The electromagnetic radiation produced by horticulture led grow lights targets the active spectrum of photosynthesis, photomorphogenesis, photoperiod and phototropism in plants. In addition to high energy efficiency and accelerated returns, horticulture led grow lights offer unprecedented spectral control, which is essential for horticultural lighting.
horticulture led grow lights are solid-state light sources that generate photosynthetically active radiation (PAR) in the spectral range of 400 to 700 nanometers (nm) to drive photosynthesis in plants. In addition, horticulture led grow lights can be used to generate electromagnetic radiation targeting the active spectrum of photomorphogenesis, photoperiod and phototropism in plants. Horticultural lighting systems have been developed to provide supplemental photoperiodic light in a greenhouse environment or a sole source of photosynthetic light in a controlled indoor environment. The use of energy and spectrally efficient LED technology in greenhouse grow lights has sparked a revolution in indoor plant lamp.
How does light affect plant growth?
Light is essential for plant growth. All plants, including flowering, fruiting and vegetable plants, are autotrophs that have evolved to use light to drive photosynthesis. Photosynthesis is the process plants use to convert water and carbon dioxide into complex carbohydrates (sugars) and oxygen. These carbohydrates, such as cellulose or glucose, provide metabolic building blocks for various biosynthetic pathways. Excess carbohydrates are used for biomass formation, including stem elongation, increase in leaf area, flowering, fruit formation, etc. The photoreceptors responsible for photosynthesis are chlorophyll, although other types of antennal photoreceptors (mainly carotenoids) also facilitate photosynthesis. In addition to driving photosynthesis, specific wavelengths of electromagnetic radiation are used as a source of information to drive photomorphogenesis (changes in plant morphology caused by light), photoperiod (response to light-dark cycles), and phototropism (growth direction). Each type of photoreceptor is sensitive to specific wavelengths and drives a different subset of light morphogenetic changes.
Chlorophyll is a key photoreceptor in green plants and comes in two main forms, A and B. Chlorophyll A is the major plant pigment, accounting for about 75% of photosynthetic activity, with peak absorption responses at 430 nm and 680 nm. Chlorophyll B, with absorption peaks at 460 nm and 640 nm, is an auxiliary pigment that collects energy and transfers it to chlorophyll A. Therefore, chlorophyll B is not independently involved in biosynthesis. In addition, the 3:1 ratio of chlorophyll A to B in plants indicates a major dependence of plants on chlorophyll A in photosynthesis. Although chlorophyll levels increase under electromagnetic radiation whose spectral components are rich in red (long wavelengths) and blue (short wavelengths), chlorophyll reflects most wavelengths in the green region (550 nm to 650 nm), which is where leaves appear Reason is green.
The carotenoid family includes beta-carotene and the major luteins (zeaxanthin, violaxanthin, and lutein). These secondary metabolites absorb light most strongly in the 450 nm to 550 nm range. Carotenoids are yellow to orange because they reflect or transmit light in the wavelength spectrum of about 550 to 650 nm. Carotenoids not only aid in photosynthesis, but also protect chlorophyll from photo-oxidation by dissipating excess light as heat when the photosynthetic area is overloaded with input energy.
Plants also have non-photoreceptor and non-photomorphogenic antenna pigments, such as anthocyanins and flavonoids. They act as sunscreens and block superoxide production in response to high-intensity blue (400-500nm) or ultraviolet (300-400nm) radiation. In plants, anthocyanins, flavonoids, and carotenoids are important bioactive antioxidants that inhibit free radicals and eliminate compounds that can lead to photobleaching and growth inhibition.
Photomorphogenesis is mediated by phytochromes, cryptochromes, and phytochrome photoreceptors. There are two isomers of phytochromes, called Pr and Pfr, which respond to 660 nm red and 735 nm infrared radiation, respectively. Different photomorphogenetic responses mediated by phytochromes are sent to metabolic pathways within plants that regulate seed germination, root development, tuber and bulb formation, leaf expansion, stem elongation, dormancy, flowering and fruit production. Cryptochromes that absorb light in the 340 nm to 520 nm range prevent hypocotyl elongation and mediate entrainment of circadian rhythms in flowering plants. Phototropins are plasma membrane-localized protein kinases that regulate phototropism, chloroplast accumulation, stomatal pore size, leaf flattening, and inhibition of leaf expansion.
What are the advantages of LED in horticultural lighting?
