Grow Lights Technical Information

The energy produced by the sun reaches the earth as electromagnetic radiation which travels in packets of energy called photons. Each photon has a characteristic energy that determines the frequency of vibration or oscillation. The distance that a photon moves during the oscillation is referred to as a wavelength and is measured in nanometers. Electromagnetic radiation spans a broad range of wavelengths - from 100 nm to about 1 mm (1,000,000 nm). A very small part of this spectrum is visible to the human eye i.e. between 380 nm and 780 nm. Electromagnetic radiation that falls in this range of wavelengths is called visible light.

Photosynthesis
The photo biochemical process that uses light as a catalyst to drive the synthesis of glucose from carbon dioxide and water is called photosynthesis. Photosynthesis is the ultimate source of metabolic energy for all plants. The photo biochemical process can be divided into two phases: light dependent reactions (light reactions) and light-independent reactions (dark reactions). In the light reactions, energy from sunlight is harvested to drive the synthesis of adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), while releasing oxygen as a waste product. In the dark reactions (so named because they do not use light), carbon dioxide is modified by the addition of hydrogen to form glucose and ultimately other carbohydrates, proteins and fats. The assembly of carbon atoms in organic molecules requires the energy-releasing cleavage of high energy bonds of ATPs and NADPHs. In this phase, the ATP loses one of its three phosphates to become ADP (Adenosine diphosphate) and the NADPH loses one electron to become NADP+. The two phases of photosynthesis are interlinked and complimentary. The energy-depleted ADPs and 12 NADPs are restored in light reactions to their high energy forms (ATP and NADPH).

The spectrum of light strongly affects the primary metabolic processes (growth and development of leaves, stems, roots, and floral organs) and production of secondary metabolites (flavonoids, terpenes, cannabinoids). Photosynthetic chlorophylls and carotenoids, and other accessory pigments, absorb more efficiently blue and red light. However, all plants have a species-specific light preference, and photoreceptors that influence their anatomical, physiological, morphological and biochemical properties can span multiple wavelengths. For horticultural lighting, one of the major challenges is to develop a light spectrum that enables efficient activation of different photoreceptors for optimal results in all aspects of plant growth, including photosynthesis, photomorphogenesis, photoperiodism and phototropism.

Red light (600 – 700 nm) offers almost twice the quantum efficiency of blue light for driving photosynthesis. Wavelengths within this range can trigger phytochrome responses related to germination (photoblasty), pigment formation, stem growth, flowering (photoperiodism), circadian rhythm entrainment, and dormancy. Red light is crucial for carbohydrate synthesis, hormone activation, improving phenolic compound concentrations to promote rooting. In general, plants exposed to more red light, in particular hyper red with its absorption peak located at 660nm, grow taller and faster than plants exposed to more blue light.

Blue light (400-500 nm) is another main contributor to photosynthesis. Blue light is also known to trigger two families of photomorphogenic photoreceptors: phototropins and cryptochromes. Wavelengths in this range have been linked to the regulation of stomata opening, chlorophyll concentration, lateral bud growth, root development, transition to flowering, enzyme synthesis, and leaf thickness. High blue irradiation cause plants to have reduced leaf internodal length, compact and bushy growth, high dry matter content and low leaf temperature (efficient transpiration). High deep blue content is needed for seedlings to start germinating and sprouting. Blue light is particularly effective at promoting stem elongation and leaf expansion. However, over exposure to the high energy blue wavelengths can inhibit plant growth.

Green light (500–600 nm) was conventionally thought to have minor importance in biology because plants reflect wavelengths in this range. However, there is strong evidence that green light can penetrate deeper into the plant canopy and, therefore, makes a unique contribution to photosynthetic carbon assimilation and biomass accumulation on both a leaf and canopy level. The absorption of green photons by anthocyanins suppresses production of superoxide, which can cause free radical damage to plant cells. Irradiation with blue light causes plants to have long petioles internodes and high leaf temperature. Green light is partially perceived by phototropins and cryptochrome. When green light is the only source it can induce a small degree of stomatal opening.

