# Experimental Study on Chromaticity Control in Visible Light Communication Systems
## 3 Research on RGB LEDs Dimming
### Simulation Analysis of Dimming  
The duty cycle is proportional to the luminous flux, and the luminous flux determines the LED luminance. The larger the luminous flux, the brighter the LED luminance. Therefore, the duty cycle is also directly proportional to the luminous brightness of the LED. Calculate and simulate the ratio of different brightness of RGB LEDs controlled by the circuit, change the ratio of brightness of a single LED, and observe the changes in color temperature and color coordinates of the RGB LEDs hybrid light source via simulation. The arrangement of the three types of LEDs is shown in Figure 1, with an observation area of 500 mm \(\times\) 500 mm and a distance of 100 cm from the emitter. In the simulation, the distance remains unchanged; only the brightness ratio of the LED is changed, and the total luminous flux of the set LED is 100 lm. (1) When the control circuit adjusts the luminous flux ratio of RGB LEDs, different color temperatures can be obtained. Simulate the changes in different color temperatures and the corresponding color coordinates based on the ratio calculated from Equations (15) to (16). As shown in Figure 2, the color coordinate \(x\) shows a decreasing trend with the increase in color temperature, while the color coordinate \(y\) fluctuates up and down with the increase in color temperature. The fluctuation is greater in the low color temperature stage and smaller in the high color temperature stage. Figure 1: The arrangement position of RGB LEDs. When the duty cycle ratios of the RGB LEDs are set to 0.6:0.28:0.12, 0.5:0.32:0.19, 0.47:0.33:0.2, and 0.45:0.34:0.21, the mixed light source on the observation surface is shown in Figure 3. At this time, the color temperature of the mixed light source is 3470 K, 5677 K, 6528 K, and 7265 K. The color coordinates are (0.4044, 0.3847), (0.3286, 0.3334), (0.3116, 0.3336), and (0.3038, 0.3099), and the total amount of light is around 88 lm. From the simulation, it can be seen that the proportion of RGB LEDs has a significant impact on the color coordinates and color temperature of the synthesized white light, with a relatively small impact on luminous flux. By changing the duty cycle of the red, green, and blue LEDs, the hybrid white light source can change from warm white to cold white. (2) Maintain a constant distance of 100 cm between the receiving surface and the light source, and simulate the changes in the average light intensity of the receiving surface under different color temperatures. As shown in Figure 4a, the average light intensity of the receiving surface under different color temperatures is around 1872 lx, with an error of less than 5 lx between the maximum and minimum values. When the color temperature is 3500 K, 5500 K, 6500 K, and 7300 K, it can be seen from the average light intensity distribution of the receiving surface simulated in Figure 4b-e that there is almost no difference. In summary, as the color temperature of the light source changes, the change in the illuminance intensity of the receiving surface is very small and can be ignored. Figure 3: Simulating light sources at different color temperatures. (**a**) Color temperature is 3470 K; (**b**) Color temperature is 5677 K; (**c**) Color temperature is 6528 K; and (**d**) Color temperature is 7265 K.  
Figure 2: Curve of color coordinates changing with color temperature. (3) Maintain a color temperature of 6500 K and simulate the analysis of light intensity on the receiving surface at different distances. As shown in Figure 5a, as the distance between the light source and the receiving surface increases, the energy received by the receiving surface becomes less and more dispersed. When the distance between the light source and the observation surface is 50 cm, 100 cm, 200 cm, and 300 cm, it can be seen from the light intensity of the receiving surface simulated in Figure 5b-e that the energy dispersion increases with the increase in distance.