3 common misunderstandings and design suggestions for LED plant lighting

introduction

Light plays a key role in the growth of plants. It is the best fertilizer to promote the absorption of chlorophyll and the absorption of various plant growth such as carrots. However, the determinant of plant growth is a comprehensive factor, not only related to light, but also inseparable from the configuration of soil and water fertilizers, growing environmental conditions and comprehensive technical control.

In the last two or three years, reports on the application of semiconductor lighting technology in three-dimensional plant factories or plant growth have emerged one after another. But after reading it carefully, there is always some feeling of uneasiness. In general, there is no question that can really recognize the role that light should play in plant growth.

First, let's get to know the spectrum of the sun, as shown in Figure 1. It can be seen that the solar spectrum is a continuum, in which the blue and green spectra are stronger relative to the red spectrum, and the visible light spectrum ranges from about 380 to 780 nm. The growth of natural organisms is related to the intensity of the spectrum. For example, most plants in the vicinity of the equator grow very fast, and the size of growth is relatively large. However, the intensity of the sun is not as high as possible, and it is selective for the growth of animals and plants.

Second, the second spectral diagram of several key absorption factors for plant growth is shown in Figure 2.

As can be seen from Figure 2, several key auxins that affect plant growth have significant differences in the absorption spectrum of light. Therefore, the application of LED plant growth lamps is not a simple matter, but rather very targeted. It is necessary here to introduce the two most important concepts of plant growth factors for photosynthesis.

Chlorophyll

Chlorophyll is one of the most important pigments associated with photosynthesis and is found in all organisms that produce photosynthesis, including green plants, prokaryotic blue-green algae (blue fungi) and eukaryotic algae. Chlorophyll absorbs energy from light, which is then used to convert carbon dioxide into carbohydrates.

Chlorophyll a mainly absorbs red light, and chlorophyll b mainly absorbs blue-violet light, mainly to distinguish between shade plants and sun plants. The ratio of chlorophyll b to chlorophyll a in the shade plants is small, so the shade plants can strongly utilize blue light and adapt to the growth of the shade. Chlorophyll a is blue-green and chlorophyll b is yellow-green. The strong absorption of chlorophyll a and chlorophyll b has two, one in the red region with a wavelength of 630-680 nm, and the other in the blue-violet region with a wavelength of 400-460 nm.

Carotenoid

Carotenoids are a general term for important natural pigments that are ubiquitous in yellow, orange-red or red pigments in animals, higher plants, fungi, and algae. More than 600 natural carotenoids have been discovered to date.

Carotenoids absorb light in the range of OD 303 ~ 505 nm, which provides the color of food and affects the body's intake of food; in algae, plants and microorganisms, its color is not covered by chlorophyll. In plant cells, the carotenoids produced, in addition to absorbing and transferring energy, help photosynthesis, while also protecting cells from the destruction of excited single-electron oxygen molecules.

Some conceptual misunderstandings

Regardless of energy saving effects, light selectivity, and light coordination, semiconductor lighting has shown tremendous advantages. However, from the rapid development of these two years, we have also seen a lot of misunderstandings in the design and application of light, mainly in the following aspects.

1 As long as the red and blue chips of a certain wavelength are combined according to a certain ratio, they can be applied to the cultivation of plants, for example, the ratio of red to blue is 4:1, 6:1, 9:1 and so on.

2 As long as it is a white light, it can replace the sun's illumination. For example, the three-color white light tube widely used in Japan, etc., the use of these spectra has a certain effect on the growth of plants, but the effect is not as good as the light source made by LED.

3 As long as the PPFD (light quantum flux density) of an important parameter of illumination reaches a certain index, for example, PPFD is greater than 200 μmol·m-2·s-1, but it must be noted when using this indicator. Plants are still male plants, to find or find the light compensation fullness of these plants, also known as light compensation points. In actual applications, the seedlings are often burned or withered. Therefore, the design of this parameter must be specifically designed according to the plant species, the environment and conditions of growth.

Regarding the first aspect, it has been introduced in the introduction that the spectrum required for plant growth should be a continuum with a certain distribution width, using a narrow spectrum of red and blue (Fig. 3(a)). The light source made of the wavelength chip is obviously unsuitable. In the experiment, it is found that the plant will be yellowish, the leaf stem is very light, and the leaf stem is very thin.

For the fluorescent lamps mainly composed of three primary colors which have been commonly used in previous years, although the white color is synthesized, the red, green and blue spectra are discrete (Fig. 3(b)), and the width of the spectrum is narrow. The spectral intensity of the continuous part below is relatively weak, and the power is still relatively large compared to the LED, etc., 1.5 to 3 times of energy consumption. Therefore, it is not as good as the LED light in use.

