Part II: Viewing Angle

In our last post, we talked about the significance of lighting for pant growth in terms of light intensity, photoperiodism and light quality. In plant growth, light intensity is expressed as photosynthetic photon flux density (PPFD). It depends on both the optical design of the fixture and the number of photons emitted by the light source.

So what are the advantages of LEDs over conventional lights in providing sufficient light intensity for plant growth?

Let’s suppose there is a plant growth environment, where the plant is growing in the nutrient solution right under the lighting fixture on the ceiling. Only the light downward can reach the plant surface and be absorbed, while the upward and sideward light cannot. Figure 1 describes the light paths of traditional light sources and LEDs without optical design. Let’s say these four lights emit the same amount of photons, and the light from a traditional fixture, due to its broad viewing angle, is largely compromised and wasted if without any reflection accessories. Moreover, the reflector itself has an absorption rate that causes light loss. If the fixture requires small viewing angle, the reflecting angle must be small too. So for a light bulb or a tube light, the light upward and sideward is reflected multiply between the reflector and the fixture, leading to huge light loss and low efficiency. While when it comes to the LEDs, it is different. The LED itself has a small viewing angle, making the utility rate much higher, even without any additional reflectors. Besides, the LED dissipates small amount of heat, so optics made of low-priced resin can be added directly onto the light source, instead of any additional reflectors. In this way, the original 120° viewing angle can be decreased to any angle smaller (which is normally 15° on the market).

 
Fig.1 Viewing angles of different types of light sources.
 

Apart from that, the diversity in the viewing angle enables the widespread application of the LED light sources in plant factories. Each plant needs its own lighting solution to meet the illuminance requirement for every stage of growth. In Figure 2, the three LED lights have different viewing angles but the same amount of photon flux. Finally they reach the same PPFD on the same illumination area. This means that for plant factories that may differ a lot in heights, it is much easier and more convenient to use LED light sources, because the flexible viewing angle choices can help reach the required photosynthetic photon flux level per unit area.    

 
Fig.2 Three LEDs at different heights achieving the same level of illuminance.