|MLA Citation:||Bloomfield, Louis A. "Question 1226"|
How Everything Works 25 May 2018. 25 May 2018 <http://howeverythingworks.org/print1.php?QNum=1226>.
What makes a metal-halide lamp so efficient is that there are relatively few ways for the lamp to waste energy as heat. While collisionally excited mercury atoms normally emit most of their stored energy as ultraviolet light—the basis for fluorescent lamps—they can't do this in a high-pressure environment. A phenomenon called "radiation trapping" makes it almost impossible for this ultraviolet light to escape from a dense vapor of mercury, so a high-pressure mercury lamp emits mostly visible light. Even without the metal-halides, a high-pressure mercury lamp emits a brilliant blue-white glow. The metal-halides boost the reds and other colors in the lamp to make its light "warmer" and more like sunlight.
Next time you watch one of these lamps warm up, observe how its colors change. When it first starts up, its pressure is low and it emits mostly invisible ultraviolet light (which is absorbed by the lamp's glass envelope). But as the lamp heats up and its pressure increases, the rich, white light gradually develops. Incidentally, if the power to a hot lamp is interrupted, the lamp has to cool down before it can restart because it only starts well at low pressures.