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HPS vs. LED Supplemental Lighting Physics: Photon Efficacy, Far-Red Kinetics & Greenhouse Heat Balance

In high-latitude European greenhouse clusters (such as Westland in the Netherlands and the Niederrhein region around Straelen in Germany), natural winter Daily Light Integral (DLI) frequently drops below 5 to 8 mol/m²/day. To maintain year-round vegetative growth, reproductive balance, and marketable yields in commercial crops like tomato (Solanum lycopersicum), sweet pepper, and cucumber, supplemental assimilation lighting is indispensable. For decades, 1000W electronic-ballast High-Pressure Sodium (HPS) luminaires served as the industry standard. However, the commercial advent of solid-state Light Emitting Diodes (LED) boasting fixture electrical photon efficiencies exceeding 3.5 to 3.8 µmol/J has triggered a profound paradigm shift in greenhouse engineering.

1. Electrical Photon Efficacy ($\eta_{photon}$) & Spectral Distribution

The core physiological metric governing assimilation lighting is Photosynthetically Active Radiation (PAR, 400–700 nm) or Extended PAR (ePAR, 400–750 nm) delivered per joule of input electrical energy:

Where $\Phi_{PPF}$ is the photosynthetic photon flux (µmol/s) and $P_{elec}$ is electrical power consumption (W = J/s). Modern comparison reveals stark physical contrasts:

• Single-Ended & Double-Ended 1000W HPS: Achieve an electrical photon efficacy between 1.8 to 2.1 µmol/J. Their broad-spectrum emission is heavily weighted in the amber/orange-yellow wavelengths (560–620 nm), which corresponds to lower human-eye photopic efficiency and moderate photosynthetic quantum yield.
• State-of-the-Art Toplighting LEDs: Utilize hyper-efficient monochromatic aluminum-gallium-indium-phosphide (AlGaInP) Deep Red (660 nm) and indium-gallium-nitride (InGaN) Royal Blue (450 nm) diodes paired with phosphor-converted broad spectrums, reaching 3.4 to 3.9 µmol/J—representing a 70% to 90% reduction in electricity consumption per delivered mol of PAR photons.

2. Far-Red (730 nm) Kinetics & The Emerson Enhancement Effect

Historically, photons between 700 and 750 nm were excluded from standard McCree PAR curves. Recent photobiological research from Zhen & Bugbee (2020) and Wageningen UR confirmed that Far-Red (FR, 700–730 nm) photons synergistically drive Photosystem I (PSI) reaction centers (P700), resolving excitation imbalances when combined with shorter 400–680 nm photons preferentially absorbed by Photosystem II (PSII, P680).

This photochemical synergy, known as the Emerson Enhancement Effect, increases canopy quantum yield while simultaneously triggering shade-avoidance morphogenic responses: cell elongation, expansion of leaf lamina area, and enhanced light interception efficiency in lower canopy layers.

3. Greenhouse Thermodynamic Heat Balance Disruption

While LEDs deliver unmatched photon efficacy, the transition from HPS to LED alters the thermal microclimate of the Venlo greenhouse:

• HPS Thermal Signature: HPS lamps emit roughly 35–40% of their input electrical energy as longwave Infrared (IR) radiant heat directly downward toward the crop canopy. This radiant energy raises canopy leaf temperature ($T_{leaf}$) by 1.5°C to 2.5°C above bulk greenhouse air temperature ($T_{air}$), actively driving leaf-to-air Vapor Pressure Deficit (VPD) and transpiration.
• LED Thermal Signature: LEDs emit essentially zero radiant infrared downward; heat is dissipated purely via upward convection and conduction through top-mounted aluminum heatsinks. Under 100% LED retrofits, growers frequently observe lower canopy temperatures ($T_{leaf} \approx T_{air} - 0.5^\circ\text{C}$), requiring supplementary bottom-pipe heating circuit activation (geroei / grow pipes) to sustain target developmental rates (DTR).

4. Economic Payback & Hybrid Lighting Architecture

To capture the energy savings of solid-state photonics without incurring excessive winter boiler gas costs for convective compensation, commercial facilities increasingly adopt Hybrid Lighting Systems (50% HPS / 50% LED). The HPS luminaires provide top radiant heat during sub-zero winter spells, while the LED bars provide high-efficiency photon boosting during spring/autumn shoulder months.