Far red light (700-800nm) is absorbed by phytochromes of the Pfr form, which has the absorption peak at 730 nm. Phytochromes are the main regulators of plant circadian rhythm and shade avoidance mechanisms. An additional channel for far red supports quick development of healthy plant biomass which enable large flower formations. Far red light works with deep red light to either delay or advance flowering based on day length perception. High far-red irradiation causes elongation of stem and petioles and allows plants to grow in a more compact form.

UV light, including UV-A (320-400 nm) and UV-B (280 - 320 nm), can cause photoinhibition of chloroplasts when applied incorrectly, resulting in reduced photosynthetic rates, low biomass production, photobleaching and death of leaves. Cryptochromes, phototropins, and members of the Zeitlupe/Adagio family can be stimulated by UV-A light. UV-B light is captured by the UVR8 photoreceptor. In small doses, both UV-B and UV-A light increase the stress tolerance of plants and cause plants to create more secondary metabolites to strengthen their defense mechanisms. Plants grown under ultraviolet radiation tend to have thick leaves and stems, and short internodes.

Artificial Lighting in Horticultural Applications
The individual sensitivities of plant photoreceptors open up opportunities for selective intervention in the metabolic processes and biochemical signalling pathways of plants using artificial light sources. Horticulture lighting is intended to simulate the plant-friendly spectrum of sunlight and provide photosynthetically, photomorphogenically, and photoperiodically active spectrums to support plant growth, development, and yield. By targeting various photoreceptors with select wavelengths and applying the optimum light recipe at every stage of a plant's growth, growers are able to invoke desired photosynthetic responses and morphological changes in their plants.

Horticulture lighting systems play several roles in plant growth:

Supplemental lighting is typically provided in greenhouses where natural sunlight fails to provide the desired daily light integral (DLI) for propagation and transplant production during light limiting conditions (e.g. winter months in northern latitudes, cloudy conditions). Supplemental lighting not only enables enhanced photosynthesis and thereby improves growth and quality of plants, it can also be used to prolong shelf-life or to alter the biochemical properties after harvesting.

Photoperiodic lighting is used to control the flowering times of certain short-day and long-day plants such as chrysanthemum, euphorbia pulcherrima, kalanchoe, gypsophilia and carnations. To induce early or out-of-season flowering for predetermined market dates, the photoperiod of a plant is modified by extending the day length to trick the plant into behaving in the desired way. In addition to flowering induction, photoperiodic lighting is also applied in the seed germination process.

Sole-source lighting is designed for full-cycle cultivation in controlled environment facilities, multi-tier vertical farms, growth chambers and containers. This means all the light is produced by artificial light sources. It is therefore essential that the spectral composition of the artificial light is balanced for optimal plant development at different growth stages from seedling, germination, flowering, fruiting and harvesting.

Types of Horticulture Lighting Installations
Top lighting—plants are illuminated from ceiling level. This type of light installation is mostly found in greenhouses which still take advantage of light from the sun and use artificial light sources to supplement natural daylight. High ceiling mounting with wide-angle beam distribution maximizes upper-canopy photon capturing efficiency and reduces fixture density. However, top lighting systems consume a considerable amount of power in order to ensure that sufficient photosynthetically active photons reach plant canopies across a distance. Traditional light fixtures such as high pressure sodium (HPS) and metal halide (MH) lights radiate a high percentage of thermal energy that can increase plant temperature and therefore must be mounted at some minimum distance above the plants.

Vertical farming—racks of plants are stacked vertically on top of each other and lighting is positioned a short distance from plants. Using controlled environment agriculture (CEA) technology, growers are able to cultivate high density crops in facilities with a low land-area footprint. In vertical farms, sunlight is not available and artificial light is the only source of light. Light fixtures for multi-layer cultivation applications must have a low profile and cannot emit infrared energy because they are mounted directly above and in close proximity of the crop. Growers have complete control over the spectrum and intensity of the sole-source lighting.

