an overhead system.

Is a 50/50 split the optimum?

No universal optimum is established. A modeled ratio belongs to its canopy and source configuration. Propose several allocations, then verify the candidate layout in the real crop.

Can a horizontal PAR sensor measure interlighting?

It measures photons reaching its own plane with its directional response. A single upward-facing sensor cannot establish the lateral light environment or total absorption of surrounding leaves.

Does interlighting reduce electricity use?

Only if the installed strategy achieves the required outcome with less electricity. Equal emitted photons may still require different power. Include annual runtime and climate loads before claiming savings.

Can an ordinary top-light fixture be installed among plants?

Only when the manufacturer confirms suitability for that orientation and environment. Optics, clearances, temperatures, protection, supports and cleaning conditions require verification for the exact configuration.

MarsEVOL Perspective: Verify the Fixture and Control Boundary

MarsEVOL approaches top lighting vs interlighting as a system-design decision. The official SOLIFY PRO page identifies a compact greenhouse supplemental fixture and 0–10 V dimming. That does not establish a dedicated interlighting configuration. Confirm mounting, optics, environmental ratings and compatibility from the proposed model’s current documentation.

For a combined proposal, request a separate interlighting specification and a documented interface to the greenhouse controls. The cover illustrates overhead SOLIFY PRO form only; it is not a customer installation or a claim of an available interlighting product.

Conclusion

A defensible top lighting vs interlighting decision connects canopy geometry to measured light delivery and crop value. Compare inputs consistently, preserve sensor orientation, separate control channels where required, and test the installed arrangement across a meaningful harvest period. Expand only when marketable production and operating performance justify the complete system.

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References