Spectral sensor control LED lighting system
New LEDs are rapidly evolving and are being used in homes, commercial buildings and other market segments such as greenhouses and vertical farms. The new LEDs offer better, richer spectral characteristics and are more efficient and less expensive than early LEDs and traditional high-pressure nano-lamp (HPS) sources.
The new LEDs are superior to traditional HPS lamps in the field of horticulture: low power consumption, low heat dissipation and long service life have opened up new opportunities for the horticultural market. The new LEDs can be placed close to plants to control light intensity and spectral properties and optimize plant growth conditions. There are two advantages to do this: First, growers increase the density of plants in the greenhouse by vertically stratifying plants (called vertical agriculture). Second, growers can optimize greenhouse lighting time and spectrum to accelerate plant growth. LED has become the preferred source of horticultural lighting, but LEDs also have limitations that require control circuitry to achieve stable illumination and optimal plant growth environment spectrum. These problems can be solved using closed-loop control driven by spectral sensors.
The value of plant growth lamps
Many plants grow only in specific areas and times, which is related to the plant's temperature and humidity requirements. Another important factor is the light source, which directly affects photosynthesis and thus affects plant growth.
For example, some studies have found that light with a wavelength of 680 nm is critical for the growth rate of tomato during germination, but has little effect after germination. In the growth phase, 650 nm light can increase the content of chlorophyll a. Therefore, each stage of plant growth (germination, growth and harvest) requires illumination of different wavelengths.
In the northern hemisphere, a greenhouse is needed if you want to grow tomatoes all year round. Protect tomatoes from the harsh winter weather, equipped with lighting systems to compensate for the lack of natural light in winter and create the best spectral properties for growth. Optimize the lighting solution so that the lighting system can reflect the daily changes in the solar spectrum, consistent with the plant's day and night cycle.
Like tomatoes, other plants also require specific spectral properties to promote growth and photosynthesis. The spectral characteristic curve includes the spectral wavelength range and illuminance. The spectrum used for horticulture is mainly visible light with wavelengths ranging from 400 nm to 700 nm, with peaks in the red and blue ranges. However, all plants require full spectrum to achieve optimal growth, and the circadian rhythm depends on the light conditions of the origin of each plant.
Because of the differences between plants of the same species, the quality of light can have a major impact on the growth and development of specific plants. Monitoring source intensity, spectrum, and circadian rhythm ensures optimal lighting conditions.
Temperature characteristics of LED emission spectrum
The spectral characteristics of red LEDs are much more affected by temperature changes than blue LEDs. In the range of 5 ° C (41 ° F) to 70 ° C (158 ° F), the brightness is reduced by nearly 40%.
At normal ambient temperatures, the characteristics of the luminaire are very close to the ideal black body curve or Planck trajectory, but when the ambient temperature rises to 80 ° C, the spectrum deviates greatly from the target value. This is a concern for horticultural lighting systems.
As described above, the growth conditions of the plant are optimized, with an emphasis on the actual output spectral curve, rather than the color of the light perceived by the human eye. Therefore, one needs to understand the spectral characteristics of LED light sources at different temperatures in order to control the spectral characteristics and brightness of the luminaire.
Optical feedback system
The normal operation of the horticultural lighting system requires adjustment of the output based on time and temperature to maintain a specific chromaticity and brightness.
One method is to stabilize the current and voltage of the LED; another method is to measure the LED temperature and feedback the temperature value to the LED driver for temperature control. This indirect adjustment relies on the aging prediction model of the phosphorous material in the LED, and also needs to classify the Led used in the production of the luminaire. The third method is closed loop adjustment. As shown in Figure 4, this solution differs from the unregulated control solution. It measures the spectral characteristics of the spectral sensor in real time and directly controls the LED driver or controller to adjust the output to ensure matching with the specified target chromaticity and brightness values.
This method is applicable to any LED source such as RGB+White, RGB White+Amber, and a light source system with four or more spectral LEDs.
Sensors can acquire spectra in a variety of ways. One is to add a light guide to the luminaire, collect light from multiple LEDs, and direct the mixed light to the sensor. The other is to design a spectral sensing unit outside the luminaire, flush with the plant, and the sensing unit can detect and react to the light source. In this case, the sensing unit can also detect and react to daylight.