YAG phosphor

YAG phosphor is a pale yellow powder that glows yellow when blue light hits it. YAG stands for yttrium aluminum garnet, Y₃Al₅O₁₂, a crystal related to garnet gemstones. A small share of its yttrium atoms, often a few percent, are replaced by cerium, and those cerium atoms do the glowing.

It's made by mixing powders of yttrium oxide, aluminum oxide and a little cerium oxide made from cerium, plus a flux that helps crystals grow. The mix is fired at around 1,500 °C in a gas that keeps the cerium in the right chemical state. The oxides react into garnet crystals with cerium built in. The fired cake is crushed, washed and sorted to particles a few micrometers across.

Each cerium atom absorbs blue light and almost instantly re-emits it as a broad yellow glow, losing a little energy as heat. In a white LED, the phosphor is mixed into the silicone over the blue chip, and the amount sets how warm or cool the white looks. YAG's light is weak in deep red, so TVs aiming for richer color often use other phosphors or quantum dots. With neodymium instead of cerium, the same crystal makes Nd:YAG lasers.