*4.1.3 Narrow spectral width*

The emission spectrum of the LED and LD is shown in **Figure** 3. The full width half maximum (FWHM) of the LED is 20 nm, while that of LD is 2.5 nm. In the illumination process, the spectral width reflects the color purity of the generated light. The requirements on spectral width are slightly different in different areas. In the LED-backlit display, the spectral width is the smaller the better, which leads to a purer emitted light color and is useful for color matching.

**Figure 3.** *The emission spectrum of LED and LD [19].*

**Figure 4.**

*(a) The phosphor temperature and output power versus input currents. (b) Quantum efficiency versus phosphor temperatures of commercially available phosphor YAG: Ce [19].*