High energy efficiency and long lifetime are the hallmark advantages of LED technology. In horticultural lighting, efficiency has another interpretation. Traditionally, horticultural lighting systems have used high pressure sodium (HPS), metal halide (MH) lamps, or in some cases, fluorescent lamps. However, the energy conversion efficiency of these light sources is very low (usually below 20%). In comparison, the wall-plug efficiency of LED chips is as high as 66%, while the radiant efficiency of phosphor-converted LEDs is much higher than 40%.
In this industry, light source or system efficiency is converted to photon efficiency, which quantifies the efficiency of an LED in producing photosynthetic photon flux (PPF) per joule of electrical energy used, rather than describing it in the human eye. PPF is the total amount of photosynthetically active photons produced by a light source, measured in micromoles per second (µmol/s). The photon efficiency of horticultural LEDs is measured in PPF/W and in µmol/J. In practice, LED grow lights can achieve a photon efficiency of 3.2 PPF/Watt, while typical HPS grow lights can only achieve 1.7 PPF/Watt.
Spectral engineering has been a central theme in horticultural lighting since the beginning. As mentioned earlier, the optical bandwidth between 400 and 700 nm is a major part of the electromagnetic spectrum and can stimulate phytochromes for photosynthesis. Even within the PAR spectrum, not all wavelengths of light are equally effective at driving plant photosynthesis. Red and blue wavelengths are most effective at stimulating photosynthesis and controlling plant morphology, while wavelengths that fall within the green portion of the PAR range have very limited effects on plant growth.
Spectral efficiency describes how much the spectral power density (SPD) of a light source overlaps with the action spectrum required for the most efficient photosynthetic response. The spectral efficiency of HPS, MH, and fluorescent lamps is poor because their SPDs contain a significant portion of photosynthetically inactive light, such as infrared radiation (IR) and ultraviolet radiation (UV). The fixed SPD of these broad-spectrum sources means that photosynthetically active radiation may be oversaturated at some wavelengths and undersaturated at others.
Better spectral control is one of the fundamental advantages that LEDs maintain over traditional horticultural lighting systems. LEDs are essentially monochromatic light sources, emitting in a narrow spectral band, producing colored light such as red, blue, or green. The narrow bandwidth spectrum emitted by the LED can be easily tuned to correspond to the photosynthetic peak of the PAR curve. Narrow-band LEDs can be converted to polychromatic light by phosphors for a wider spectrum to support full-cycle growth of plants. Multi-channel LEDs in RGB, RGBA or RGBW combinations can be superimposed and mixed to make up any color in the LED, enabling unprecedented spectral flexibility and efficiency.
Unlike metal halide and high pressure sodium lamps, which dissipate large amounts of infrared energy (heat) in their radiant beams, LEDs do not radiate thermal infrared energy in their spectrum. The absence of radiant heat allows for maximum photon irradiance close to the plant canopy, which ultimately leads to better photosynthetic productivity while saving space and energy. The high radiant heat flux of HPS grow light lamp requires a certain distance between the light source and the plants, so these lamps can only be used in overhead lighting applications. LED technology enables new strategies, such as interlighting, to achieve uniform photosynthetic illuminance throughout the canopy without generating unnecessary heat.
How are horticulture led grow lights made?
The epitaxial layers of horticulture led grow lights are made of direct bandgap semiconductors because they have a higher probability of radiative recombination than semiconductors with indirect bandgap. The two main semiconductor families are nitride diodes and phosphide diodes. Indium Gallium Nitride (InGaN) produces electromagnetic radiation in the shorter wavelength part of the visible spectrum and is therefore used to make white, green, cyan, blue and royal blue diodes. Red, orange-red, and amber light can be produced using LEDs formed from phosphide semiconductors such as aluminum indium gallium phosphide (AlInGaP), whose small band gap enables the diode to generate longer wavelength radiation.
InGaN epilayers are grown on sapphire, silicon carbide (SiC), or silicon substrates (wafers), while AlInGaP epilayers are grown on gallium arsenide (GaAs) or gallium phosphide (GaP) substrates. High-quality epitaxial growth depends on the lattice matching of the substrate material to the InGaN or AlInGaP layer. Any mismatch between the substrate and the semiconductor layer can lead to microcracks (thread dislocations). This type of atomic defect causes the recombination between electrons and holes to occur non-radiatively, compromising the internal quantum efficiency (IQE) of the LED. Threading dislocations form at the highest density on silicon and sapphire-based GaN LEDs. Compared to chips with silicon or sapphire substrates, SiC substrates generate far fewer dislocations and yield an efficiency advantage of 5% to 10%.
