Outline
Basic definition of ‘light’
Some definitions
Not all parts of the PAR range are created equal
What light is used for in controlled environment agriculture
Types of supplemental light
Commonly used supplemental lamp types
Light measurement
Light transmission
Considerations when choosing a lamp type
Design of artificial lighting for CEA
Light
Plants use radiant energy from the sun to photosynthesize and, subsequently, to grow. The term, ‘Light’, as it is commonly used, refers to the range of wavelengths of electromagnetic radiation visible to the human eye – approximately 380 to 770 nanometers (nm). Coincidently, this corresponds closely to the wavelengths plants use for growth (approximately 400 to 700 nm) which is called ‘Photosynthetically Active Radiation’, or PAR. Not all radiation from the sun (or, for that matter, from artificial lighting) is in the PAR range. Both shorter (in the ultra-violet range) and longer (in the infra-red, or heat, range) radiation can be emitted from PAR sources and the amount of each must be considered and sometimes managed.
DLI –Daily Light Integral, the number of moles of PPF photons irradiating one square meter in one day
Intensity –Irradiation onto a surface expressed in units of micromol per meter squared persecond. Photon flux onto a surface
Lamp –The physical supplemental light source; commonly called the “bulb”
Luminaire –Complete lighting fixture, including the reflector, lamp, and ballast
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Mole –From chemistry, 6.022 x 10^23 atoms, or light photons (6.0 x 10^23 or 6.0 x 1023 means that there are 23 places after the 6....so 6,022,000,000...14 more 0's)
PAR –Photosynthetically-Active Radiation (400 to 700 nm)
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PPF –Photosynthetic Photon Flux – the number of micromols of photons of PAR wavelength irradiating one square meter of area every second
Radiation –Photons leaving a lamp or luminare irraditon, photons striking a surface per square foot or square meter
Not all parts of the PAR range are created equal
Photosynthetic action spectrum
Plants have multiple pigments that absorb light energy and utilize that energy during photosynthesis, including chlorophylls a and b, carotenoids, phycoerythrin, phycocyanin, and other more minor pigments. Scientists have quantified the amount of light each pigment can absorb at each wavelength, creating absorption spectrum graphs for each pigment. There is a temptation to match the light source to a pigment absorption spectrum and one would use Chlorophyll a as the pigment to match because it is the most important pigment for photosynthesis. The absorption spectrum is not the complete story however. There is another factor that must be considered, called the action spectrum. The action spectrum is the yield of oxygen produced when a plant is exposed to a single wavelength. This does not match the absorption spectrum exactly; the current theory for this is that not all pigments are as efficient in converting light energy to photosynthesis products. A “blue” photon, for example, carries more energy than a “red” photon, but each leads to the same amount of photosynthetic product. Therefore, when thinking about using supplemental lighting to aid plant growth, the photosynthetic action spectrum should be considered in conjunction with the spectrograph showing the relative quantity of each wavelength produced by the supplemental light.
Plant morphogenesis and light
Morphological changes associated with differences in light quality (color) are categorized into two main classes: blue light responses and red light responses (aka, phytochrome response). Examples of blue light responses include the elongation of the hypocotyls, stomatal opening and phototrophism while examples of red light responses include things that are day length sensitive such as germination, flowering, and preparation for dormancy. Therefore, the amount of blue and red light in supplemental light sources should be considered because they can have a strong effect on the growth and development of the plant.
Plant color and light
Plant secondary metabolites (those produced as pigments) and light quality can influence plant color. Many secondary pigments are produced in plants, including beta carotene, phenolics and anthocyanins. These pigments are used by the plant to help dissipate surplus electromagnetic energy that strikes the leaves. This extra energy is often from the ultra-violet portion of the spectrum. Anthocyanins are familiar because of the red color they contribute to leaves during the fall when leaves of deciduous plants are preparing for winter dormancy. They are also responsible for the red/purple color of some annual and perennial bedding plants and lettuce cultivars. The glazings (covering material) over controlled environments can filter out some of the light wavelengths that are responsible for anthocyanin production. For example, glass filters out some of the shorter ultra violet rays in the range that is called UV-B but allows UV-A to pass through. Polyethylene film allows both UV-A and UV-B to pass through and reach the plants. Research has demonstrated the increased anthocyanin production by baby leaf lettuce showing a 30% increase by the application of supplemental lighting (Li and Kubota, 2009). Therefore, supplemental light can be utilized that includes wavelengths that induce the production of these pigments.
