Plants can grow only when photosynthesis is greater than respiration. This critical point is called light compensation point. For example, if the light compensation point of tomato is 51.6 μ mol / S / m2, the effective radiation of photosynthesis must be greater than 51.6 μ mol / S / m2 to work.
Lux is the intensity of human perception. For plants, lux is measured by the intensity of the spectrum contained in the lamp that is effective for photosynthesis. This is called photosynthesis effective radiation, referred to as par or PPF for short. There is no conversion between PPF and lux. PPFD is the strength per unit area of PPF
Generally, the 20W T8 tube LED can only provide PPFD of about 24 μ mol / S / m2 at a height of 30cm from the plant. If it is used to grow tomatoes, at least 3 lamps can be used. But the tomato will grow up. In the practical application, the foreign greenhouse can only get 90 μ mol / S / m2 PPFD by using 630W led hanging in the 4m high temperature room. The price of 600W agricultural sodium lamp can reach about 120 μ mol / S / m2, and the price is about 1 / 5 of LED. Therefore, agricultural sodium lamp is mainly used in the greenhouse.
When PPFD reaches this point, the photosynthetic rate of plants is the fastest, but beyond this point, there will be no more growth and even reaction. The light saturation point of tomato is 1998 μ mol / S / m2. At present, the maximum power lamp in the application is 1000W double end agricultural sodium lamp, PPF 2100 μ mol / s. when it is hung in a 6-meter-high greenhouse, the PPFD is only about 200 μ mol / S / m2, and the sunlight PPFD at noon in summer is more than 2000 μ mol / S / m2, so any artificial light source on the earth can not replace the sunlight.
The light intensity of the plant's fill light is far from that of the natural sunlight. As a reference, the sunlight intensity at noon on a sunny day is more than 100000 lux, that at morning and evening on a sunny day is 20000-40000 lux, that at cloudy and cloudy days is 5000-30000 lux, and that near the window but without direct sunlight is 2000-8000lux. Generally, the illumination required for the growth of green plants is between 20000-50000 lux. In contrast, the illumination provided by perpetual lamp is very low, and it can not meet the normal lighting requirement of plants.
The LED form of plant growth lamp is usually made of a combination of red LED and blue LED, making the form of perpetual lamp and spotlight. Although the LED technology has been able to achieve higher luminous efficiency (100lm/w), the luminous efficiency of red LED and blue LED used in perpetual lamp can only reach 20lm/w, much lower than that of ordinary fluorescent lamps (50-90lm/w), and the price per unit power is much more expensive than fluorescent lamps. According to the calculation, if we want to use LED perpetual lamp to reach the illumination of normal growth in 40cm area, it will take about 1000W LED and cost more than 10000 yuan. It's better to use fluorescent lamp or microwave sulfur lamp.
The experiment also showed that in the distance of tens of centimeters, the fluorescent light source would cause significant temperature rise to the plant and culture medium, and lead to dryness. The main cause of temperature rise is radiation, so it is not easy to block. At the same time, the temperature of a single fluorescent lamp is as high as 60-90 ℃ when it works stably. If a large number of fluorescent lamps are installed intensively in order to improve the illuminance, effective heat dissipation measures must be specially taken for the lamp itself. Although LED has the advantage of small heating, but high-power LED and LED light group also need to consider heat dissipation.
Conclusion: if the plants are used to replace sunlight and provide illumination for plant growth, the existing perpetual lamp is not yet practical.
Supplement: in fact, in addition to light supplement, providing a higher concentration of carbon dioxide (gas fertilizer) is more effective for plant growth. If you grow small plants, put them in a foam box, lay a layer of light stone or Lan Shi on the bottom and pour some water. Then put a plant on it, and put a glass plate on the foam box. This can provide plants with high temperature, high humidity and carbon dioxide rich environment. This kind of stuffy plant grows very big and fast.