PPFD is the optical quantum flux density, which refers to the effective radiation flux density of light in photosynthesis, which represents the total number of light quantum incident on the leaf stems of plants in the wavelength range of 400-700 nm per unit time and unit area. Its unit is μE·m-2·s-1 (μmol·m-2·s-1). Photosynthetically active radiation (PAR) refers to total solar radiation with a wavelength in the range of 400-700 nm. It can be expressed either by photon or by radiant energy.

The light intensity reflected by the illuminometer used in the past is brightness. However, since the spectrum of plant growth changes depending on the height of the light source from the plant, the light coverage surface, and whether the light passes through the blade or the like, it is used as light when studying photosynthesis. Strong indicators are not precise enough, and most of them are now using PAR.

Generally, the positive plant PPFD> 50 μmol·m-2·s-1 can initiate the photosynthesis mechanism; while the plant PPPD only needs 20 μmol·m-2·s-1. Therefore, when purchasing LED plant lights, you can purchase the number of LED plant lights based on this reference value and the type of plants you plant. For example, if a single LED plant bulb PPFD is 20 μmol·m-2·s-1, more than 3 LED plant bulbs are needed for planting positive plants.

Several semiconductor lighting designs

There are two basic reference methods for semiconductor lighting for plant growth or planting.

At present, the indoor planting mode is very hot in China. This model has several features:

The function of 1 LED lamp is to provide the full spectrum of plant illumination, which requires the luminaire to provide all the illumination energy, and the production cost is relatively high;

The design of 2LED plant growth lamps needs to consider the continuity and integrity of the spectrum;

3 It is necessary to effectively control the time of illumination and the intensity of illumination, such as factors such as letting the plants rest for a few hours, the intensity of the irradiation is not strong enough or too strong;

4 The whole process needs to imitate the conditions required for the actual optimal growth environment of the plants, such as humidity, temperature and CO2 concentration.

It has a good outdoor planting mode for outdoor greenhouse cultivation. The characteristics of this model are:

The function of 1 LED lamp is to supplement the light. The first is to strengthen the light intensity of the blue and red areas in the case of sunshine during the day, to promote the photosynthesis of plants, and to compensate and promote the plants when there is no sunshine at night. Growth rate;

2 The fill light needs to consider which growth stage the plant is in, such as the nursery period or the flowering result period.

Therefore, the design of LED plant growth lights should first have two basic design patterns: all-weather lighting (indoor) and plant growth fill (outdoor). For indoor planting, the design of LED plant growth lights needs to consider three aspects, as shown in Figure 4. It is not possible to directly package the chips of the three primary colors in a certain proportion.

For example, a spectrum of the seedling stage, considering its need to strengthen the growth of roots and stems, strengthen the bifurcation, and the use of the light source is indoors, so the spectrum can be designed in the form of Figure 5.

For the design of the second type of LED plant growth lights, the design is mainly for the light-assisted design of the base planted in the outdoor greenhouse. The design idea is shown in Figure 6.

The authors suggest that more planters use the second option to use LED lamps to promote plant growth.

First of all, China's outdoor greenhouse cultivation has decades of experience, a large amount of bread, large in the south and north, has a good greenhouse cultivation technology foundation, providing a large number of freshly marketed fruits and vegetables for surrounding cities, especially in soil and water and fertilizer. A wealth of research results have been achieved in planting.

Secondly, this light-filling scheme can greatly reduce the unnecessary consumption of energy, and at the same time can effectively improve the yield of fruits and vegetables. In addition, China has a vast geographical area and is very convenient for promotion.

As a scientific research of LED plant lighting, it also provides a broader experimental base. Figure 7 is an LED auxiliary lamp developed by the research team for filling light in a greenhouse. The spectrum is shown in Figure 8.

According to the above design ideas, the research team conducted a series of experiments, and the experimental results were very significant. For example, the seedling extension lamp, the original lamp used is a fluorescent lamp with a power of 32 W and a seedling cycle of 40 days. We provide a 12 W LED lamp with a shortening of the seedling cycle to 30 days, and effectively reduce the temperature of the lamp in the nursery workshop, saving air conditioning power consumption. The strength, length and color of the seedlings are better than the original seedlings. For the nursery of common vegetables, good verification conclusions have also been obtained, which are summarized in the following table.

Among them, PPFD in the light-filling group: 70~80 μmol·m-2·s-1, red-blue ratio: 0.6-0.7; in the natural group, the PPFD value in the day ranged from 40 to 800 μmol·m-2·s-1, red Blue ratio: 0.6~1.2. It can be seen that the above indicators are better than the natural growth of the seedlings.

in conclusion

This paper introduces the latest developments in the application of LED plant growth lamps in plant cultivation and planting, and points out some misunderstandings in the application of LED plant growth lamps in plant cultivation and cultivation. Finally, the technical ideas and solutions for the development of LED auxiliary lamps used in plant cultivation and planting are introduced. It should be pointed out that there are also some factors to be considered in the installation and use of the lamp, such as the distance of the lamp from the plant, the irradiation range of the light and how to cooperate with normal water, fertilizer and soil.