Incandescent, fluorescent, metal halide, high-pressure sodium, and emerging LED lights have all been used or are being used in the greenhouse for light. Among these types of light sources, high-pressure sodium lamps have higher luminous efficacy, longer service life, and higher overall energy efficiency, occupying a certain market position, but high pressure sodium lamps have poor lighting continuity and low safety (including mercury). Problems such as not being able to reach close range are also prominent. Some scholars take a positive attitude toward LED lamps in the future or can overcome the low performance of high pressure sodium lamps. However, LED's are expensive, light-filling technology is difficult to support, light-filling theory is not perfect, and LED plant fill light product specifications are confusing, which makes users doubt the application of LEDs in plant light-filling applications. Therefore, the article systematically summarizes the previous research results and its production and application status, and provides reference for the light source selection and application in the greenhouse.
Differences between high-pressure sodium lamps and LED lighting
1. The principle of light emission and the difference in external structure
The high pressure sodium lamp consists of mercury, sodium, helium arc tube wicks, glass bulbs, and getters from the inside to the outside. Because of its different core ballasts, it is divided into high-voltage sodium lamp and electronic high-pressure sodium lamp. Different-pressure high-pressure sodium lamps need to use ballasts with corresponding specifications. LED is also called a light emitting diode. The core part is a wafer composed of a P-type semiconductor and an N-type semiconductor. There is a transition layer between the P-type semiconductor and the N-type semiconductor, which is called a P-N junction. When the current flows from the anode of the LED to the cathode, the semiconductor crystal emits light of different colors from purple to red. The intensity of the light is related to the current. According to the luminous intensity and operating current can be divided into ordinary brightness (luminous intensity <10 mcd), high brightness (luminous intensity of 10 ~ 100 mcd) and ultra-high brightness (luminous intensity> 100 mcd) and other types. Its structure is mainly divided into four blocks: the structure of the light distribution system, the structure of the heat dissipation system, the drive circuit, and the mechanical/protective structure.
2. Irradiation range and spectral range differences
The high-pressure sodium lamp tube has a light emitting angle of 360[deg.], and most of them must be reflected by the reflector before being irradiated to a specified area. The spectral energy distribution is roughly red, orange, yellow, green and blue (only a small part). According to different light distribution designs of LEDs, the effective light emission angles can be roughly classified into three categories: ≤180°, 180°~300°, and ≥300°. The LED light source has wavelength tunability, and can emit monochromatic light with narrow light waves, such as infrared, red, orange, yellow, green, blue, etc., and can be arbitrarily combined according to different needs.
3, the application of conditions and life differences
The high pressure sodium lamp is the third generation of lighting source. It has a wide range application under the conventional alternating current, high luminous efficiency, and strong penetrability. The maximum life span is 24000h, and the minimum is also maintained at 12000h. The sodium lamp is accompanied by heat generation while it is being illuminated, so the sodium lamp is a heat source. In the process of use, there is also a self-extinguishing problem. LED as the fourth generation of new semiconductor light source, using DC drive, life can reach more than 50000 h, and the attenuation is small, as a cold light source, can be close to the plant irradiation. Compared with LED and high-pressure sodium lamp, it is pointed out that LED has higher safety, does not contain harmful elements, and is more environmentally friendly.