White LED

A white LED, as used in a TV backlight, is a small package a few millimeters across. Look closely and you'll see a white plastic cup filled with a yellow coating. Underneath sits a chip that glows bright blue.

It's assembled in a few steps. The lead frame is a stamped metal frame, usually with a molded white plastic cup that reflects light upward. The blue LED chip is glued to the bottom of the cup, and fine bond wires connect its contacts to the frame. Then the cup is filled with silicone mixed with YAG phosphor powder, and the silicone is cured into a clear, flexible solid. Silicone is used rather than epoxy because it doesn't yellow under intense blue light and heat.

When current flows, the chip gives off blue light. Some passes straight out; the rest is absorbed by the phosphor and re-emitted as a broad yellow glow. Our eyes see blue plus yellow as white. Many TVs use other phosphors, such as separate red and green ones, or quantum dots, for purer colors. Nichia sold the first white LEDs of this type in 1996.

In an LED strip, a row of these sits under lenses.