the Seidel test, the high concentration of fluorescein prevents fluorescence from occurring; only when the fluorescein is sufficiently dilute (eg, because of aqueous leaking from a lesion) is fluorescence seen.

Common fluorescent lights are gas discharge tubes containing mercury vapor. The discrete mercury spectrum is converted to a continuous spectrum by coating the inside of the glass tube with fluorescent materials, from which this light source gets its name.

Phosphorescence

In most cases, electrons remain in elevated energy states for very short periods. However, the elevated state is sometimes “metastable”; that is, the electron may remain in the elevated state for several seconds, minutes, or perhaps even days before dropping back down. Such a process describes how light is produced by phosphorescence, which is essentially identical to fluorescence except that the transitions take longer. Light resulting from fluorescence stops immediately after removal of the exciting energy, whereas light resulting from phosphorescence persists long after the exciting energy ceases.

Lasers

An electron may stay in a metastable state for minutes or longer. However, a photon of appropriate frequency passing near such an electron will stimulate the electron immediately to drop to a lower state and radiate an identical photon (see Fig 8-15C). Such stimulated emission is the basis of the light emission in lasers. In fact, the word laser was initially an acronym for Light Amplification by Stimulated Emission of Radiation (LASER).

Lasers use an active medium with an appropriate metastable state. Energy is introduced into the active medium in a variety of ways. For example, optical pumping uses a bright incoherent light source to excite a large number of electrons into the metastable state. The active medium is inside a resonator cavity, which typically has a fully reflecting mirror on one end and a partially reflecting one on the other; this design causes light to make numerous passes through the active medium, producing more photons by stimulated emission with each pass (Fig 8-18).