How to convert light intensity into an electrical quantity How to measure the light intensity of different light sources?

Reply:
Take a red, green, blue LED.
The determination of light intensity can be crucial, for example, when designing the lighting of a room or preparing to take a photo. In the age of the Internet of Things (IoT), determining light intensity also plays an important role in so-called smart agriculture. In this context, a key task is to monitor and control important plant parameters to promote optimal plant growth and accelerate photosynthesis. Therefore, light is one of the most important factors. Most plants generally absorb red, orange, blue, and violet wavelengths of light 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 part of the spectrum and the intensity of light exposure in different growth stages, growth can be maximized, ultimately increasing yield.
A circuit design for measuring light intensity in the visible spectral range for experiments in plant photosynthesis. Three different colored photodiodes (green, red, and blue) are used here, and they 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.

The circuit shown consists of three precision current-to-voltage converters (transconductance amplifiers), one for each color (green, red, and blue). The output of the current-to-voltage converter is connected to the differential input of a sigma-delta analog-to-digital converter (ADC), providing the measurement as digital data to the microcontroller for further processing.

Convert 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 graph shows the characteristic curves of output current versus light intensity for red (CLS15-22C/L213R/TR8), green (CLS15-22C/L213G/TR8), and blue (CLS15-22C/L213B/TR8) photodiodes.

Figure 1. Circuit Design for Measuring Light Intensity

Figure 2. Current vs. Light Intensity for 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 determined individually by the feedback resistor RFB. To do this, 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,PP is the desired full output voltage range (full scale, peak-to-peak); INTMAX is the maximum light intensity, which is 120,000 lux for direct sunlight.

Current-Voltage Conversion

High-quality current-to-voltage conversion requires that the bias current of the op amp be as small as possible, since the output current of the photodiode is in the picoamp range, and large bias currents can cause considerable errors. The offset voltage should also be small. The AD8500 from Analog Devices is ideal for such applications, with a typical bias current of 1 pA and a maximum offset voltage of 1 mV.

Analog to digital conversion

For further processing of the measured value, the photodiode current converted into a voltage must be provided to the microcontroller as a digital value. An ADC with multiple differential inputs, such as the 16-bit ADC AD7798, can be used for this purpose. Therefore, the output code for the measured voltage is as follows:

This equation is incorrect and should be changed to:

digital code
GAIN
in
AIN = input voltage,
N = number of digits,
GAIN = gain factor of internal amplifier,
VREF = external reference voltage.

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 efficient, making this circuit ideal for battery powered portable field applications.

in conclusion

Error sources such as the bias current and offset voltage of the device must be considered. Moreover, the amplification factor inside the ADC converter will affect the signal quality (the offset voltage of the transconductance amplifier will be multiplied by the gain inside the ADC to amplify the error of the offset voltage), thus affecting the final sampling result. Using the circuit shown in Figure 1, it is relatively easy to convert light intensity into electrical values for further data processing.