Key takeaways in 30 seconds
- → PPE = PPF (µmol/s) ÷ Power (W) — the lamp efficiency in µmol/J
- → HPS: 1.6–1.9 µmol/J / Professional LED: 2.5–3.5 µmol/J
- → Always compare system PPE (driver included), not LED-only PPE
- → L70 ≥ 50,000 h — essential durability criterion
- → Demand an LM-79 or TM-21 report before any purchase
Why watts and lumens are useless
Watts measure electrical consumption — not photon output. A 600 W HPS lamp and a 400 W LED can produce the same PPF (photon flux), with a 200 W difference in consumption.
Lumens measure brightness as perceived by the human eye, with a weighting factor favouring yellow-green (555 nm). In horticulture, this weighting is irrelevant. A lamp that is very "bright" to our eyes may be poorly efficient for plants.
Number of diodes: pure marketing argument. 1,000 poor-quality diodes are worth less than 100 Samsung LM301H diodes.
The only question that matters:
For every unit of electricity I spend, how many photons useful for photosynthesis does my lamp produce? That is exactly what PPE measures.
PPE — Photon Production Efficiency
Definition:
PPE (Photon Production Efficiency) is the ratio between the PAR photon flux emitted (PPF, in µmol/s) and the electrical power consumed (in watts).
PPE (µmol/J) = PPF (µmol/s) / Power (W)
Example: 300 W lamp producing 900 µmol/s → PPE = 900 / 300 = 3.0 µmol/J
Lamp PPE vs system PPE
The distinction is critical and often exploited commercially:
LED-only PPE
Measured directly on the LED chip, without driver, at 25°C. Does not represent real operating conditions. Can be 10–20% higher than system PPE.
System PPE (what to demand)
Includes driver losses (~10%), thermal losses and degradation at real operating temperature. Representative value for your greenhouse conditions.
PPE comparison table by technology
| Technology | Typical PPE (µmol/J) | Lifespan (L70) | Assessment |
|---|---|---|---|
* System PPE (driver included). 2025 values — rapidly evolving market.
Beyond PPE: the often-overlooked criteria
Flux degradation: L70 and LM-80
The PPE of an LED decreases over time (thermal and chemical degradation of semiconductors). The LM-80 standard defines the measurement protocols for this degradation, and TM-21 allows projection of lifespan beyond measured data.
L70
Time after which luminous flux reaches 70% of initial value. L70 ≥ 50,000 h is the minimum for a professional installation.
L90
Time after which flux reaches 90% of initial value. More demanding — the best LEDs (Samsung LM301H) show L90 ≥ 36,000 h.
Junction temperature and thermal management
The PPE of an LED drops significantly with temperature. A chip at 85°C can lose 15–25% PPE compared to 25°C. A good heatsink is not a luxury — it is the condition for the stated PPE to be real.
How to read a horticultural LED data sheet
Always demand
- • System PPE (lamp + driver) in µmol/J
- • Total PPF in µmol/s
- • LM-79 test report (photometric)
- • LM-80 data and TM-21 projection
- • IP rating (IP65 minimum for greenhouses)
- • LED brand (Samsung, Osram, Lumileds...)
Warning signs
- • PPE stated without specifying measurement conditions
- • No third-party LM-79 report (accredited laboratory)
- • "Full spectrum" without SPD (Spectral Power Distribution)
- • Wattage without corresponding PPF
- • No information on the LED components used