**5. Color conversion phosphors**

Commercial, industrial, and residential lighting are promptly shifting towards a phosphor-converted white light system. The most preferred way to generate white light from LEDs/LDs is to use phosphors that have the ability to absorb the high energy (short wavelength) photons and down-convert them into lower energy (longer wavelength) ones. The chemical composition of phosphor plays an essential role in display, SSL, and VLC. In the 19th century, phosphor's name arose as a generic term for a material that glows in the dark. Phosphor is a kind of photoluminescent material that emits visible light when exposed to certain radiation. The luminescence process of the phosphor is shown in **Figure 5(a)**.

*Blue Laser Diode-Based Visible Light Communication and Solid-State Lighting DOI: http://dx.doi.org/10.5772/intechopen.100106*

**Figure 5.** *(a) The luminescence process of phosphor. (b) Jablonski diagram of downconversion luminescence process.*

The luminescence process occurs when the activator absorbs the radiant energy and goes to a higher energy state. As the excited state is unstable, the excited photon emits absorbed energy in the relaxation state and falls back to a lower energy state. The Jablonski diagram of the downconversion phosphor luminescence process is shown in **Figure 5(b)**.

The properties associated with ideal phosphor are color quality, CRI, thermal stability, emission spectrum, photoluminescent lifetime, etc. Recent studies have highlighted that phosphor-based white light quality depends on photoluminescent material, size, composition, and arrangement with the package. The absorption rate depends on the crystal structure, particle size, and particle size distribution of phosphor. The difference between the spectral position of the peak of absorption and emission spectra is called stock shift, expressed in wavelength. The extent of the stock shift depends on the characteristic of phosphor. During the illumination process, the phosphor cannot absorb all excitation energy; some of the energy is reflected or transmitted. The most absorbed energy will be released in the form of light, and the rest of the energy may be converted into heat; therefore, in this process, emitted energy is always less than the absorbed energy, and thus the wavelength of excitation light is always shorter than that of emitted light. The phosphor's efficiency depends on how much relaxation (loss of energy) occurs during activation and emission.

Phosphors are increasingly utilized in lighting devices, such as compact fluorescent lamps, LEDs, and LDs. Nowadays, phosphors are using everywhere and extensive research have been carried out to find proper color conversion phosphors. The color conversion phosphor that can be excited by the blue or ultraviolet laser diode is shown in the **Table 2**.
