Agricultural lighting

Demand for light in plants            

Light is the only source of energy for plant photosynthesis. Light intensity, light quality (spectrum) and the periodic changes of light have a profound impact on the growth and development of crops. Among them, light intensity has the greatest impact on plant photosynthesis.            

Light intensity            

Light intensity can change crop morphology, such as flowering, internode length, stem thickness and leaf size and thickness. The requirements for light intensity of plants can be divided into light, light, and weak light plants. Vegetables are mostly photophilic plants, and their light compensation points and light saturation points are relatively high. The relevant requirements of light intensity of crops in artificial light plant factories are the important basis for choosing artificial light sources. Understanding the light requirements of different plants is essential for designing artificial light sources and improving the production performance of the system.            

Light quality            

Light quality (spectral) distribution also has important effects on plant photosynthesis and morphogenesis (Fig. 1). Light is part of radiation, and radiation is an electromagnetic wave. Electromagnetic wave has the characteristics of wave and quantum (particle). The quantum of light is called photon (photon), also known as light quantum in horticulture. Radiation with wavelength ranging from 300 to 800 nm is called physiological active radiation, while radiation with wavelength ranging from 400 to 700 nm is called photosynthetic active radiation (PAR).            

The two most important pigments in plant photosynthesis are chlorophyll and carotenes. Fig. 2 is the spectrum absorption spectra of photosynthetic pigments, in which chlorophyll absorption spectra are concentrated in red and blue bands. The illumination supplementary system is based on the spectral requirements of crops to supplement light artificially to promote plant photosynthesis.            

Photoperiod            

The relationship between photosynthesis and photomorphogenesis and day length (or photoperiod time) is called plant photoperimetry. Photoperiod is closely related to the number of photoperiod. The number of hours of illumination refers to the time when the crop is irradiated by light. Different crops need to have a certain number of light hours to achieve flowering and fruiting. According to the different photoperiod, it can be divided into long-sunshine crops, such as Chinese cabbage, which need more than 12-14 hours of illumination at a certain stage of their growth; short-sunshine crops, such as onions, soybeans, which need less than 12-14 hours of illumination; medium-sunshine crops, such as cucumbers, tomatoes, peppers, which need longer or shorter sunshine. They can blossom and grow strong.            

Among the three elements of environment, illumination intensity is an important basis for choosing artificial light source. At present, there are many ways to express illumination intensity, mainly including the following three kinds.            

(1) Illumination refers to the surface density of the luminous flux received on the irradiated plane (the luminous flux per unit area), in units: Lux (lx).            

(2) photosynthetically effective radiation illuminance PAR, unit: W/m.            

(3) Photosynthetically active light quantum flux PPFD or PPF is the quantum number of photosynthetically active radiation arriving or passing per unit time and area in units of micromol/(ms). It mainly refers to the light intensity of 400~700nm directly related to photosynthesis. It is also the most commonly used index of light intensity in the field of plant production.            Light source analysis of typical complementary light system            

Artificial light supplement is to increase the light intensity or prolong the time of illumination in the target area by installing a light supplement system, so as to realize the light demand of plants. Generally speaking, the supplementary light system includes the lighting equipment, the circuit and its control system. Light source including several incandescent lamp, fluorescent lamp, metal halide lamp, high-pressure sodium lamp and LED type. The incandescent lamp electro-optic efficiency is low, low photosynthetic efficiency, has been out of the market, so this paper do a detailed analysis.            

Fluorescent lamp            

It belongs to the type of low pressure gas discharge lamp. The glass tube is filled with mercury vapor or inert gas, and the inner wall of the tube is coated with phosphor. The light color varies with the fluorescent materials coated in the tube. The fluorescent lamp has good spectral performance, high luminous efficiency and low power. Compared with incandescent lamp, the fluorescent lamp has longer lifetime (12000h) and lower cost.            

Because the fluorescent lamp itself has less calorific value, it can be illuminated close to plants, which is suitable for three-dimensional cultivation. However, the distribution of fluorescent lamp spectrum is unreasonable. The most commonly used method in the world is to add reflective cover to increase the effective light source composition of crops in cultivation areas as far as possible.            

Adv-agri Company of Japan has also developed a new supplementary light source HEFL. HEFL belongs to the category of fluorescent lamp. It is the general name of cold cathode fluorescent lamp (CCFL) and external electrode fluorescent lamp (EEFL), and it is a kind of mixed electrode fluorescent lamp. HEFL lamp tube is very thin, its diameter is only about 4 mm, and its length can be adjusted from 450 mm to 1200 mm according to cultivation requirements. It is an improved version of conventional fluorescent lamp.

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