In the NIST technique, an HID lamp sits in an intense beam of X-rays. The X-rays penetrate the lamp's ceramic housing but are partially absorbed by mercury gas in the lamp, casting a shadow in the beam. A special digital camera behind the lamp captures a high-resolution, 2D image of this X-ray shadow showing the density of mercury atoms in the discharge. From the mercury distribution, researchers can determine the temperature distribution in the lamp. The technique has been used to quantify processes that consume power without producing light. Research indicates this method could be practical in industrial laboratories using small-scale X-ray sources.
HID lamps produce 26% of the nation's light output but consume only 17% of the electricity used for lighting. Better efficiency could save lots of money: HID lamps consume roughly 4% of U.S. electricity, equal to about $10 billion annually.
The highly efficient lamps have two electrodes in a ceramic tube that contains small amounts of mercury and metal-halide salts. An electric current between the electrodes heats the lamp, vaporizing the mercury and metal-halide salts, and producing a gas of electrically charged particles, or plasma. Metal atoms, excited by collisions with electrons in the plasma, emit light at many different wavelengths, producing a bright white light.