Most people think artificial supplementary lighting is only used in greenhouses with insufficient sunlight. However, some areas have abundant sunlight and do not need supplementary lighting. I will provide some analytical data on this issue for your reference.
First, the conclusion is given:
1) The light quality of sunlight is not suitable for photosynthesis.
2) The amount of light in most greenhouses is insufficient and unstable.
3) The photosynthesis efficiency of greenhouses in sunny areas is not high.
4) Without artificial supplementary lighting, achieving continuous and stable yield and quality assurance in greenhouse cultivation is impossible.
1. Light quality of sunlight
The figure below is a spectrum of the full range of standard sunlight AM1.5G wavelength. Absorption lines or absorption bands appear at some wavelengths. This is because when sunlight passes through the solar atmosphere, it is absorbed or scattered by some elements or molecules in the atmosphere, resulting in a decrease in light intensity at these wavelengths, forming a depression. Therefore, it can be seen that the spectral wavelength range of standard sunlight is 280-4000nm.
The standard solar spectrum is a commonly used spectral standard in sunlight and solar energy research and applications.
(Note: AM1.5G is a standard sunlight spectrum, representing the spectral characteristics of sunlight when it propagates vertically in the atmosphere. AM stands for atmospheric mass, 1.5 means that the path length of sunlight through the atmosphere is 1.5 atmospheric masses, atmospheric mass is a dimensionless value with a zenith angle of 48.2 degrees, and G stands for global.)
For plant illumination research, the wavelength range is 350-850nm.
The following figure is the spectrum of AM1.5G in this band:
The radiation content of this band:
Ultraviolet light (350-399nm): 9.34%
Blue light (400-499nm): 21.21%
Greenlight (500-599nm): 23.22%
Red light (600-700nm): 21.62%
Far-red (701-850nm): 24.61% Red-blue ratio R:B=1.02
For plant photosynthesis research, the wavelength range is 400-700nm, also known as PAR.
The following figure is the spectrum of AM1.5G in the PAR band:
Radiation content in this band:
Blue light (400-499nm): 32.33%
Greenlight (500-599nm): 35.40%
Red light (600-700nm): 32.27%
R:B=1.02
The following figure is the distribution diagram of photons in the PAR band
From the above data, the light quality of sunlight is compared with our planting test data. The growth rate and photosynthesis efficiency of plants under artificial lighting are much higher than in sunlight.
Although the light quality of sunlight is not good, it meets the second of the three principles of spectral technology: light quantity takes precedence over light quality.
So, what is the actual light quality of sunlight on the ground?
The light quality of sunlight in different regions from 23°N (blue), 39°N (red), and 44°N (gray) is different.
The above figure is a normalized spectrum diagram. It can be seen from the figure that the higher the latitude, the higher the blue light component. However, this figure cannot explain the red-to-blue ratio. It expresses the trend. We are concerned about the sunlight data in the greenhouse. Let's discuss it below.
2. The quality of sunlight in the greenhouse
Regardless of the type of greenhouse, sunlight is attenuated, mainly due to the influence of structural parts, greenhouse film or glass, etc., and the spectrum of sunlight also changes.
The following figure shows the spectrum changes inside the greenhouse (red) and outside (blue), using absolute spectrum data to draw the map.
Calculating the above figure, it is concluded that the amount of sunlight in the greenhouse is attenuated by more than 35%.
Even a Venlo greenhouse will have more than 28% less sunlight.
The attenuation of light quality by greenhouse translucent materials is mainly ultraviolet and blue light.
From the relative spectrum calculation inside and outside the greenhouse, the ultraviolet and blue light of sunlight are partially absorbed, so the red-to-blue ratio in the greenhouse will increase.
Many factors affect the light quality of sunlight in the greenhouse. The most uncontrollable factor is that sunlight itself is affected by seasons, weather, dust accumulation of translucent materials, etc. The conclusion is that the light quality is constantly changing for planting under sunlight alone, and the planting process must be constantly adjusted, which greatly affects the quality and yield of finished plants.
3. The amount of sunlight in the greenhouse
How to calculate the amount of sunlight in the greenhouse?
First, let's introduce a concept: DLI.
DLI: Daily accumulation of sunlight, which means the molar amount of sunlight per square meter per day, used to measure the light accumulation of crops. Unit: mol/d/m2
DLI is related to geographical location. DLI affects the rate of photosynthesis and plant growth. The response of plant growth to DLI varies depending on species and varieties.
The DLI outside the greenhouse is different from the DLI inside the greenhouse, usually with a difference of 4-8mol/d/m2
The DLI in the greenhouse needs to be measured for a long time, which is an important light parameter to ensure greenhouse planting.
Greenhouse DLI calculation:
DLI=Σ 0.0036*PPFDi*hi (i=12...n)
Where: different periods, unit: hours Different plants have different requirements for DLI.
DLI in different planting areas also varies greatly.
The DLI index calculation itself is not related to the light quality of sunlight. We can measure the illuminance in the greenhouse and then use the XD factor to convert PPFD. The influence of sunlight quality on DLI will be reflected to a certain extent.
XD factor: In the wavelength range of 400-700nm, when the spectral morphology is determined, the light source can convert PPFD by measuring the illuminated surface's illuminance value (LX). This conversion constant is the XD factor.
Conversion method: PPFD = illuminance value (LX) / XD factor, where illuminance unit LX: lm/m2, PPFD unit: umol/s/m2
Note: The XD factor is related to the spectral form of the light source. Different spectral forms of the same light quality have different XD factors. Haoliang Solid Light Source Research Institute provides XD factors.
The following XD factors are provided for reference:
23° north latitude, XD factor: 57
39° north latitude, XD factor: 55.4
44° north latitude, XD factor: 55
Note: The precise XD factor requires professional calculation.
For example: the average daily sunlight illumination in a greenhouse in Guangdong in a certain season is 13000, then PPFD=13000/55=228 umol/s/m2
If the effective illumination time of sunlight in the greenhouse is 7 hours, the DLI in the greenhouse is 0.0036*228*7=5.74 mol/d/m2
For DLI in the greenhouse below 6, it is considered a low light level.
4. Do areas with better lighting conditions need artificial supplementary lighting?
From the XD factor, it can be seen that the higher the latitude, the more blue light components, the smaller the XD factor, the larger the PPFD value calculated at the same illumination level, which also leads to a lower red-blue ratio of the spectrum and worse light quality.
The light quality impact of sunlight can be observed from natural phenomena, such as the higher the altitude, the greater the PPFD, but the shorter the plant growth.
If the DLI in the plantable area reaches 45 mol/d/m2, the PPFD peak of sunlight will be greater than 2000 umol/s/m2, and the amount of light will cause light stress to the plants. The plants will close the stomata of the leaves and stop photosynthesis. In general, the photosynthesis efficiency of plants will be reduced, and the daily energy supplement of plants is not sufficient. At the same time, the high blue light content itself has a stress effect on plant physiology, and high blue light also affects the taste of fruits and vegetables, making them sour or bitter.
High-quality agricultural products are the primary sign of modern agricultural planting. In areas with high levels of sunlight, greenhouse planting requires artificial lighting to adjust the light quality, otherwise, the input-output ratio cannot be achieved.
Conclusion
Bio-optics uses the theory of photon quantum calculation to completely derive the algorithm model of greenhouse lighting, making greenhouse lighting a controllable planting lighting technology.
If there is no application of artificial lighting technology in the greenhouse, it cannot be said to be a modern agricultural planting technology, let alone the application of intelligent greenhouse technology.