Interlighting (intracanopy lighting)—a multi-directional, and typically linear lighting solution designed to provide photosynthetically active lighting along the side or within the foliar canopy in a greenhouse. Light sources are placed in between the plants and the leaves, which prevents mutual shading and encourage previously shaded leaves to photosynthesize. As with vertical farming, intracanopy lighting requires a "cold" light source so that plants will grow safely.


Photosynthetically Active Radiation (PAR)
Electromagnetic radiation over the spectral range of 400 nm to 700 nm is called photosynthetically active radiation (PAR), as predominantly the wavelengths in this range are used by plants to drive photosynthesis. Plant grow lights are primarily designed to target this spectral range, and in some cases provide additional electromagnetic radiation to activate photoreceptors which have absorbance wavelengths outside the PAR region. A plant grow light must convert as much electrical energy as possible into PAR energy. As such, horticulture lighting systems are evaluated by their ability to stimulate photosynthesis. Rather than quantifying the photopic sensitivity by "luminous flux", radiation flux of a grow light is converted into photosynthetic photon flux (PPF) in the PAR region. Assessing interaction between radiation and plants is focused on the quantities of hotosynthetically active photons that fall on plant canopies. Listed below are the most important metrics in horticulture lighting.

Photosynthetic photon flux (PPF) measures the amount of photons emitted by a light source in the PAR region, and is expressed in micromoles per meter squared per second (µmol/s). PPF tells the photon output of a light source, it however does not provide the amount of photons that land on the plants.

Photon efficacy (PPF/W), expressed in micromoles per joule (µmol/J), provides a clear picture of the energy conversion efficiency of a horticulture lighting system or a light source. This metric indicates the total number of photosynthetically active photons generated by one joule of electrical energy.

Photosynthetic photon flux density (PPFD) is a measure of photosynthetically active photons that fall on a square meter of the target area per second. The unit of this metric is expressed as "μmol/m2/s". PPFD is the most important metric with regard to lighting deployment because it provides a field measurement of the number of photons that are incident on a plant canopy. It therefore makes sense to collect this data in order to make sure that neither energy is wasted nor photosynthesis is underdriven. Every plant species has a light saturation point beyond which the rate of photosynthesis stops rising and additional light becomes wasted. The sun delivers approximately 2000 µmol of PPFD at sea level on a bright sunny day. However no plant species on earth can absorb this full quantity of photon flux. A PPFD between 400 and 800 μmol/m2/s is sufficient to drive photosynthesis in plants.

Daily light integral (DLI) quantifies the total number of photons that a plant absorbs over a 24-hour period and is measured in moles of photons per square meter per day (mol/m2/d). DLI is an important variable because the cumulative PPF delivered during a day has a profound effect on plant branching, rooting, stem thickness and flower number. The ideal DLI varies widely between species, cultivation environments, and at different growth stages. For example, to achieve peak production tomato crops require a DLI of 30-35, whereas a DLI of 13 is sufficient for vegetable seedling production, and propagation of cuttings needs only 4-6 mol/m2/d.

PPF : photosynthetic photon flux(μmol/s) the total amount of light (PAR) that is produced by a light source each second.

Efficacy: µmol/J Efficacy refers to how efficient a horticulture lighting system is at converting electrical energy into photons of PAR.
With the PPF and the input wattage, you can calculate the efficiency.

PPFD: Photosynthetic Photon Flux Density(μmol/s.m²) the amount of light that actually reaches your plants and algae within the PAR region or the number of photosynthetically active photons that fall on a given surface each second.

DLI: Daily Light Integral(mol/d.m²) the total amount of light that is delivered to a plant every day. DLI is a cumulative measurement of the total number of photons that reach the plants and algae during the daily photoperiod.

PAR: Photosynthetically Active Radiation is a region of the light spectrum (400 to 700 nm) that plants utilize for the process of photosynthesis. PAR is not a measurement or metric but defines the range of light needed to support photosynthesis.