Application of LED light source in greenhouse fill light

“Everything grows on the sun”, and illumination is one of the essential elements for crop photosynthesis. The quality of light conditions directly affects the yield and quality of crops. In nature, the sun's illuminance varies with geographic latitude, season, and weather conditions. In addition to the above factors, the illumination in the greenhouse is also closely related to the structure of the greenhouse, management measures and the light transmission properties of the materials. Due to factors such as greenhouse cover materials, dust and structural shading, the lighting conditions in the greenhouse are much worse than those in the open field, generally only 30%-70% of the open field, especially in the winter and early spring, the sun's elevation angle is low. The sunshine time is short, and the illumination in the greenhouse often cannot meet the demand of crop growth. Artificial light supplement has become an inevitable choice for many greenhouses with controlled environment.

At present, the greenhouse artificial light supplement mainly considers three aspects: First, the requirements for illuminance. Based on the main crop light compensation points, the light compensation points of tomato, cucumber and pepper are 3000Lx, 2000Lx and 1500Lx respectively, and the light saturation points are 70,000Lx, 55000Lx and 30000Lx. Therefore, the greenhouse light illuminance generally requires 1000Lx in the crop canopy. -3000Lx. The second is the requirement for light quality. The photosynthesis of crops mainly uses blue-violet light of 400nm-500nm, red orange light of 600nm-700nm and a small amount of far-infrared light of 700nm-800nm. The greenhouse fill light is generally different for crops, for R/B (red/blue) and R/. FR (red/far red) has specific requirements. The third is the requirement for the photoperiod. The phenomenon of day and night alternating and recurring in nature forms a photoperiod, and crops adapt to this change in light and long evolution. However, before and after the winter solstice or even cloudy days, the illumination time often fails to meet the growth and development needs of the crop, and artificial light is needed to increase the illumination time. In recent years, artificial light supplementation in greenhouses has become an important means of facility horticultural production, and various artificial light sources have also been rapidly developed.

1 Common light sources and characteristics of greenhouse fill light

At present, the commonly used greenhouse artificial light source mainly includes fluorescent lamps, high pressure sodium lamps, low pressure sodium lamps, metal halide lamps and the like. In recent years, with the successful development of high-power LEDs, new energy-saving light source LEDs have also attracted widespread attention.

Ordinary fluorescent lamps provide more green light, about 50%, and most of the rest are red and blue, which account for about 25% of the total spectrum, and the far red spectrum is very low. Fluorescent lamps have high luminous efficiency and long life (about 12000h), but the power is small (currently 28W and 36W are commonly used), and many lamps are needed to meet certain illumination levels. Because it contains a lot of green light, it is easy to cause crops to grow. Currently used in plant tissue culture.

The sodium lamp is a gas light source that emits mercury and sodium vapor, and is classified into a low pressure sodium lamp and a high pressure sodium lamp. The low-pressure sodium lamp appears earlier, the sodium vapor pressure does not exceed a few Pa, and its luminescence spectrum is concentrated at 589 nm yellow light, which can usually only be used with other light sources. The luminous efficiency is up to 200lm/W and the life is long (about 9000h). The standard operating sodium vapor pressure of the high pressure sodium lamp is about 10 kPa, and there are more red orange light and less blue green light in the luminescence spectrum. It has high luminous efficiency, high power and long life (about 12000h). It is currently used more in greenhouse light supplementation. However, since the high-pressure sodium lamp is a heat source, the surface temperature is high and the amount of heat is large, so that the crop cannot be irradiated at a close distance.

A metal halide lamp is a gas illuminating light source in which a mixed vapor of mercury and a rare metal halide generates an electric arc to discharge light. Different spectra can be exhibited by changing the composition of the metal halide. The luminous efficiency is high, the power is large, and the life is long (5000h~20000h). However, there is mercury in the filler in the lamp, which will pollute the environment when the lamp used is broken or discarded. The spectrum contains a lot of far red light, which generates a large amount of heat and cannot illuminate the crop at close range.

LED (Light Emitting Diode) light source is a new energy-saving light source developed in recent years. Compared with artificial light sources such as incandescent lamps, fluorescent lamps and high pressure sodium lamps, LEDs have significant advantages:

· DC low voltage power supply. The forward voltage of low-power color LEDs is usually 1.5V-2.8V, and the forward voltage of high-power LEDs is usually 3V-4V, which is much smaller than the safe voltage.

· Energy saving. The sodium lamp and the metal halide lamp are gas discharge illuminating lamps, and the metal element vapor is discharged by heating and heating to emit light. The LED is a solid-state light source that emits light without heating. It is a cold light source, so it reduces the amount of electricity consumed in heating.

· Monochrome light source with high luminous efficiency. LEDs emit monochromatic light with a half-wave width of ±20 nm, which can accurately provide the desired spectrum for plants without wasting power to emit unwanted spectra of plants such as yellow and green.

· Small size and flexible application. Lightweight and short luminaires can be designed for large application space.

· Environmental protection. LED is a solid light source that does not contain harmful substances such as mercury. It does not cause pollution during installation and use, and its waste can be recycled. The LED light source is an environmentally friendly green light source.

·long life. LED is a solid light source, there is no loose part inside, no vulnerable parts such as glass and filament, and easy to burn parts. It has high mechanical strength, vibration resistance and impact resistance, and its service life can reach more than 50,000h.

The obvious advantages of LED light source have attracted the attention of scholars in the Netherlands, Japan, the United States and other countries, especially the successful development of high-power LED in recent years, which laid the foundation for LED application in the greenhouse. 

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