# Experimental Study on Chromaticity Control in Visible Light Communication Systems
## 3 Research on RGB LEDs Dimming
### Dimming Experiment  
This experiment uses hexagonal RGB LEDs beads to form a luminescent light source and uses the HP350 spectral illuminometer manufactured by Hangzhou Double Color Intelligent Testing Instrument Co., Ltd., which is produced in Hangzhou, Zhejiang, China, to measure the light color parameters. When the three-color LED is operated at full current, the light color parameters are shown in Table 1, and the relative spectral power distribution is shown in Figure 6.  
Adjust the duty cycle of the LED and measure the brightness of the LED. The variation curve of the measured LED brightness with the duty cycle is shown in Figure 7. The brightness of the green light changes significantly with the duty cycle, while the changes in the red and blue light are relatively close, with relatively small changes compared to the green light. Figure 4: Light intensity distribution under different color temperatures. (**a**) The variation of light intensity with color temperature; (**b**) Color temperature is 3500 K; (**c**) Color temperature is 5500 K; (**d**) Color temperature is 6500 K; and (**e**) Color temperature is 7300 K.  
Figure 5: Light intensity distribution at different distances. (**a**) The variation of light intensity with distance; (**b**) Distance is 50 cm; (**c**) Distance is 100 cm; (**d**) Distance is 200 cm; and (**e**) Distance is 300 cm. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**LED Type** & \begin{tabular}{c} **Chromatic** \\ **Coordinates** \\ \end{tabular} & \begin{tabular}{c} **Peak Wavelength** \\ **(nm)** \\ \end{tabular} & \begin{tabular}{c} **FWHM (nm)** \\ \end{tabular} & \
\begin{tabular}{c} **Luminous Intensity (cd)** \\ \end{tabular} &
\begin{tabular}{c} **Luminous Flux (lm)** \\ \end{tabular} \\ \hline Red LED & (0.6894, 0.3099) & 630 & 20 & 1.8–2.0 & 67.9 \\ Green LED & (0.1689, 0.7232) & 520 & 20 & 5.0–6.0 & 178.1 \\ Blue LED & (0.1434, 0.0416) & 460 & 20 & 1.0–1.5 & 29.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Three LED parameters. Figure 6: The relative spectral power distribution curve of RGB LEDs. human eye. The average color temperature error is 0.609%, and the difference between the measured and calculated values is small. Therefore, the color temperature adjustment in the experiment is relatively stable. The theoretical color coordinates of the synthesized light source are obtained from Equations (3) to (4), and the actual color coordinates are measured using a spectral illuminometer. Figure 10 shows the variation curve of the measured and calculated color coordinates with color temperature. The error of the calculated excellent coordinate \(x\) is less than 0.0007, and the error of the color coordinate \(y\) is less than 0.0014. The measured color coordinate is in good agreement with the calculated color coordinate. The color coordinates have a significant impact on low color temperature light sources, while they have a relatively small impact on high color temperature light sources, which is consistent with the simulation results mentioned above. The duty cycle of RGB LEDs obtained from Equations (13) to (15) can obtain mixed white light with different color temperatures. As shown in Figure 11, the duty cycle of the red, green, and blue LEDs varies with color temperature. It can be seen in Figure 11 that as the color temperature increases, the duty cycle of the red LED decreases, while the duty cycle of the green LED and blue LED increases.