Artificial Light for Greenhouse Crops: LED, HPS, and How to Get It Right

by Paramount Seeds on Jul 27 2026

Please note: Paramount Seeds is a seed supplier. We do not sell, supply, or endorse specific artificial lighting products or manufacturers. The information in this article is provided for educational purposes to help growers make informed decisions about their lighting programs.

Light is the engine of greenhouse crop production. When natural light is insufficient — whether due to season, latitude, or weather — artificial lighting bridges the gap between what the sun provides and what your crops need to perform. Getting lighting right is one of the highest-leverage decisions a greenhouse grower can make. This guide covers the fundamentals of artificial lighting, the key technologies available, and how to apply them to your specific crops.

Understanding Light in the Greenhouse Context

PAR, PPFD, and DLI — The Metrics That Matter

Before selecting lighting equipment, it's essential to understand the metrics used to quantify light for plant growth:

PAR (Photosynthetically Active Radiation) — the wavelength range of light (400–700nm) that plants use for photosynthesis.
PPFD (Photosynthetic Photon Flux Density) — the intensity of PAR light measured at the plant canopy level, expressed in μmol/m²/s.
DLI (Daily Light Integral) — the total amount of PAR light received over a 24-hour period, expressed in mol/m²/day.

DLI = PPFD (μmol/m²/s) × photoperiod (hours) × 0.0036

Understanding mol/m²/day

The unit mol/m²/day (moles of photons per square metre per day) can seem abstract at first. Here's what it actually means:

A mole in this context is a count of photons — specifically, 6.022 × 10²³ photons (Avogadro's number).
Per square metre refers to the area of the plant canopy receiving that light.
Per day means the total accumulated over a full 24-hour period.

A worked example: A greenhouse LED fixture delivering 300 μmol/m²/s at canopy level, running for 18 hours, delivers a DLI of:
300 × 18 × 0.0036 = 19.4 mol/m²/day

Add 6 mol/m²/day of natural winter light and your total DLI reaches approximately 25 mol/m²/day — within the optimal range for tomato and cucumber production.

Natural Light as the Baseline

In most greenhouse production regions, natural DLI varies significantly by season. In northern latitudes, winter DLI can drop to 5–10 mol/m²/day — well below the minimum for productive greenhouse vegetable crops. For more on how lighting interacts with your growing system, see our hydroponics vs. aquaponics guide.

Lighting Technologies: LED vs. HPS

High-Pressure Sodium (HPS)

Proven track record — decades of data on crop response across all major greenhouse vegetables
High output — HPS fixtures deliver high PPFD at relatively low capital cost per μmol
Heat output — HPS generates significant radiant heat, beneficial in cold climates but problematic in warm climates or summer production
Spectrum — heavy in the yellow-orange range (550–650nm); lacks the blue spectrum that influences plant morphology
Efficiency — typically 1.0–1.7 μmol/J
Lifespan — 10,000–18,000 hours before significant lumen depreciation

LED (Light Emitting Diode)

High efficiency — modern greenhouse LEDs deliver 2.5–3.5+ μmol/J
Tunable spectrum — specific red:blue ratios; can include far-red, UV, and other wavelengths
Low heat output — minimal radiant heat at the canopy; fixtures can be placed closer to the crop
Long lifespan — 50,000+ hours with minimal lumen depreciation
Higher capital cost — payback typically within 3–5 years depending on electricity rates and operating hours
Dimming capability — most LED systems can be dimmed and controlled dynamically

Which to Choose?

For new installations, LED is the clear choice in most situations. HPS remains viable for operations with existing infrastructure, low electricity costs, or cold climates where heat output is a benefit.

DLI Targets by Crop

Tomato

Minimum DLI: 20 mol/m²/day
Optimal DLI: 25–35 mol/m²/day
Maximum photoperiod: 18 hours
Notes: Highly responsive to increased DLI up to ~30–35 mol/m²/day. See our tomato plant spacing guide for canopy management context.

Browse Tomato Varieties →

Cucumber

Minimum DLI: 20 mol/m²/day
Optimal DLI: 25–30 mol/m²/day
Maximum photoperiod: 18–20 hours
Notes: High-light crop with strong yield response to increased DLI. Intercanopy lighting beneficial in long-season production.

Browse Cucumber Varieties →

Lettuce

Minimum DLI: 12 mol/m²/day
Optimal DLI: 17–22 mol/m²/day
Maximum photoperiod: 20 hours
Notes: Well suited to sole-source LED production. Tipburn risk increases at very high DLI or under poor airflow — see our tipburn guide. Far-red light can accelerate growth and increase fresh weight at harvest.

Browse Lettuce Varieties →

Pepper

Minimum DLI: 20 mol/m²/day
Optimal DLI: 25–30 mol/m²/day
Maximum photoperiod: 16–18 hours
Notes: Some varieties are day-length sensitive for flowering. High DLI supports fruit set and color development in colored bell pepper production.

Browse Pepper Varieties →

Microgreens

Optimal DLI: 12–20 mol/m²/day depending on species
Maximum photoperiod: 16–18 hours
Notes: Lower light requirements than mature vegetable crops. LED preferred due to low heat output. See our microgreens production guide for crop-specific DLI guidance.

Photoperiod Management

Most greenhouse vegetables are day-neutral for flowering — they flower based on age and development rather than day length.
Avoid continuous light for tomato — most varieties develop leaf curl and interveinal chlorosis under 24-hour lighting. Maintain a minimum 6-hour dark period.
Use dynamic lighting control — modern LED systems with light sensors can automatically adjust output based on real-time natural light levels.
Night interruption lighting — for photoperiod-sensitive crops, a brief period of low-intensity light during the night can substitute for a full extended photoperiod.

Spectrum Considerations

Blue light (400–500nm) — promotes compact growth, thick leaves, and stomatal opening. Minimum 10–15% blue recommended for most crops.
Red light (600–700nm) — most efficient wavelength for photosynthesis; most LED fixtures are red-dominant.
Far-red light (700–800nm) — accelerates growth through the Emerson enhancement effect; particularly valuable in lettuce production.
Green light (500–600nm) — penetrates deeper into the canopy; full-spectrum white LEDs include green and are preferred by some growers for worker comfort.

Practical Lighting Design Considerations

Uniformity — aim for less than 20% variation in PPFD across the growing area.
Mounting height — LED fixtures can be mounted closer to the crop than HPS. Intercanopy LED systems in tall crops dramatically improve light distribution to lower leaves.
Reflective surfaces — white or reflective greenhouse cladding, floors, and walls significantly improve light utilization efficiency.
Energy management — invest in smart controls, light sensors, and dimming capability to minimize energy waste.
Heat management — account for lighting heat output in your climate control calculations.

The Bottom Line

Note: Paramount Seeds does not supply artificial lighting products or equipment. For lighting procurement, consult specialist greenhouse lighting suppliers and request photometric plans specific to your greenhouse dimensions and crop requirements.

Artificial lighting is a precision tool — not a switch you flip and forget. The right technology, the right DLI target, the right spectrum, and the right controls all work together to deliver consistent, high-quality crops year-round.

Tomatoes → Cucumbers → Lettuce → Peppers → Contact Us →

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