Metal halide lamp | Metal halide lamp facts

       Metal halide lamps, is a type of lamp belonging to the high-intensity discharge (HID) family of lamps. They deliver high light output for their compact size. They are often preferred in lighting systems for their characteristics such as compact, efficient and powerful light source. The efficiency and improved efficacy and light color of metal halide lamps are owed to the addition of rare earth metal salts to the mercury vapor lamp. This type of lamp was originally created for industrial lighting applications sometime in the late 1960s. Now, metal halide lamps are available in various sizes and configurations for residential, commercial, and industrial applications.

Like any other lamps, metal halide lamps perform under high pressure and temperature, and needs special fixtures to function safely. Since metal halide lamps are small compared to a fluorescent or incandescent lamp with the same light level, small reflective luminaires must be used to direct the light such as in applications outdoor lights, flood lights, or office and industrial buildings and warehouses.

The applications of metal halide lamps are for general lighting purposes and specific applications requiring ultraviolet (UV) or blue-frequency light. Since these type of lamps are able to operate in a wide spectrum, they are rapidly gaining preference for indoor applications, sports arena or facilities, and reef aquarists as light source for corals. Metal halide lamps are also widely used in professional lighting fixtures where they are also popularly known as MSD lamps. They come in various power ratings such as 150-, 250-, 400-, 575- and 1200-watt ratings. They are also referred to as intelligent lighting. Practical applications are LCD, DLP, and film projectors wherein metal halide lamps are indispensable light source.

Metal halide lamp operation

       How do metal halide lamps operate? Metal halide lamps generate light by passing an electric arc through certain mixture of gases. They are very much similar to other gas-discharge lamps like mercury-vapor lamps. Inside a metal halide lamp, the compact arc tube has a mixture of high-pressure argon, mercury and metal halides variety. The variety of the mixture of halides affect the characteristic of light produced, the correlated color temperature and intensity, that is, the blue light may appear darker shade of blue, or a red light will appear deeper tone of red. The argon gas component inside the lamp is responsible for the fast striking of the arc across the two electrodes when there is a source voltage passing through the lamp. This is due to the fast ionization characteristic of argon. The increasing heat and pressure produced by the arc vaporizes the mercury and other metal halides, thereby producing the light.

Metal halide lamp components

• Arc tube - The metal halide lamp consists of iodides or, in some instances, bromides of various metals like scandium, sodium, indium and sodium in European Tri-Salt types, thallium, and recently dysprosium for high color temperature. For low color temperature, tin is used. For high power rated movie lighting fixtures, holmium and thulium is used. While gallium and or lead for special high ultraviolet or printing applications. The color of the light produced by metal halide lamps depends on the mixture of the metals inside every lamp. For festive or theatrical effects and green light, pure iodides of thallium are used. To achieve a blue light, the major component is indium. To reduce the arc impedance, alkali metals usually sodium or potassium is used. This permits the arc tube to become relatively long like in metal halide ballasts.

• Outer bulb - An outer glass bulb fitting protects the components inside the metal halide lamp including its support frame and arc tube from oxidation or rusting, heat loss. Also, it shields to prevent the short wavelength ultraviolet light produced by the vapor discharge of mercury from escaping. The vapor discharge is transmitted by the fused silica inner bulb. Older, single ended or single base models use soda or borosilicate glass bulb that blocks the arc tube from escaping. Other contemporary single ended and double ended models use specially doped ultraviolet stop fused silica outer bulbs. There are two tungsten electrodes doped with thorium inside fused quartz arc tube, which are sealed at each end. The current is then flowed to them by molybdenum foil seals in the fused silica. The light is actually produced inside the arc tube.

• Ballasts - To produce the proper voltage to the arc, metal halide lamps need electric ballasts in order to control the arc current. Some metal halide lamps have a third electrode to start the arc when the lamp is first switched on just like any other mercury vapor lamps. This is why upon switching on, the light flickers. Pulse-start metal halide lamps have no starting electrode, instead, they have an ignitor to create a high-voltage pulse to make the arc. On cold strike, the needed voltage is from 1-5 kilovolts, while hot restrike requires 30 kV to start an arc. The American National Standards Institute (ANSI) have established standards and parameters for lamp-ballast system for metal halide components. Newer electronic ballasts have ignitor and ballast included into one package. The ballasts need high-frequency in order to power the lamps. These ballasts are more efficient as compared to iron ballast or line frequency ballast because of lesser loss. The lamp efficacy is not affected by high-frequency operation like in fluorescent lamps.

Risk of lamp explosion

       Just like any other device, HID arc tubes diminish in strength over time depending on the factors like thermal stress, mechanical vibration, and chemical attack. This is noticeable by the discoloration of the arc tube, light consumption, and thermal increase. The tube becomes weaker through time until it finally breaks. Sometimes, new bulbs or tubes break down even though they are new. This is attributed to the factory faults like microscopic cracks but the possibility is very minimal. Manufacturers usually proceed the lamps for seasoning and quality control before they are released in the market.

Eye Hazard

       While it is undisputed that the metal halide lamps are best source of lighting for reef aquarium, there are also drawbacks with respect to the eye hazard of the device especially to the hobbyist due to constant exposure. One of the ill-effects to health is blurred vision which can be traced to photokeratitis or commonly known as snow blindness. Snow blindness is an illness attributed to unprotected exposure to ultraviolet radiation. That is why in school gymnasiums, where HIDs are often used, people who are constantly exposed to the bulb may experience eye and skin injuries.