Phosphor Coating

Phosphor coating is a critical optical component in white LED bulbs, consisting of phosphor particles suspended in a silicone or epoxy matrix. This coating is precisely deposited on or around the LED semiconductor chip to absorb a portion of the blue light emitted by the chip and re-emit it at longer wavelengths (typically yellow, green, and red), combining with the remaining blue light to produce white light. The thickness, concentration, and distribution of the phosphor layer directly control the color temperature (CCT), color rendering index (CRI), and luminous efficacy of the LED.

The working principle is based on photoluminescence, specifically Stokes shift. When high-energy blue photons (typically from a GaN-based LED chip) strike the phosphor particles, electrons in the phosphor are excited to higher energy states. As they return to their ground state, they emit lower-energy photons in the yellow to red spectrum. This converted light mixes with the unconverted blue light to create white light. The specific phosphor composition determines the spectral output and color quality.