a phosphor
/ FOSS-for /
A phosphor is a solid that glows: feed it energy — ultraviolet light, a beam of electrons, or blue light — and it gives that energy back out as visible light of its own colour. You have met phosphors without knowing it: the white powder coating the inside of a fluorescent tube, the yellow layer that makes a white LED white, the glowing dots of an old television screen, the glow-in-the-dark stars on a bedroom ceiling. Almost every phosphor is a ceramic — a transparent host crystal with a tiny sprinkle of a special impurity that does the actual glowing.
The mechanism is a two-part team. The bulk material is the host lattice (for example Y2O3, YAG, or a sulphide); dissolved into it in small amounts is an activator ion (such as europium Eu3+ for red, cerium Ce3+ or europium Eu2+ for broad yellow-white, terbium Tb3+ for green, or manganese). When a photon or electron arrives, it lifts an electron in the activator to a higher energy level; that electron then relaxes and drops back down, releasing a new photon. Because some energy is always lost to heat and vibration on the way, the emitted photon has lower energy — a longer wavelength — than the one absorbed, a shift called the Stokes shift. The colour of the glow is set by the activator's energy levels and how the surrounding crystal squeezes them.
Phosphors are the quiet workhorses of lighting and displays. In a fluorescent lamp they convert the tube's ultraviolet into white light; in a white LED a YAG:Ce phosphor turns part of a blue chip's light into yellow, and blue-plus-yellow reads as white; in scintillators they turn X-rays and gamma rays into visible flashes a detector can count. The honest limits: a phosphor's efficiency falls as it gets hot (thermal quenching) and as it is driven harder (saturation), which is a real design headache for high-power LEDs. And a naming trap worth clearing up — 'phosphor' has nothing to do with the chemical element phosphorus; the word is older and simply means 'light-bearer'.
A white LED is really a blue chip with a dab of yellow YAG:Ce phosphor on top. The phosphor absorbs some of the blue light and re-emits it as broad yellow; the blue that slips through mixes with the yellow, and your eye reads the blend as white light.
A phosphor is a colour converter: it swallows high-energy light and hands back light of a lower energy, softer colour.
Two things to keep straight: a phosphor is not made of the element phosphorus, and a phosphor cannot up-convert for free — by the Stokes shift the emitted light is almost always lower in energy (longer wavelength) than the light that excited it, so you cannot turn red into blue this way.