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- How does a halogen bulb work?
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How does a halogen bulb work?
Lifetime and Efficiency of Halogen Bulbs
A halogen bulb is often 10 to 20 percent more efficient than an ordinary incandescent bulb of similar voltage, wattage, and life expectancy. Halogen bulbs may also have two to three times as long a lifetime as ordinary bulbs, sometimes also with an improvement in efficiency of up to 10 percent. How much the lifetime and efficiency are improved depends largely on whether a premium fill gas (usually krypton, sometimes xenon) or argon is used.
How a Halogen Bulb Works
A halogen bulb is an ordinary incandescent bulb, with a few modifications. The fill gas includes traces of a halogen, often but not necessarily iodine. The purpose of this halogen is to return evaporated tungsten to the filament.
As tungsten evaporates from the filament, it usually condenses on the inner surface of the bulb. The halogen is chemically reactive, and combines with this tungsten deposit on the glass to produce tungsten halides, which evaporate fairly easily. When the tungsten halide reaches the filament, the intense heat of the filament causes the halide to break down, releasing tungsten back to the filament.
This process, known as the halogen cycle, extends the life of the filament somewhat. The halogen cycle keeps the inner surface of the bulb clean, which lets halogen bulbs stay close to full brightness as they age.
In order for the halogen cycle to work, the bulb surface must be very hot, generally over 250 degrees Celsius (482 degrees Fahrenheit). The halogen may not adequately vaporize or fail to adequately react with condensed tungsten if the bulb is too cool. This means that the bulb must be small and made of either quartz or a high-strength, heat-resistant grade of glass known as "hard glass."
Since the bulb is small and usually fairly strong, the bulb can be filled with gas to a higher pressure than usual. This slows down the evaporation of the filament. In addition, the small size of the bulb sometimes makes it economical to use premium fill gases such as krypton or xenon instead of the cheaper argon. The higher pressure and better fill gases can extend the life of the bulb and/or permit a higher filament temperature that results in higher efficiency.
Any use of premium fill gases also results in less heat being conducted from the filament by the fill gas, meaning more energy leaves the filament by radiation, meaning a slight improvement in efficiency.
Halogen Bulb Failure
Halogen bulbs usually fail the same way that ordinary incandescent bulbs do, usually from melting or breakage of a thin spot in an aging filament.
KEEP IT CLEAN
DIMMING A HALOGEN LAMP