Many kinds of artificial lamps may be used to provide light in horticultural operations. Most lamps are capable of fueling photosynthesis, if they are bright enough. However, there is considerable waste of energy if the light source does not provide optimal color combinations. LED arrays are more efficient than High Pressure Sodium lamps for fueling photosynthesis. This white paper explains why this is so.
The choice of light source is very important for horticultural operations. Natural sunlight is almost perfect for field crops which happen to be native to the area where they are being grown. However, non-native plants being grown in greenhouses or other artificial conditions require artificial lights. There are advantages and disadvantages to each of the varieties of lamps. High Pressure Sodium (HPS) lamps and Light-Emitting Diode (LED) lamps share the advantages of energy efficiency and low cost of use per lumen produced. However, the LED lamps have several advantages over HPS lamps, in terms of plant production.
HPS emits very little usable light for plants, compared to LEDs
The spectrum of light produced by HPS lights is heavy in the green and yellow regions, with low quantities in the orange-red and blue-violet ranges. This is sketched out in the graph below. Very little of the green and yellow light can be used for photosynthesis by most plants. As a result, more total light –and therefore more total energy- is required to produce acceptable results in the crops than would be needed if the correct spectrum of colors were used. By contrast, LED horticulture lights can be constructed with any combination of colors that might be desired.
How HPS emits light that is not healthy for plants
Sodium lamps produce visible light that is mostly in the yellow range, along with a large amount of infra-red radiation in the form of non-visible heat. High Pressure Sodium lamps produce some light outside of the yellow range, which is why they can be used in horticulture. Low Pressure Sodium lamps usually produce yellow light with no other colors. The strong yellow light and intense heat are not useful fuel for photosynthesis, and they can produce considerable stress for the plants.
HPS lights are not optimized for specific plant growth and photosynthesis
HPS lights are optimized for visible light output, generally. They are very good for lighting up areas cheaply. They are excellent for directional lighting where the color of the light is not important but where contrast is needed, such as along roadways or sidewalks. They are especially good for streetlights that are near observatories and similar structures where light pollution would be a problem. The golden yellow light does not travel very far and there is very little blue light, so the light “stays” in the area where it is wanted instead of spreading up into the sky. (Harder, 2007) This very same lack of blue light that cuts down on light pollution makes the HPS lamp less useful for photosynthesis.
Much of the light-energy in the heavily-yellow-weighted spectrum of the HPS is wasted. The fact remains that the use of HPS for horticulture is a compromise, using a light source that was never designed to be optimized for photosynthesis.
All sources of artificial light on the market today produce some heat, in addition to light. The more heat the lamps produce, the more energy must be expended to produce the light. The more heat the lamps produce, the more energy must be expended to cool the growing area, too. In addition to this, light fixtures which produce a large amount of excess heat must be kept away from delicate plant tissues such as leaves to prevent burn damage. Therefore the fixture must be placed higher in the air, relative to the expected height of the leaves. The added height increases the cost of the housing for the plants.
HPS lamps produce more excess heat than LEDs do, when the two kinds of lamps are putting out the same amount of light. In some situations, the LEDs’ relative lack of heat cuts down on leaf-burn.This has an added bonus. It is possible to safely place LED lamps under the leaves of plants.Why would someone want to do this? In an unpublished monograph, Dale N. Moss presented the preliminary data of a study of the optimum lighting for leaves, conducted with support from NASA and Connecticut Agricultural Experiment Station.In this monograph, he explained that maximumphotosynthetic productivity can be achieved if the chloroplasts on both the upper and the lower surfaces of the leaves are also exposed to light (Moss, c1963). LED lamps can be readily used for this purpose. HPS lamps cannot.
LED Arraysfor the Specific Wavelengths that Plants Need
LED grow lights have a large advantage over other artificial light sources for horticulture: the lamp arrays can be optimized for any ratio of colors the manufacturer desires. By choosing different materials for the semiconductor part of the lamp itself (the component that actually emits the light) or different color filters on the casing (the “bulb” part),practically any color of light can be produced, including infrared and the ultraviolet ranges. LED lamps for horticultural use are made up of combinations of bulbs in various colors, according to the variety of plant being grown. For most
plants, this is a combination of reds and blues (“LED 1” in the diagram). For some of the grass crops, only red is required (“LED 2” in the diagram). (Tikhomirov, A.A.,1994; Bugbee, B.,1994)
Several varieties of artificial light have been used for horticultural purposes, and many studies have been conducted to determine which specific light sources are the most effective and efficient. Greenhouse operators in northern countries and the space agencies share an interest in determining which sources produce the best growth and which sources require the least resources in terms of energy, space, and cooling. High Pressure Sodium lights produce intense light at relatively low cost. Therefore, they are widely usedin commercial horticulture. However, they do not produce the ideal array of colors.
Also, HPS lamps produce a huge quantity of heat. As a result of the heat, more energy must be expended to cool the growing area and the lights must be kept a certain distance from the tops of the plants to prevent burning. By contrast, LED grow lights use less electricity directly, they can be easily customized with the correct ratios of colors for the species of plant being grown, they produce less waste heat (reducing the need for cooling mechanisms), and they do not burn the leaves of the plants below them — even with extended leaf-on-bulb contact, in many cases.
Bugbee, B. (1994). Effects of Radiation Quality, Intensity, and Duration on Photosynthesis and Growth.In T.W. Tippets (Ed.).NASA-CP-95-3309, International Lighting in Controlled Environments Workshop, Kennedy Space Center, FL: National Aeronautics and Space Administration.
Buil, C (undated). Spectral Calibration.On Spectroscopy, CCD & Astronomy. Http://www.astrosurf.com/buil/us/spe2/hreso14.htm
Campbell, T. (2012).White Paper: The Right Lighting for Photosynthesis: Which Wavelengths are Most Important and Why? Prepared by the author for Blu LED.
Geiger, D.R. (1994). General Lighting Requirements for Photosynthesis. InTibbits, T.W., International Lighting in Controlled Environments Workshop, National Aeronautics and Space Administration, John F. Kennedy Space Center, Kennedy Space Center, Florida.
Harder, S. (2007). White Paper: Metal Halide (MH) vs High Pressure Sodium (HPS). Paper concerns outdoor lighting such as street lamps. Accessed online at www.darkskysociety.org/handouts/white_paper–mh_vs_hps.pdf
Moss, D.N. (c1963).Optimum Lighting of Leaves. Connecticut Agricultural Experiment Station with financial support from NASA, unpublished preliminary data, N63 16152.
Roberts, L.M. (2010). Spectrum of a typical High Pressure Sodium (HPS) lamp.High_Pressure_Sodium_Lamp_Spectrum.jpg.LMRoberts at en.wikipedia.
Tikhomirov, A.A. (1994). Spectral Composition of Light and Growing of Plants in Controlled Environments.In T.W. Tippets (Ed.).NASA-CP-95-3309, International Lighting in Controlled Environments Workshop, Kennedy Space Center, FL: National Aeronautics and Space Administration.
Photographs are copyright of Itotombo Studio, and are used by permission.
All charts and diagrams are composed by the author with data from cited materials as indicated.