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PART III: Light Quality

Compared with the factor of LED viewing angle we discussed last time, our topic today–light quality is a more significant index in terms of which LEDs have a revolutionary advantage over conventional light sources. The emergence and growth of LED technology has stimulated the development of modern agriculture and resulted in many new forms of agriculture, such as vertical farming.  

Biologically, plants can perceive light within certain range of wavelength, just as the human eye does. The luminosity function is used to describe the human eye’s sensitivity to lights of different wavelengths. Plants also have their own “luminosity function”, which is hugely different from that of the human eye. Several decades ago, people found through studies that plants have certain “luminosity function” (to be precise: absorption function) towards light. Gates (1965) explained in “Spectral Properties of Plants” why plants absorb lights of different types. The absorbance properties of chlorophyll a, chlorophyll b and carotene are shown in Figure 1. Plants can well absorb blue (400 nm to 500 nm) and red light (600 nm to 700 nm), yet are less receptive to green light (600 nm to 700 nm). That is to say, not all the lights within the photosynthetically active radiation range (400 nm to 700 nm) are effective in plant growth in the same way or to the same extent. The most efficient light source is the one which emission property best matches the plant’s spectrum absorption property.    

Figure 1. Absorbance spectrum of plants (Gates et al. “Spectral Properties of Plants”,1965).

Before LEDs, conventional light sources, including metal halide lights, fluorescent lights and high pressure sodium lights, have limited emission properties due to the way they illuminate. It is difficult to change or tune the spectrum when manufacturing them. For the plants, part of the light (ie.500 nm-600 nm) from conventional light sources may be useless, thus compromising the efficiency, and increasing the operation cost of the plant factory. Besides, conventional lights such as metal halide lights emit infrared light over 700 nm and produce heat, which can be harmful to plant growth. This way, more limitations are placed for the lighting design in plant factories, and growers will have to set the lights away from the plants.

Different from conventional light sources, LEDs illuminate in a much more flexible way. The emission spectrum of an LED is determined by the semiconductor material that makes up the LED chip, so different materials can be used to make LEDs of different spectrum properties (Figure 2). Therefore, LEDs are highly customizable because they can be made diverse to meet needs of different plants in different growing stages. In most cases, plants need combination of lights, such as blue light and red light. Packaged LEDs can be made according to the needs, with chips of different spectrum properties or chips combined with phosphor. Compared to the single color LED in Figure 2, the combined light source has almost the same manufacturing process (only except the added material cost). This greatly promotes the popularization of special spectrum LEDs. 

Figure 2. Emission spectra of LED light sources made of different materials
(Toyoki Kozai et al. “LED lighting for urban agriculture”).


To wrap up this chapter, LED lighting technology provides specialized light sources for plants’ needs, which can utilize the maximum light energy transferred from electric energy, and can help cut the production and operation cost for plant factories.

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