Ask a Physicist Answers

We're confused about the difference between fluorescent and halogen bulbs. Fluorescent bulbs are classified as hazardous waste because they contain mercury. Do halogen bulbs also contain mercury? - LR and APM, Washington, DC

BulbsThese two types of bulbs are quite different from one another and only fluorescent bulbs contain any mercury. A halogen bulb is a modern variation on the old-fashioned incandescent bulb and uses thermal radiation from a white-hot tungsten filament to produce its light. In contrast, a fluorescent bulb produces light without heat, at least in principle, and begins that process with an electric discharge in mercury vapor.

The path to white light in a fluorescent bulb is somewhat complicated. The bulb contains a thin vapor of mercury and other gases and electrons are injected into that vapor by heated metal electrodes at the bulb's two ends. An electric current is then driven through the mercury vapor, causing the mercury atoms to emit light. But the light they emit is primarily invisible ultraviolet light, a seemingly foolish waste of energy. However, this ultraviolet light never leaves the bulb and instead allows the bulb to emit a carefully tailored spectrum of visible light.

The ultraviolet light is absorbed by a thin coating of white phosphor powder on the inside surface of the glass envelope. That powder uses energy from the ultraviolet light to emit a mixture of visible wavelengths through the phenomenon known as fluorescence. While some of the ultraviolet light energy is wasted as thermal energy, most of it becomes visible light. The color of that light is determined primarily by the mixture of chemicals in the phosphor powder. There are a variety of common phosphor mixtures that give us cool white, warm white, deluxe cool white, and deluxe warm white, among others. The warmer mixtures imitate incandescent lighting while the cooler mixtures are more like daylight. Most bulbs are labeled according to their phosphor mixture. And a trip to Los Vegas will point out that not every fluorescent lamp produces white light; advertising signs based on colored fluorescent lamps are everywhere.

A halogen bulb is quite similar to an ordinary incandescent: both use electric currents to heat their tungsten filaments to white heat. But the lifetime of any filament is limited by its tendency to sublime-its tungsten atoms evaporate directly from solid to gas and then deposit themselves as a black smudge on the top inner surface of the bulb. However, in a halogen bulb, a tungsten recycling process constantly rebuilds the filament. That recycling, which depends on halogen compounds in the small, hot glass tube, prolongs the filament life so dramatically that the filament can operate several hundred degrees hotter than that of an ordinary bulb and still have an excellent operating life. A halogen bulb's hotter filament gives it a whiter, more energy-efficient light.

Although a halogen bulb does contain toxic bromine and iodine, those chemicals are common in seawater and pose little health threat in such small quantities. However, the mercury in fluorescent lamps is less innocuous and it can and should be distilled out of discarded fluorescent lamps for recycling and reuse.

Answered by Lou A. Bloomfield of the University of Virginia