The two most commonly used artificial lights in commercial greenhouse operations are HPS (High Pressure Sodium Lamp) and LED (Light Emitting Diode).
Background HPS bulbs produce light by transmitting electricity back and forth through an "arc tube". An external ballast (a device that regulates the lamp current) is required to provide voltage for the "start pulse". The start pulse starts the flow of electrons in the arc tube-generating weak light and heat. The heat vaporizes liquid mercury and sodium into gases. The gas enters the arc flow and shrinks the arc, thereby generating more heat and light. At this time, the HPS bulb will emit a very bright orange/yellow spectrum.
LEDs generate light through electroluminescence in semiconductor materials. One semiconductor is composed of more negatively charged particles (ie electrons), and the other is composed of more positively charged particles. Current always flows in one direction between semiconductors-from the more negative side to the more positive side. A photon is emitted where these two semiconductors meet. The color of the light emitted by the LED depends on the type of semiconductor through which the electrons pass.
HPS HPS lamps have been around for a while, so they are trusted by many growers. Their upfront cost is much lower than LED lights, which makes them beneficial to growers with limited budgets. Although you can't customize the spectrum of HPS lamps, they can produce a lot of light-which is good for the flowering stage of plant growth. Unfortunately, the high intensity light from HPS lamps generates a lot of heat. This may be an advantage in some northern climates, but in many cases, greenhouses equipped with HPS lights require a lot of additional cooling, which may increase operating costs. In addition, HPS lamps have limited dimming capabilities, which limits the control of light intensity.
The average life of HPS lamps (LC90) is 20,000 hours. This means that the light intensity of their bulb will be reduced to 90% of its potential after 20,000 hours. In contrast, the LC90 of an LED bulb is 50,000 hours. HPS lights must be replaced more frequently than LEDs, which increases annual operating costs.
With the advancement of LED technology, LED LED plant growth lights are becoming more and more popular. Many growers like LED lights because they require much less energy to operate, and the output is comparable to HPS lights. They also do not dissipate heat like HPS lamps, which means your HVAC system can be downsized, thereby reducing energy costs. Although their upfront costs are much higher, LEDs save growers a lot of operating costs.
In addition, LEDs can produce a more suitable spectrum than HPS. As shown in the figure below, HPS is limited by the gas (sodium) emitted mainly in the yellow/red region (550 to 620nm). LEDs are more flexible and can emit more blue light (400-500nm), which is very valuable for photosynthesis and photomorphogenesis.
Growers can mix and match LED colors according to their growth conditions or the growth stage of their crops. The LED also has a 0-100% dimming ability, which can fully control the light intensity. The dimming and spectrum customization functions also allow growers to try different intensities of color LED combinations according to the growth stage. Therefore, growers can choose to shape their plants in a certain way to suit their environment.
Spectral distribution For LEDs and HPS in greenhouses, one thing to consider is the type of luminaire. HPS lamps are limited by their technical nature, because you need a wide reflector to make this technology effective. Otherwise, most of the energy you spend to acquire photons in the canopy will be scattered and wasted in space.
LEDs are more directional than HPS, so the photons they produce will reach your tree canopy directly. This may also be a disadvantage of LEDs. It requires proper professional advice and dimensions to ensure that your canopy will have a uniform light intensity redistribution so as not to overlook certain parts of the canopy. Fortunately, most LED manufacturers understand the importance of shading in a greenhouse environment. You can find some "strip fixtures" that fit under the truss or your structure to minimize shadows.
You want to strive for an even light level from artificial lights. HPS lamps produce more shadows, especially because they require wide reflectors.
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