by Mike Rycroft, Now Media –
Greenhouse, tunnel, and indoor agriculture is increasing worldwide. Appropriate lighting within indoor plant farms can significantly improve plant growth, extend growing seasons, and positively affect plant health.
Light emitting diodes (LEDs) were initially used as indicator lamps in electrical equipment. However, this technology has so developed that it is now used for illumination purposes, and as such can be used to influence indoor plant growth.
Plants need light for growth, and growing cycles are influenced by the level of light and the daily amount of light received. In indoor agriculture, the substitution of natural light with artificial light can influence growing cycles. Supplemental lighting has been in use for many years in commercial greenhouses to ensure a consistent daily amount of light, and to optimise growth seasons. Growth and development are affected by two things: the wavelength and the amount of light received daily.
Light spectrum and growth
Figure 1 shows the spectral distribution of daylight. Not all wavelengths of light produce photosynthesis. Photosynthetically active radiation (PAR), which has a wavelength of between 400 and 700 nm, is the spectrum that artificial lighting for horticulture is based on. Research has shown that radiation in the UV and IR regions is also useful for plant pest and disease control.
Plants do not use the PAR band for growth but rely on rather narrow spectral bands within the range. This is illustrated in Figure 2. Light absorption is greatest in the red and blue bands. Increasing the amount of light in these bands, above the overall light level, promotes growth stages of plants.
Daily amount of light received
This depends not only on the level of light received but also on the duration. Because light levels vary, the daily amount of light received is expressed as the Daily Light Integral (DLI) over the whole year. DLI is the amount of PAR received each day as a function of light intensity and duration. With daylight as the source, the DLI will vary both seasonally and with weather conditions. Artificial lighting is used as a supplement to daylight to produce a consistent DLI over the whole year.
Lighting designers and horticulturalists tend to use different terminology. While lighting engineers speak in terms of lumens and illuminance, horticulturalists speak in terms of photosynthetically active radiation (PAR) and photosynthetic photon flux density (PPFD). Instead of lumens, there are micromoles, and instead of luminous flux, there is quantum flux. DLI is measured in terms of moles of light per m2 per day, or mol·m-2·d-1.
Artificial lighting for horticulture and agriculture
Artificial lighting is used in two ways: Supplemental lighting and indoor lighting.
Supplemental lighting (SL)
This is used in addition to daylight lighting in greenhouses to provide a boost of the spectral bands which are important to growth, and to compensate for variations in daily sunlight hours within seasons and overcast conditions.
Indoor lighting (AL)
Artificial lighting is used as the only light source and provides spectral balance and DLI control. AL is used in vertical farming and indoor horticulture. The type of lighting used varies with the application (see Figure 3).
Artificial lighting systems
Traditionally, high-intensity discharge (HID) and sodium lamps have been used for this purpose. Although effective in providing a high intensity light, no control over the spectrum was possible, and it was not possible to generate artificial light which met all wavelength requirements for optimum plant growth. The advent of LEDs has made it possible to tune the spectrum of agricultural lights to vary the number of different spectra which affect plant growth and development.
It is possible to produce LEDs in a range which covers the full PAR spectrum (Figure 4). Lighting systems are constructed by combining LEDs of different colours to provide the required spectral balance.
Red/blue light systems (R/B)
Because red and blue are the colours seen to affect growth the most, some systems are based on combinations of red and blue LEDs. This produces a purple, or shade of light which is useful for supplemental lighting. The ratio of red to blue varies from 5:1 to 8:1 depending on the application.
Full spectrum luminaires (FSL)
Selected spectral (R/B) luminaires have a problem in that human vision is impaired under these conditions and examination of the condition of plants and the execution of tasks becomes difficult. To overcome this, “full spectrum” (FSL) LED luminaires have been developed. It has also been found that plants fare better when the full spectrum of PAR light is used.
FSLs attempt to produce a spectral spread which is suitable for human vision, but which still contains high levels of red and blue light. The luminaires contain spectral components in the full range from blue to red and contain a segment of lower level green light. Fortunately, the characteristic of existing LEDs can be used to provide a manageable spectrum. Figure 5 shows the spectrum of a cool white LED.
This is significantly different from the spectrum of sunlight and has the advantage of a high level of blue wavelengths but is deficient in the red part of the spectrum. Adding red LEDs allows the spectral balance to be achieved, while maintaining a significant portion of the mid-range spectrum. FSLs available consist of a mixture of white, red, and blue LEDs with the addition of IR and UV in some cases.
Fixtures come in a variety of shapes, including strips, tubular, rectangular cubical, flat boards, and circular units. The choice of unit depends on site requirements and application. LEDs are also available in different mounting forms.
LEDs are mounted in a housing with individual lenses/reflectors for each device.
Surface mount devices (Quantum boards)
Full spectrum and variable spectrum LED SMD quantum boards are constructed of a mix of many low-level LEDs of different wavelengths. A typical example contains LEDs in the following ratio: 288 white, 12 red, 4 UV.
Chip on Board (COB)
COB LEDs are built by mounting many tiny LEDs directly onto a substrate in close proximity. Together, these diodes create one single module, providing a uniform light source essentially operating as one big LED chip, giving a high intensity source of light.
White COB LEDs are often mounted in combination with groups of individual red and blue LEDs to provide an FSL having a relatively small area.
Variable spectrum systems
In fixtures containing a mixture of different coloured LEDs, it is possible to vary the spectral composition by switching groups of LEDs on and off, or selectively dimming groups of LEDs. Basic systems make use of a switch on each fixture, while more complex systems use centralised dimming control systems.
Remote control of lighting fixtures using radio or wired LAN from a centralised control unit, allows individual control over lighting units or groups of units to vary spectrum, photoperiod, and intensity. The units adjust the ratio of the different light colours to influence plant height, shape, size, color, flavour and nutrition. Advanced systems make use of light sensors in daylit greenhouses to accurately adjust the amount of artificial light to achieve the optimum DLI.
Quantum dot (QD) LEDs for horticulture
QDLEDs combine a standard high-emission blue or violet LED with a coating of photoluminescent QD material to produce light of the required colour or combination of colours. Quantum dot LEDs overcome the difficulties of other LEDs by combining the required quantum dot materials (different coloured phosphors) in a photoluminescent QDLED (Figure 9). This produces a multi-wavelength output from a single device. Using this technology, it is possible to produce a true full-spectrum LED of the required composition.
QLEDs may also be constructed from strips of photo-luminescent QD material placed on top of a row of LEDS to produce QD strip-lights.
 AC/DC Dynamics: “Solutions/LED growlights”, www.acdc.co.za
 R Blackey: “Enriching horticultural lighting for faster growth and better crops”, LED Professional, April 2019.
 California lightworks: “Light spectrum and plant growth”, https://californialightworks.com
 Lumigrow: “Full spectrum led grow lights: The truth you need to know”, https://lumigrow.com
 “Think Beyond White LED Grow Lights”, https://thegreensunshineco.com
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