An overview
Since the invention of light-emitting diodes (LEDs), there has been a significant rise in the amount of photons that can be converted from electric energy; nevertheless, this is getting close to reaching a physical limit. At the point that all of the input energy is transformed into energy in the form of photosynthetic photons, the theoretical maximum efficiency is achieved. The efficiency of blue LEDs can reach up to 93%, it is possible for phosphor-converted "whites" to reach 76%, and red LEDs can reach 81%. Because of these enhancements, new prospects for horticulture lighting have become available. In this article, we will discuss the following topics: (1) the underlying physics and efficiency of LEDs; (2) the current efficacy of LEDs; (3) the effect of spectral quality on crop output; and (4) the potential efficacy of horticulture fixtures. This can be accomplished by optimising the spectral impacts on plant morphology, which differ from species to species. This will allow for advancements in the conversion of photons to yield. On the other hand, spectrum effects on photosynthesis are very comparable across species; nevertheless, the currently accepted definition of photosynthetic photons, which ranges from 400 to 700 nanometres, might require some modification. Current droop, heat droop, driver (power supply) inefficiencies, and optical losses are the four characteristics that are inherent to all LED fixtures. The top limit of LED fixture efficacy is established by multiplying the LED package efficacy by these four factors. Considering the current state of LED technology, the calculations suggest that the efficacy limitations for white and red fixtures are 3.4 µmol J−1, while for blue and red fixtures, the efficacy limits are 4.1 µmol J−1. By including optical protection against water and excessive humidity, these values are reduced by around 10%. There are tradeoffs between peak efficacy and cost, which we detail.
Physics
The term "efficiency" refers to ratios that have the same units in both the numerator and the denominator, and these ratios can be stated as a percentage. The term "LED efficiency" refers to the ratio of the optical power output to the electrical power input, expressed as watts per watt or percent. The term "efficacy" refers to ratios that are expressed in a variety of units. The term "efficacy" is used in the field of horticultural lighting to describe the amount of photons that are produced per second per watt of input electricity. Due to the fact that a watt is equal to a joule per second, this can be simplified to µmol per time. The Planck–Einstein relation, which is often referred to as Planck's equation, is a mathematical expression that describes the relationship between photon energy and wavelength. Using this equation, we can see that the relationship between energy and wavelength is inversely proportional. Equation used to convert between efficiency and efficacy, as well as to compute the highest potential photosynthetic photon efficacy for a particular spectrum. This equation is used to convert between efficiency and efficacy.
In order to determine the influence that photons have on plants in relation to the amount of electrical power that is input, we obtain the relevant units by converting the efficiency of LEDs into their efficacy. This is in accordance with a different physical law known as the Stark–Einstein Law, which holds that for every photon that is absorbed, only one molecule is actually capable of reacting. It is possible to restate this law by stating that one photon is capable of excitating one electron. The scope of photon efficacy in this work is restricted to photons with wavelengths ranging from 400 to 700 nanometres, with the exception of far-red LEDs, which incorporate photons with wavelengths as high as 800 nanometres. It is common for LED package makers to report efficacy in lumens per watt because this is a meaningful metric for human illumination. However, this metric is not applicable for horticultural lighting because it is a measurement of photons weighted for human vision based on the human eye's sensitivity to different colours.
An LED package, which is the LED chip contained within a housing, is what is meant by the term "LED" in this article. In addition to facilitating mechanical and electrical connections to the fixture, the housing or packaging also serves as a heat channel, influences the distribution of photons, and incorporates the phosphor layer for white LEDs (for more information, see below). LED packages are the target audience for LED performance standards. When LED packages are incorporated into a fixture, this is referred to as an LED fixture.
LEDs' fundamental efficiency at their core
There are three sub-efficiencies that contribute to the overall efficiency of LEDs (LED packages), which is the product of these three sub-efficiencies:
The ratio of the emitted photon energy, expressed in electron volts, to the applied voltage (Vphoton/Vf) is the definition of electrical efficiency. This ratio is influenced by the LED's internal electrical resistance.
The internal quantum efficiency, also known as the photon per electron, refers to the process by which electrons are converted into photons. This process is influenced by non-radiative recombination routes, which include impurities and microphysical defects.
The ratio of photons that leave the LED semiconductor material to the total number of photons that are generated is referred to as the photon extraction efficiency. This ratio is influenced by both internal reflection and reabsorption. The phrase "package losses" is used in the LED industry to refer to any losses that occur during the process of harvesting photons from an LED package. The LED package types might have a wide range of variations in these.
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