Q How to measure the light intensity of different light sources? A、
Take a red light, green light, blue light LED.
The determination of light intensity can be crucial, for example, when designing the lighting of a room or preparing for a photo shoot. In the era of the Internet of Things (IoT), determining light intensity also has an important role to play in so-called smart agriculture. In this context, a key task is to monitor and control important plant parameters in order to promote optimal plant growth and accelerate photosynthesis.
Therefore, light is one of the most important factors. Most plants typically absorb light at red, orange, blue and violet wavelengths in the visible spectrum. Light in the green and yellow wavelengths of the spectrum is generally reflected and does not contribute much to plant growth. By controlling the intensity of some of the spectrum and light exposure during different growth stages, growth can be maximized, ultimately increasing yield.
Figure 1 shows a circuit design for measuring light intensity in the visible spectrum range for an experiment on plant photosynthesis. Three different colors of photodiodes (green, red and blue) are used here, which respond to different wavelengths. The light intensity measured by the photodiodes can now be used to control the light source according to the requirements of the specific plant.
Figure 1. Circuit design for measuring light intensity
The circuit shown consists of three precision current-to-voltage converters (transconductance amplifiers), one for each color (green, red and blue). The outputs of the current-voltage converters are connected to the differential inputs of a sigma-delta analog-to-digital converter (ADC), thus providing the measured values as digital data to the microcontroller for further processing.
Conversion of light intensity to current
Depending on the intensity of the light, more or less current will flow through the photodiode. The relationship between current and light intensity is approximately linear, as shown in Figure 2. The graph shows the characteristic curves of output current versus light intensity for the red (CLS15-22C/L213R/TR8), green (CLS15-22C/L213G/TR8) and blue (CLS15-22C/L213B/TR8) photodiodes.
However, the relative sensitivities of the red, green and blue diodes are different, so the gain of each stage must be determined separately by the feedback resistor RFB. For this purpose, the short-circuit current (ISC) of each diode must be obtained from the datasheet and then the sensitivity S (pA/lux) is obtained at the operating point determined from it. RFB is calculated as follows.
VFS,P-P indicates the full output voltage range required (full scale, peak-to-peak); INTMAX indicates the maximum light intensity, which is 120,000 lux for direct sunlight.
Current-to-voltage conversion
High-quality current-to-voltage conversion requires that the bias current of the op-amp be as small as possible, because the output current of the photodiode is in the picoamp range, and a large bias current can cause considerable errors. ADI’s AD8500 is ideal for such applications, with a typical bias current of 1 pA and a maximum detuning voltage of 1 mV. Analog to Digital Conversion
In order to further process the measured values, the photodiode current converted to voltage must be provided to the microcontroller as a digital value. For this purpose an ADC with multiple differential inputs, e.g. 16-bit ADCAD7798, can be used. thus the output code for the measured voltage is as follows.
Among them
AIN= input voltage.
N = Number of bits
GAIN = Gain factor of the internal amplifier.
VREF= External reference voltage.
To further reduce noise, common mode and differential filters are used for each differential input of the ADC.
All of the components described are very power-efficient, making the circuit ideal for battery-powered portable field applications.
Conclusion
Error sources such as the bias current and the detuning voltage of the device must be considered. Furthermore, the amplification factor inside the ADC converter affects the signal quality (the detuned voltage of the transconductance amplifier is multiplied by the internal gain of the ADC, amplifying the error in the detuned voltage), which affects the final sampling result. Using the circuit shown in Figure 1, light intensity can be converted to electrical quantities for further data processing with relative ease.
