## 4 Results and Discussion
The maximum temperature difference is 4.89 \({}^{\circ}\)C, which is 4.6% lower than the filament color temperature of 2200 K. The junction temperature of the 255-mm filament with a color temperature of 2700 K is 104.63 \({}^{\circ}\)C, and the maximum temperature difference is 14.3 \({}^{\circ}\)C, which is 13.7 % lower than that of the filament with a color temperature of 2200 K.  
Thermal resistance is one of the parameters that can best measure the heat dissipation of LED devices. It represents the ratio between the temperature difference between the two ends of the object and the stable power of the heat source when the heat flow is transferred along the heat sink. The calculation method is shown in Formula (2):  
\[R=\frac{T_{j}-T_{c}}{P} \tag{2}\]  
where \(T_{j}\) represents the junction temperature, \(T_{c}\) represents the temperature of the thermocouple glue point, and the unit of thermal resistance is \({}^{\circ}\)C/W. When the filament is made, the surface of the filament is fully covered by fluorescent glue and the contact surface with the chip is well sealed, so there is no contact thermal resistance due to the thermal convection of the interface itself. Formula (2) can be directly used to calculate the thermal resistance of the LED. The calculation result of thermal resistance is shown in Figure 5.  
Under the same lining condition, the thermal resistance change law is similar to the junction temperature, and it decreases for the filaments built for higher CCT. It shows that the heat dissipation capacity of LED filaments is different under different CCT. The heat dissipation capacity of LED filaments with low CCT is poor, and the junction temperature and thermal resistance are higher than those with high CCT. The color temperature of the LED filament is directly related to the phosphor content of each color added when preparing the filament. In order to better analyze the relationship between the content of each color powder of the phosphor and the thermal performance, the color content of the experimental sample filaments measured by the EVERFINE integrating sphere is shown in Table 2, and the color coordinates of the filaments with different color temperatures are shown in Figure 5. When the input light interacts with the phosphor, there is a complicated process of electricity, heat, light energy conversion and light tracking. According to the law of the conservation of energy, when short-wavelength light excites the phosphor, the black body is heated, part of the energy is converted into long-wavelength light and the other part is dissipated in the form of heat. It can be seen from Table 2 that the higher the color temperature of the filament, the lower the red powder content and the higher the green powder content. The red phosphor CASN: Eu\({}^{2+}\) has a strong absorption and blocking effect, which reduces the absorption of the green phosphor LuAG: Ce\({}^{3+}\) green luminescence, so the higher the proportion of the red phosphor, the greater the heat loss. Figure 6 shows the offset of the color coordinates for different color temperatures. The lower the color temperature, the stronger the absorption of the red phosphor CASN: Eu\({}^{2+}\), and the overall color coordinate shifts to the red spectrum in the lower right corner. When energy conversion is performed, the heat absorption is significantly increased, which leads to an increase in junction temperature and thermal resistance and an increase in heat loss. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Color Ratio(\%)**} & \multirow{2}{*}{**CCT**} & \multicolumn{3}{c}{**175 mm**} & \multicolumn{3}{c}{**255 mm**} \\ \cline{2-7}  & & **2200 K** & **2400 K** & **2700 K** & **2200 K** & **2400 K** & **2700 K** \\ \hline Red ratio (\%) & 34.7 & 29.8 & 24.6 & 30.1 & 27.6 & 24.7 \\ Blue ratio (\%) & 1.7 & 2.8 & 2.0 & 2.1 & 2.0 & 2.9 \\ Green ratio (\%) & 63.6 & 67.4 & 73.4 & 67.8 & 70.4 & 72.4 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Proportion of phosphor color.