Lighting options for controlling photoperiod

Features - Greenhouse Structures

When selecting the best lighting option for controlling greenhouse photoperiod, equipment cost is not the only factor to consider.

Subscribe
June 9, 2011

The long operational life of light emitting diode (LED) lamps reduces the procurement, disposal and labor costs associated with bulb replacement.Photoperiod is the number of hours of light in a 24-hour period. A small change in photoperiod can mean the difference between vegetative and reproductive growth in many flowering species.

When plants flower as a response to a photoperiod shorter or longer than a particular duration they are said to be short- or long-day plants, respectively. Supplemental lighting is used to extend the natural photoperiod in greenhouses. When plants are exposed to at least 10 footcandles of light, the lighting source is insignificant. In this case, an incandescent lamp is preferred. These lamps are affordable, effective and easy to install.

Incandescent lamps are also effective at inducing a photoperiodic response. These lamps are rich in far red (700 to 800 nanometers or nm) light and have a red/far red ratio of 0.4 to 0.5. In some crops the low red/far red ratio causes stem elongation.


Turning out the lights
The Energy Independence and Security Act of 2007 called for energy efficiency standards of light bulbs to increase by 30 percent in the United States, effectively phasing out most common types of incandescent lamps by 2012 through 2014. Predicted energy savings include $13 billion in energy costs and preventing 100 million fewer tons of carbon dioxide from entering the atmosphere.

The light “bulb” is the housing and the “lamp” is what produces the light. The United States is not breaking new ground with the 2007 legislation. Australia began phasing out incandescent lamps in 2009, followed by the European Union, the Philippines and Argentina. Brazil and Mexico are expected to follow.


The classic incandescent
The light output from an incandescent lamp is not affected by ambient temperature and the bulb’s life expectancy is slightly affected by the number of on/off cycles. The lamps are fairly inefficient, costing more over the lamp’s effective lifetime compared to other lamps.

Only 5-10 percent of the electricity used is transformed into visible light. Thus, much of the electricity used to generate light results in heat. When electricity passes through the lamp’s metal filament, its temperature rises to approximately 4,100°F


Compact fluorescent lamps
Compact fluorescent lamps (CFL) are made of glass tubes filled with gas and a small amount of mercury. The average amount of mercury in each compact fluorescent is approximately 5 milligrams, which is enough to cover a ballpoint pen tip. Because the lamps contain mercury they require special disposal. Some retail stores accept the lamps for disposal (http://search.earth911.com/?what=CFL).

A cool white fluorescent (typical of compact fluorescent lamps) contains blue, some red, and almost no far red light. One complaint surrounding these lamps is the color of light. Consumers have reported that compact fluorescent lamps are not as appealing to the human eye. This is due to the lamps color temperature. Lower color temperatures (degrees Kelvin) give off a warmer light (yellowish). Higher color temperatures give off a cooler (bluish) light.

Horticulture researchers at Michigan State University have tested compact fluorescent lamps as an alternative to incandescent lamps for photoperiodic response. With some plant species where compact fluorescent lamps were used as the sole lighting source, flowering was delayed sometimes up to two to three weeks. When compact fluorescent and incandescent lamps were used in a 1:1 ratio, the flower inhibition was overcome. This may be the result of the red/far red ratio decreasing from 8.4 to approximately 0.6. Growers should use caution if considering a switch to compact fluorescent as their sole lighting source because of the impact on flowering.


A new lamp for the socket
The light emitting diode (LED) is more like that of a computer chip than a light “bulb”. It is a solid-state semiconductor device. It has been estimated that in 2008, LEDs accounted for 7 percent share of the global lighting market. By 2020, LEDs are estimated to account for 75 percent of global lighting.

LEDs may seem new, but they were invented in the 1920s. However, a visible light (380 nm to 780 nm) version was not developed until the early 1960s and it was red (~660 nm) in color. Additional wavelengths were developed through the 1970s. The first high-brightness blue LED was developed in the early 1990s.

The color of light emitted by a LED is determined by the type of semiconductor material and the impurities used to form the LED. With the addition of a phosphor coating to the blue LED, the white LED was created.


LED advantages
In addition to being short waveband specific, LEDs offer many positive attributes over traditional lighting sources. LEDs are more efficient than incandescent and fluorescent lamps and are essentially equivalent to high-intensity discharge (HID) lamps. White LEDs are less efficient because the phosphor coating must interact with the base color to create white light. There exists potential for significant cost savings with LEDs over current incandescent lamps.

Unlike traditional lamps, LEDs generally do not “burn out”. Instead, the metric used is the LEDs “lifetime” which is the time (in hours) required for the light output to drop below a percentage of the original maximum intensity under optimal operating conditions.

Growers will generally replace LEDs when the light output drops below 90 percent. For example, a Philips’ GreenPower LED flowering lamp will provide a lifetime of approximately 6,000-10,000 hours at 90 percent and operate on 16-18 Watts of electricity. This LED lamp fits into a standard E27 fitting, thus making it a drop-in replacement for any incandescent lamp fitting.

The long operational life of LEDs reduces the procurement, disposal and labor costs associated with bulb replacement. LEDs also offer the advantage of turning on and off unlike compact fluorescents. LEDs emit little or no radiant heat. Unlike fluorescent lamps, LEDs do not contain mercury.


LED challenges
The primary obstacle that has prevented mass LED installations in greenhouses has been cost. As with most products, the price of LED technology will decrease as more products emerge on the market. This has not prevented early-adopters from using the technology. LEDs are being used in Europe and are making their way to North America.

For LEDs, many factors need to be considered when determining the return on investment. How much will the lamps cost (or save) over their lifetime? The variables include energy, labor, disposal and environmental savings. Also, what is the lamps influence on crop production? Will crops flower earlier? What effects will LEDs have on flower quality/quantity? Can the same desired effect be achieved within a shorter timeframe? Will the lamps provide greater control over flower/production scheduling?

A lighting cost calculator is being developed by Hort Americas so growers can compare different lamps with user-supplied inputs such as lamp specifications and local energy costs.


Johann Buck is technical services manager, Hort Americas LLC, (469) 532-2369; www.hortamericas.com.