Gas-discharge lamps are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas, a plasma. Typically, such lamps use a noble gas; (argon, neon, krypton, and xenon) or a mixture of these gases. Some include additional substances, like mercury, sodium, and metal halides, which are vaporized during startup to become part of the gas mixture. In operation the gas is ionized, and free electrons, accelerated by the electric field in the tube, collide with gas atoms. Some electrons in the atomic orbitals of these atoms are excited by these collisions to a higher energy state. When the excited electron falls back to a lower energy state, it emits a photon of light of a characteristic frequency. The color of the light produced depends on the emission spectra of the atoms making up the gas, as well as the pressure of the gas and other variables. Gas discharge lamps can produce a wide range of colors. Some lamps produce ultraviolet radiation which is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface. The fluorescent lamp is perhaps the best known gas-discharge lamp.