Horticulture led grow lights can be divided into two categories: full-spectrum LEDs and narrow-band LEDs. Full (or wide) spectrum LEDs provide the spectral composition of sunlight without thermal radiation and wavelength waste. The formulations of these LEDs focus on the blue and red regions while providing additional wavelengths such as far red and green to support full cycle cultivation and complete plant development. Narrowband LEDs provide monochromatic output to maximize the most desired wavelengths of light. These LEDs are available in deep blue (450 nm), ultra red (660 nm), far red (730 nm) and green (530 nm) colors. Violet LEDs are neither full spectrum nor narrowband LEDs, but combine the key wavelengths of red and blue into a single package and are standard on the market. Violet LEDs can also be mixed with broad-spectrum lime LEDs to increase yield (fresh weight) and antioxidant levels, while producing high-quality white light to aid visual inspection and plant harvesting.
Full-spectrum LEDs and violet LEDs utilize wavelength conversion and color mixing to achieve the desired wavelength mixing. The LED chips are coated or dispensed with a phosphor mixture whose function is to down-convert a portion of the short wavelengths to longer wavelengths. Therefore, these LEDs are called phosphor-converted LEDs (PC-LEDs). In PC-LED architectures, Stokes losses due to phosphor down-conversion account for a large portion of the total LED energy waste. Narrowband LEDs are direct emitters and do not undergo phosphor downconversion, so they do not suffer from Stokes losses.
Phosphor-converted LEDs and narrow-band LEDs are typically encapsulated in silicone. The difference is that in PC-LEDs, the phosphor is mixed with a silicone polymer to act as a downconverter and protective encapsulant, while in narrow-band LEDs, a transparent silicone polymer is used to keep contaminants out and protect the chip Protected from mechanical shock. Silicon encapsulation has high thermal stability, light stability and chemical resistance. However, in practical applications, additional protection of LEDs is required because the high moisture and gas permeability of silicones can be a degradation factor for diodes in high-humidity cultivation environments.
Types of horticulture led grow lights
PLCC type medium power LEDs (surface mount devices that consume less than 1 watt) are the most popular light sources for general and horticultural lighting because of their relatively higher efficiency and lower cost than other types of packages. However, this type of LED is highly susceptible to accelerated performance degradation and premature failure. As a result, very competitive initial costs often do not translate into good return on investment (ROI), long payback periods and peace of mind. PLCC is short for Plastic Leaded Chip Carrier. The chips for medium-power LEDs using this architecture are mounted on a silver (Ag)-coated metal leadframe molded into a plastic housing in which a reflective cavity is formed to improve light extraction. The cavity is filled with a transparent or phosphorescent hybrid silicone polymer to encapsulate the chip. The electrical connection and thermal path between the LED chip and the lead frame are made by wire bonding. The cavity or plastic housing of inexpensive products is made of polyphthalamide (PPA) or polycyclohexyl terephthalate (PCT), which have poor resistance to photooxidation and thermal degradation. The silver leadframe plating is susceptible to corrosion due to interactions with sulfur-containing contaminants that can penetrate into the LED through the silicone encapsulation. The wire bonds used in PLCC packages may break. Inefficient heat conduction paths can lead to a concentration of heat flux that can introduce high thermal stress into the LED.
High-power LEDs fabricated on ceramic substrates have a robust thermal conduction path capable of delivering high photosynthetic photon flux density (PPFD) to the plant canopy. High-power LEDs can be driven at currents ranging from hundreds of milliamps to over one amp and produce photosynthetic photon fluxes of over 10 µmol/s from a single package. Large chips or multi-chip arrays are mounted on a ceramic substrate that is metallized with thermal vias for efficient heat dissipation. Excellent PPF maintenance and wavelength stability justify the higher cost of these ceramic-based horticultural LEDs.
Chip-on-board (COB) LEDs provide a large light emitting surface (LES) that provides high and uniform PPFD values across the entire canopy. COB LED packages consist of a dense array of LED chips that are die-bonded to a metal core printed circuit board (MCPCB) or ceramic substrate. This large, low thermal resistance substrate allows better thermal contact with a flat, clean heat sink. The removal of the intermediate substrate reduces the thermal resistance of the package. Efficient thermal design allows COB packages to operate at high current densities and deliver PPFs up to hundreds of micromoles per second.
Chip Scale Package (CSP) LEDs eliminate wire bonds and submounts with a flip-chip architecture. This technology significantly reduces thermal resistance within the package, reducing package size and cost. CSP LEDs fundamentally address the performance degradation factors of PLCC-type mid-power LEDs, making them an attractive solution for the horticultural lighting industry.