Current situation and trend of fill lighting metho

Current situation and trend of lighting methods in plant factories

plant factories combine modern industry, biotechnology, nutrient solution cultivation and information technology to implement high-precision control of environmental factors in the facilities. They have the advantages of full closure, low requirements for the surrounding environment, shortening the harvest period of plants, saving water and fertilizer, no pesticide production, and no outward discharge of waste. The unit land use efficiency is 40-108 times that of open field production, Among them, intelligent artificial light source and its light environment regulation play a decisive role in its production efficiency

light, as an important physical environmental factor, plays a key role in the regulation of plant growth and development and material metabolism. "One of the main features of plant factories is full artificial light source and intelligent regulation of light environment" has become a common consensus in the industry

the demand of plants for light

the life activities of plants cannot be separated from light. Light is the only energy source of plant photosynthesis. The periodic changes of light intensity, light quality (spectrum) and light have a profound impact on the growth and development of crops, of which light intensity has the greatest impact on plant photosynthesis

light intensity

the intensity of light can change the morphology of crops, such as flowering, internode length, stem thickness, and leaf size and thickness. Plants' requirements for light intensity can be divided into light loving, medium light loving and weak light tolerant plants. Most vegetables belong to light loving plants, and their light compensation points and light saturation points are relatively high. In artificial light plant factories, the relevant requirements of crops for light intensity are an important basis for selecting artificial light sources. Understanding the lighting needs of different plants is extremely necessary for designing artificial light sources and improving the production performance of the system

light quality

the distribution of light quality (spectrum) also has an important impact on plant photosynthesis and morphogenesis (Fig. 1). Light is part of radiation, which is an electromagnetic wave. Electromagnetic wave has the characteristics of wave and quantum (particle). The quantum of light is called photon, which is also called quantum of light in the field of horticulture. Radiation with a wavelength range of 300-800nm is called physiologically active radiation of plants; The radiation with a wavelength range of 400-700nm is called photosynthetically active radiation (PAR) of plants

the two most important pigments in plant photosynthesis are chlorophyll and carotenes. Figure 2 shows the spectral absorption spectrum of each photosynthetic pigment, in which the chlorophyll absorption spectrum is concentrated in the red and blue bands, and the lighting system is to artificially fill light according to the spectral needs of crops to promote plant photosynthesis

photoperiod

the relationship between photosynthesis and photomorphogenesis of plants and the length of day (or photoperiod time) is called photoperiodicity of plants. Photoperiodicity is closely related to light hours, which refers to the time when crops are exposed to light. Different crops need a certain number of light hours to bloom and bear fruit after completing the photoperiod. According to the different photoperiod, it can be divided into long sunshine crops, such as cabbage, which need more than 12-14h of light hours at a certain stage of its growth; Crops with short sunshine, such as onions and soybeans, need less than 12-14h of light hours; Crops with medium sunshine, such as cucumber, tomato and pepper, can bloom and bear fruit under long or short sunshine

(1) illumination refers to the area density of luminous flux received on the illuminated plane (luminous flux per unit area), in Lux (LX)

(2) photosynthetic effective irradiance par, unit: w/M

(3) photosynthetic effective photon flux density PPFD or PPF is the number of photons of photosynthetic effective radiation arriving or passing on unit time and unit area, unit: μ mol／（m?·s）。 It mainly refers to the light intensity of 400-700nm directly related to photosynthesis. It is also the most commonly used indicator of light intensity in the field of plant production

light source analysis of typical fill light system

artificial fill light is to increase the light intensity or prolong the light time in the target area by installing fill light system, so as to realize the light demand of plants. Generally speaking, the fill light system includes fill light equipment, circuit and its control system. Fill light source mainly includes incandescent lamp, fluorescent lamp, metal halide lamp, high pressure sodium lamp, led and other common types. Due to the low electro-optical efficiency and low photosynthetic efficiency of incandescent lamps, they have been eliminated by the market, so this paper will not make a detailed analysis

fluorescent lamps belong to the type of low-pressure gas discharge lamps. The glass tube is filled with mercury vapor or inert gas, and the inner wall of the tube is coated with fluorescent powder. The light color varies with the fluorescent materials coated in the tube. Fluorescent lamps have good spectral performance, high luminous efficiency, low power, long service life (12000h) and relatively low cost compared with incandescent lamps. Due to the small heat output of the fluorescent lamp itself, it can be close to plants for lighting, which is suitable for three-dimensional cultivation. However, the spectral layout of the fluorescent lamp is unreasonable. The more commonly used method in the world is to add reflectors to increase the effective light source components of crops in the cultivation area as much as possible. Japanese adv agri company has also developed a new fill light source HEFL, which actually belongs to the category of fluorescent lamps. HEFL is the general term of cold cathode fluorescent lamps (CCFL) and external electrode fluorescent lamps (EEFL), and is a hybrid electrode fluorescent lamp. The HEFL tube is extremely thin, with a diameter of only about 4mm, and the length can be adjusted from 450mm to 1200mm according to the cultivation needs. It is an improved version of the conventional fluorescent lamp

