The topic of a previous post was Photosynthetic Active Radiation (PAR) and Photosynthetic Photon Flux Density (PPFD). PAR is the band of wavelengths between 400 nm and 700 nm. PPFD is a measure of the photons hitting the plant. This post will focus on another important measure for deciding on a grow light, grow light efficiency.
PPFD is a measurement of light intensity within the PAR wavelengths and is measured in µmol/m2s1. The efficiency of a grow light on the other hand is measured in µmol/J. Sometimes written as umol/j, μmol/j, umolj-1 or PPF/W. Grow light efficiency is also referred to as photosynthetic photon efficiency/efficacy (PPE). Thus, µmol/J is the amount of micromoles of photons produced per joule of energy used.
The power draw of a light is measured in watts and watts is the joules consumed per second (J/s). Therefore, if you know the wattage of a single light source and the Photosynthetic Photon Flux (PPF) value, then the efficiency of the light can be calculated. For example: 1 100 µmol/s and 500 W is (1 100 µmol/s) / (500 J/s) = 2.2 µmol/J.
More is more
Philips lighting launched a series of grow lights in 2017 with an efficiency of 3 µmol/J, showing the world what is possible and what efficient lighting will be available in the near future.
A PPE of 1.5 umol/J is considered efficient. Anything above 2.0 umol/J is considered very efficient. For comparison, many (high pressure sodium) HPS fixtures on the market have an efficiency of 1.3 umol/J or less, with some of the best HPS grow lights approaching 1.7 umol/J. The best ceramic metal halide (CMH) grow lights have an efficiency of just under 2.0 umol/J. HPS and CMH efficiencies will increase in the near future, but LED efficiencies will increase at a more rapid rate.
Grow light efficiency importance
Efficiency is why wattage does not matter for determining the light intensity of an LED grow light or any grow light. More efficient grow lights will emit more useful light while drawing fewer watts.
Efficiency may not be as important to some growers. Indoor farmers with a large operation should care about efficiency more than smaller growers since a difference of less than 1 umol/J will have a huge impact on their electricity consumption. The cost difference for an average home grower between a semi-efficient and very efficient LED grow light may not be a huge factor, but as most growers do care about their electricity consumption, every watt being pulled from the wall is important.
The higher the PPE value, the more effective the light fixture at converting electricity into photosynthetic photons and the less energy is consumed to achieve required PPFD values (light levels).
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