How to convert light intensity into an electrical quantity!

Q: How to measure the light intensity of different light sources?
  A: Use photoelectric LEDs that respond to red, green, and blue light to measure.
   The determination of light intensity may be crucial, for example, when designing the lighting of a room or preparing to take a photo. In the Internet of Things (IoT) era, determining light intensity also plays an important role in "smart agriculture". A key task of the latter is to monitor and control important plant parameters to promote plant growth and accelerate photosynthesis.
   Therefore, light is one of the important factors. Most plants usually absorb red, orange, blue, and violet wavelengths of light in the visible spectrum. The light of the green and yellow wavelengths in the spectrum is generally reflected and does not contribute much to plant growth. By controlling part of the spectrum and light irradiation intensity in different growth stages, the growth can be promoted as much as possible, and finally the yield can be increased.


   Figure 1 shows a circuit design used to measure light intensity in the visible spectrum for experiments on plant photosynthesis. Three photodiodes of different colors (green, red and blue) are used here, which respond to different wavelengths. The light intensity measured by the photodiode can now be used to control the light source according to the requirements of the specific plant.
   Figure 1: Circuit design used to measure light intensity
   The circuit shown is composed of three precise current-voltage converters (transconductance amplifiers), one for each color (green, red, and blue). The output of the current-voltage converter is connected to the differential input of the sigma-delta analog-to-digital converter (ADC), thereby providing the measured value as digital data to the microcontroller for further processing. Conversion of light intensity to electric current
  Depending on the light intensity, 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 figure shows the output current and light intensity characteristic curves of red light (CLS15-22C/L213R/TR8), green light (CLS15-22C/L213G/TR8) and blue light (CLS15-22C/L213B/TR8) photodiodes.


   Figure 2: Current and light intensity characteristic curves of red, green and blue photodiodes
    However, the relative sensitivities of red, green and blue diodes are different, so the gain of each stage must be individually determined by the feedback resistor RFB. To this end, the short-circuit current (ISC) of each diode must be obtained from the data sheet, and then the sensitivity S (pA/lux) at the operating point determined by it. RFB is calculated as follows:



  VFS, P-P represent the required full output voltage range (full scale, peak-to-peak value); INTMAX represents the light intensity, for direct sunlight, it is 120,000 lux. Current-voltage conversion
   High-quality current-voltage conversion requires the bias current of the operational amplifier to be as small as possible, because the output current of the photodiode is only a few picoamps, and a large bias current will cause considerable errors. The offset voltage should also be small. The AD8500 is an ideal choice for this type of application. Its bias current is typically 1pA and the offset voltage is 1mV. Analog-to-digital conversion
   In order to further process the measured value, the photodiode current must be converted to a voltage and provided to the microcontroller as a digital value. For this you can use an ADC with multiple differential inputs, such as the 16-bit ADCAD7798. Therefore, the output code of the measured voltage is as follows:



   Among them: AIN = input voltage, N = number of bits, GAIN = gain coefficient of the internal amplifier, VREF = external reference voltage.
   In order to further reduce noise, common mode and differential filters are used on each differential input of the ADC. All the components described are very power-saving, making this circuit very suitable for battery-powered portable field applications. in conclusion
You must consider error sources such as the bias current and offset voltage of the device. Moreover, the internal amplification factor of the ADC converter will affect the signal quality (the offset voltage of the transconductance amplifier will be multiplied by the internal gain of the ADC to amplify the error of the offset voltage), thereby affecting the final sampling result. Using the circuit shown in Figure 1 can relatively easily convert light intensity into electrical quantities for further data processing.