The relatively closed production system of greenhouses will play an important role in meeting the demand for food growth in the future. In recent years, insufficient greenhouse light has been paid more and more attention. On the one hand, the greenhouse light transmittance is reduced due to the orientation, structure and covering material characteristics of the greenhouse, and on the other hand, the greenhouse crops are insufficiently illuminated due to climate change. For example, continuous rainy weather in winter and early spring, frequent foggy weather, etc. Insufficient light directly adversely affects greenhouse crops, causing serious losses to production. Plant grow light can effectively alleviate or solve these problems.
Incandescent lamps, fluorescent lamps, metal halide lamps, high pressure sodium lamps, and emerging LED lamps have all been used in greenhouse light supplementation. Among these types of light sources, high-pressure sodium lamps have higher light efficiency, longer service life, higher overall energy efficiency, and occupy a certain market position, but high-pressure sodium lamps have poor illumination and low safety (including mercury). Problems such as inaccessible proximity are also prominent.
Some scholars have a positive attitude toward LED lights in the future or can overcome the problem of insufficient performance of high-pressure sodium lamps. However, LED is expensive, the fill light technology is difficult to match. The fill light theory is not perfect, and the LED plant fill light product specifications are confusing, which makes users question the LED application in plant fill light. Therefore, the paper systematically summarizes the research results of previous researchers and the status quo of their production and application, and provides reference for the selection and application of light sources in greenhouse fill light.
The high-pressure sodium lamp has an illumination angle of 360°, and most of it must be reflected by the reflector to reach the designated area. The spectral energy distribution is roughly red orange, yellow-green, and blue-violet (only a small part). According to the different light distribution design of LED, the effective illumination angle can be roughly divided 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.
The high-pressure sodium lamp is the third-generation illumination source. It has a wide range of conventional alternating current, high luminous efficiency, and strong penetrating power. The maximum life is 24000h and the minimum can be maintained at 12000h. When the sodium lamp is illuminated, it is accompanied by heat generation, so the sodium lamp is a kind of heat source. There is also a self-extinguishing problem. As the fourth generation of new semiconductor light source, LED adopts DC drive, the life can reach more than 50,000 h, and the attenuation is small. As a cold light source, it can be close to plant irradiation. Compared with LED and high-pressure sodium lamps, it is pointed out that LEDs are safe, contain no harmful elements, and are more environmentally friendly.