What light is used for in controlled environment agriculture
Growth. Adding more plant biomass and enabling enough extra energy to flower/fruit if the conditions are conducive to flowering. This is the primary use of supplemental lighting in CEA production. Plants can be grown faster and more consistently.
Detecting the change of seasons. Quality (portion of the spectrum) and perceived length of day triggers processes such as flowering and dormancy. Poinsettias are a crop where the quality and duration of light is very important for the flowering process and is controlled very carefully in order to ensure the plants will flower during the limited season they are in demand.
Control the compactness of the plant. A lower light level will cause some plants to become taller with more space between sets of leaves (increases the length of the internodes).
Germination. Some seeds need light to overcome dormancy.
Increasing the production of colorful pigments. Plants that are red or purple can have that colored portion made more dramatic by the use of supplemental light.
Types of supplemental light
Fluorescent, high intensity discharge (high pressure sodium, metal halide, low pressure sodium), incandescent, light-emitting diodes (LED), microwave.
Note: A luminaire is a lighting fixture complete with the light source or lamp, the reflector for directing the light, an aperture (with or without a lens), the outer shell or housing for lamp alignment and protection, an electrical ballast, if required, and connection to a power source. (Sylvania)
Fluorescent – sub-categorized into cool-white or warm-white describing the predominant portion of the color spectrum the lamp yields. Most fluorescents used for plant production are cool-white. They are further categorized into high output (HO) and very high output (VHO). Fluorescent lamps are more commonly used in growth chambers. Greenhouse use is limited because of the lower quantity of light as well as the increased area of shade that the fixtures produce compared to HID lamps. Fluorescent lamp types are described by the diameter of the lamp in eights of an inch. For example, a T8 lamp is 8/8” or 1” wide and a T12 is 12/8” or 1 ½” wide. Both T8 and T12 lamp styles are common in growth chamber use.
‘Grow Lux’ are a special type of fluorescent lamp that has been marketed by lighting manufacturers. This type of lamp emits more light in the blue and red portion of the spectrum (the visual output looks somewhat purple) than the standard fluorescent lamp. Little scientific evidence exists that suggests such light sources are better than a comparable cool white fluorescent light source.
The current trend is to transition existing fluorescent lamp instillation to ‘T5’, a newer lamp style with a different chemical that is used to produce the light. It has a smaller diameter which requires the retrofit of existing fixtures. This update is being made in chambers across the country because the lamp is more efficient than T8 or T12 and has a longer life. New lamp instillations typically use the T5 lamp style.
It is important to note that light output is sensitive to temperature. Max output is around 100F (38C). It may be necessary to have the ability to heat and cool the air that surrounds a large bank of fluorescent lamps to ensure that the lamps operate at their maximum light output. A further reason to invest in the ability to regulate the air temperature surround fluorescent lamps is that temperatures above those that produce maximum light output (100F) can shorten the life of the lamp.
Light Emitting Diodes - LED – The newest product to hit the market. There is much research happening at universities and in the lighting industry across the country investigating the proper mixture of wavelengths and thus diodes to use for various applications. This lighting system would be best suited for growth chamber use as the arrays would block a significant amount of sunlight if used in a greenhouse. There are many engineering challenges that are being addressed currently. They include the removal of heat from the fixtures, reducing the extremely high production cost and the design of the array such that the fixture does not take up so much space so that it may be used in greenhouse production. The use of more intense diodes is being developed and may be available in the next decade.
