*4.2.1 Saturation and heat*

The phosphor film heats up when an intense light from an LD penetrates through it. The phosphor temperature and output power versus input current are shown in **Figure 4(a)**. It can be seen that after the input power exceeds the critical value, the temperature of phosphor suddenly increases while the output power decreases. The part of optical energy is converted into heat due to quantum efficiency loss and stock shift loss. This heat can cause the limitation of the lifetime of phosphor and thermal quenching. The quantum efficiency versus phosphor temperature of commercially available phosphor YAG: Ce is shown in **Figure 4(b)**. As the temperature increases, the QE remains stable, and above a certain temperature, the QE starts decreasing.
