Metal halide aquarium lights are a type of high intensity discharge (HID) lamps specifically designed for use with aquariums.
HID bulbs contain tungsten electrodes inside a quartz or alumina enclosure. An inert gas facilitates the current “strike” across the electrodes. The arc of electricity quickly heats metal ions contained within the bulb forming plasma that intensifies the light produced by the arc and lowers the energy threshold required to maintain the arc by lowering the effective resistance. As a result they take several minutes to come up to full brightness and must be allowed to cool at least several minutes after going off before being lit again.
Metal halide aquarium lights are frequently used to promote the growth of aquatic plants, invertebrates, and especially coral by providing plenty of the invisible ultraviolet light they need. The characteristic bright shafts of light also make stunning and distinctive shimmers in the aquarium. They have a higher output per watt than incandescent or fluorescent lights and are a popular type of high end aquarium lighting. (continued below...)
Metal halide lights, like fluorescent lights, require a special piece of equipment to drive them called a ballast. The ballast mediates the electrical current between the outlet and the lights. It delivers the high voltage required the start the lights, then regulates the power for optimum operation. There are several types of ballast to choose from, including high end digital models.
Most metal halide aquarium lights come with a ballast situated within the unit instead of as a separate unit. These are easier to set up and use, but tend to run hotter because the ballast will produce some heat in addition to the bulbs. Good fixtures include a heat shield and plenty of clearance to protect the aquarium from heat. The bulbs used need to be compatible in size, type, and wattage with the ballast and fixture specifications. Metal halide aquarium lights are named for the metal halides contained within that are the sources the plasma forming metal ions. A metal halide is an ionic salt of a metal (the cation) and a halide like fluorine, chlorine, bromine or iodine (the anion).
The bulb enclosure of most of these lamps contains a pressurized mix of argon, mercury and several of these metal halides. Since no two metals have the same atomic electron configuration, the spectrum of electromagnetic radiation produced in the ultra high heat and pressure environment of the bulb will be unique. By mixing different metals and their proportions in the bulb, manufacturers produce bulbs that provide a broad spectrum of light, combining the various wavelengths of the sources to make light which appears white, but actually contains all colors, just as one is accustomed to receiving from the sun. This is why a prism produces a rainbow of color – the white light is divided into its component colors by the prism.
Because of this broad spectrum these lamps often have a high color-rendering index (CRI), which is a measure of how well colors are perceived under the light as compared to natural light. The overall tendency of the light to emphasize red-orange colors or blue-green colors is called correlated color temperature. High values like 5000 K look cool or bluish and low values like 2700 K look warm or yellowish. Daylight is about 5000-6000 K but these bulbs will look too cool next to typical indoor lighting. New bulbs usually take about 100 hours of use to stabilize in color and output.