Instantaneous light
This is a difficult subject because many different industries have their own measurement systems in place to describe various qualities of light and because each industry has a separate use for the radiation, the units that the measurements are taken in are not easy (and sometimes almost impossible) to convert. For example, lighting in parking lots is measured in foot-candles which describes how bright the light appears to the human eye. This first type of measurement describes instantaneous light or the amount of light that hits the surface per second. Lighting manufacturers historically report the output of their products in lumens which are similar to foot-candles in that they describe light as it is sensed by the human eye. This unit is often converted to lumens per square meter or lux. Measurement units that best describe PAR light are called micro-moles per square meter per second (µmol *m-2s-1) and plant growth parameters are reported in scientific papers in µmol. A mol of light represents 6.022 x 1023 photons and is thus a direct quantity of radiation from the sun. Though it is possible to convert between units, it is not very accurate because the portion of the spectrum that is being measured is different. For plant production, it is essential that a light sensor be used to accurately measure the output of the lamp because the human eye is not very sensitive to differences in light output that can greatly affect the plant. The sensor should be placed at the level of the crop and care should be taken that it does not get shaded by the growing plants.
SI unit is Lux. 1 fc = 10 lux = 0.2 µmol m-2 s-1
Full sunlight 10,000 fc, 2000 µmol m-2 s-1. Overcast day could be around 1000 fc or 200 µmol m-2 s-1
Light through a window 100-2000 fc
A useful web tool to convert units produced by a growth chamber manufacturer: http://www.egc.com/useful_info_lighting.php
Daily light integral (DLI)
The next category of light that is essential to understand and consider for CEA production is daily light integral or the amount of PAR light (400-700 nm) that the plant receives in a 24-hour period which is reported as moles of PAR light per day (mol m-2 d-1). An analogy for understanding the difference between instantaneous light and DLI is of the rate at which water comes out of a tap vs. the amount of water in the pool at the end of the day. The pool is analogous to the DLI in that it reports all the water that is collected over a period of time while the quantity of water flowing from the tap describes an instantaneous measurement. Receiving a consistent amount of daily PAR is instrumental in producing a predictable and consistent amount of plant growth.
Research at Cornell University has shown that the application of a consistent DLI allows controlled environment production of a crop that is identical and predictable year-round. A computer algorithm has been developed to facilitate this controlling of DLI through the use of movable shade and supplemental light. Further research has shown that the addition of carbon dioxide can reduce the amount of supplemental light that is required and the incorporation of this phenomenon into the computer program to control lights and shade has been made. A stand-alone controller will be available for commercial purchase. For more information about this research and the development of the controller:
To view a PDF of this topic: Download LASSI description
An extension article from Purdue about measuring DLI:
http://www.extension.purdue.edu/extmedia/HO/HO-238-W.pdf
Measurement sensors
Sensors should be periodically re-calibrated to ensure accurate measurements. Calibration frequency is suggested to be every other year.
PAR meter – Instantaneous measurement with quantum sensor. These meters range in quality and price varying from $200-1400. A PAR meter is an essential tool to evaluate the light output of artificial lighting and can help to determine if a lamp is performing poorly and is in need of maintenance.
Spectroradiometer – Measures the output of the light source at fixed nanometer divisions, often every 2nm. This is more useful in a research situation.
Light transmission
In the US, the daily light integral received outside can vary from nearly 0 to more than 65 moles per day and depends on location, season and cloud cover. The daily light integral averaged over the Earth is about 26 mols per square meter per day.The DLI received at the plant level inside a greenhouse depends on how much light is transmitted through the structure which is affected by the type of glazing material, amount of supporting structure that is above the plants, and the cleanliness of the glazing.
Considerations when choosing a lamp type
Longevity of the lamp
PAR output
Conversion efficiency of electricity to PAR (look at mol/kWh which is the useful output/costly input)
Initial cost/ replacement cost of bulb and ballast
Design of artificial lighting for CEA
Computer programs created for architectural use modified for greenhouse applications exist to assist in the placement of HID lamps to assure uniform light distribution such as photopia from LTI optics. http://www.ltioptics.com/Photopia/overview.html
Or:
Photometrics pro
http://www.photometricspro.com
Reference
Li, Q., Kubota, C., 2009. Effects of supplemental light quality on growth and phytochemicals of baby leaf lettuce.Environmental and Experimental Botany. 67(1),59-64.