metal halide lamp is a high-intensity discharge lamp that can excite different elements to produce different wavelengths by adding various metal halides (tin bromide, sodium iodide, etc.) to the discharge tube on the basis of high-pressure mercury lamp [5]. Halogenated lamps have high luminous efficiency, high power, good light color, long service life and large spectrum. However, due to its low luminous efficiency, its contact with people is closer and closer to high-pressure sodium lamp, and its service life is shorter than that of high-pressure sodium lamp. At present, it is only used in a few plant factories

high pressure sodium lamp belongs to the type of high pressure gas discharge lamp. High pressure sodium lamp is a high-efficiency lamp filled with high-pressure sodium vapor in the discharge tube and added with a small amount of xenon (Xe) and halide of mercury lamp metal. As high-pressure sodium lamp has high electro-optic conversion efficiency and low manufacturing cost, high-pressure sodium lamp is the most widely used in light supplement applications in agricultural facilities at present, but its spectrum has the disadvantage of low photosynthetic efficiency, resulting in low energy efficiency. On the other hand, the spectral components emitted by high-pressure sodium lamps are mainly concentrated in the yellow orange light band, lacking the red and blue spectra necessary for plant growth

through comparison, the characteristics of different fill light sources can be clearly understood, as shown in Table 1

mobile light filling device

the intensity of light is closely related to the growth of crops. Three dimensional cultivation is often used in plant factories, but limited by the structure of cultivation frames, the distribution of light and temperature between layers is uneven, which will affect the yield of crops and the harvest period is not synchronized. Beijing No.1 company has successfully developed a manual lifting and lighting device in 2010. Its principle is to rotate the small film reeler by shaking the handle to drive the transmission shaft and the winder fixed on it to rotate, so as to achieve the purpose of retracting and releasing the steel wire rope. The steel wire rope for hanging the lighting lamp is connected with the winding wheel of the lifter through multiple groups of reversing wheels, so as to achieve the effect of adjusting the height of the lighting lamp. In 2017, the above-mentioned companies designed and developed a new mobile fill light device, which can automatically adjust the fill light height in real time according to the needs of crop growth. The regulating device is now installed on the three-layer light source lifting three-dimensional cultivation rack. The top layer of the device is the level with the best illumination, so it is equipped with high-pressure sodium lamp; The middle layer and the bottom layer are equipped with a lifting adjustment system, and LED lamps are installed, which can automatically adjust the height of the fill light according to the detection signal of the light sensor, so as to provide a suitable lighting environment for crops

compared with the mobile fill light device customized for three-dimensional cultivation, the Netherlands has developed a horizontally movable fill light device. In order to avoid the influence of the shadow of the fill light lamp on the growth of plants under the sun, the fill light lamp can be used to realize the transformation of the company from a single main business to a multi main business along the water square to push it to both sides of the support with large oil output of the oil return pipe through the telescopic slide, so that the sun can fully shine on the plants; On rainy days without sunlight, push the fill light lamp to the middle of the bracket to make the light of the fill light lamp fill the plants evenly; Moving the fill light horizontally through the slide on the support avoids frequent disassembly and moving of the fill light, reduces the labor intensity of employees, and effectively improves work efficiency

design idea of typical crop lighting system

from the design of mobile lighting device, it is not difficult to see that the design of lighting system in plant factory usually takes the light intensity, light quality and photoperiod parameters in different crop growth periods and the appropriate resolution as the core content of the design, which will be beneficial to the resolution end regulation of measurement. It relies on intelligent control system to implement and achieve the ultimate goal of energy conservation and high yield. At present, the lighting design and construction of leafy vegetables have gradually matured. For example, leafy vegetables can be divided into four stages: seedling stage, middle growth stage, late growth stage and end treatment; Fruits and vegetables can be divided into seedling stage, vegetative growth stage, flowering stage and harvesting stage. In terms of the light intensity attribute of supplementary light, the light intensity at seedling stage should be slightly lower, at 60-200 μ Mol/(M? · s), and then gradually increase. Leafy vegetables can be up to 100-200 μ Mol/(M? · s), fruits and vegetables can reach 300-500 μ Mol/(M? · s) to ensure the requirements of light intensity for plant photosynthesis in each growth period and realize the needs of high yield; For light quality, the ratio of red to blue is very important. In order to increase seedling quality and prevent overgrowth, the ratio of red to blue is generally set at a low level [(1-2): 1], and then gradually reduced to meet the needs of plant light morphogenesis. The ratio of red to blue light of leafy vegetables can be set (3-6): 1. In terms of photoperiod, similar to light intensity, it should show an increasing trend with the extension of growth period, so that leafy vegetables have more photosynthetic time for photosynthesis. The lighting design of fruits and vegetables will be more complex. In addition to the above basic rules, we should focus on the photoperiod setting of flowering period, and we must promote the flowering and Fruiting of vegetables to avoid backfiring. It is worth mentioning that the light formula should include the light environment settings for end treatment. For example, continuous light supplementation can significantly improve the yield and quality of hydroponic leafy seedlings, or UV treatment can significantly improve the nutritional quality of sprouts and leafy vegetables (especially purple leaf and red leaf lettuce)

in addition to optimizing the lighting of selected crops, the light source control system of some artificial light plant factories has also developed rapidly in recent years. This control system is generally based on B/S architecture, and realizes remote control of temperature, humidity, light and CO2 concentration in the process of crop growth through WiFi