What are Halogen Light Bulbs?
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. Problems with uneven filament evaporation and uneven deposition of tungsten onto the filament by the halogen cycle do occur, which limits the ability of the halogen cycle to prolong the life of the bulb. However, the halogen cycle keeps the inner surface of the bulb clean. This 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 common GU10 halogen 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.
Lifetime and Efficiency of Halogen Lamps
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.
Halogen Bulb Failure Modes
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.
Thin spots can develop in the filaments of halogen bulbs, since the filaments can evaporate unevenly and the halogen cycle does not redeposit evaporated tungsten in a perfect, even manner nor always in the parts of the filament that have evaporated the most.
However, there are additional failure modes.
One failure mode is filament notching or necking. Since the ends of the filament are somewhat cool where the filament is attached to the lead wires, the halogen attacks the filament at these points. The thin spots get hotter, which stops the erosion at these points. However, parts of the filament even closer to the endpoints remain cool and suffer continued erosion. This is not so bad during continuous operation, since the thin spots do not overheat. If this process continues long enough, the thin spots can become weak enough to break from the weight of the filament.
One major problem with the “necked” ends of the filament is the fact that they heat up more rapidly than the rest of the filament when the bulb is turned on. The “necks” can overheat and melt or break during the current surge that occurs when the bulb is turned on. Using a “soft-start” device prevents overheating of the “necks”, improving the bulb’s ability to survive “necking”. Soft-start devices will not greatly extend the life of any halogen bulbs that fail due to more normal filament “thin spots” that run excessively hot.
Some halogen bulbs may usually burn out due to filament end necking, and some others may usually burn out from thin, hot spots forming in the filament due to uneven filament evaporation/recovery. Therefore, some models may have a significantly extended life from “soft-starting” and some other models may not.
It is generally not a good idea to touch halogen bulbs, especially the more compact, hotter-running quartz ones. Organic matter and salts are not good for hot quartz. Organic matter such as grease can carbonize, leaving a dark spot that absorbs radiation from the filament and becomes excessively hot. Salts and alkaline materials (such as ash) can sometimes “leach” into hot quartz, which typically weakens the quartz, since alkali and alkaline earth metal ions are slightly mobile in hot glasses and hot quartz. Contaminants may also cause hot quartz to crystalize, weakening it. Any of these mechanisms can cause the bulb to crack or even violently shatter. If a quartz halogen bulb is touched, it should be cleaned with alcohol to remove any traces of grease. Traces of salt will also be removed if the alcohol has some water in it.
Since the hotter-running quartz halogen bulbs could possibly violently shatter, they should only be operated in suitable fully enclosed fixtures.
Use of Halogen Bulbs with Dimmers
Dimming a halogen bulb, like dimming any other incandescent lamp, greatly slows down the formation of thin spots in the filament due to uneven filament evaporation. However, “necking” or “notching” of the ends of the filament remains a problem. If you dim halogen lamps, you may need “soft-start” devices in order to achieve a major increase in bulb life.
Another problem with dimming of halogen lamps is the fact that the halogen cycle works best with the bulb and filament at or near specific optimum temperatures. If the bulb is dimmed, the halogen may fail to “clean” the inner surface of the bulb. Or, tungsten halide that results may fail to return tungsten to the filament. Halogen bulbs have sometimes been known to do strange and scary things when greatly dimmed.
Halogen bulbs should work normally at voltages as low as 90 percent of what they were designed for. If the bulb is in an enclosure that conserves heat and a “soft-start” device is used, it will probably work well at even lower voltages, such as 80 percent or possibly 70 percent of its rated voltage. However, do not expect a major life extension unless soft-starting is used. Even with soft-starting, do not expect to more than double or possibly triple the life of any halogen bulb already rated to last 2,000 hours or more. Even with soft starting, the life of these bulbs will probably not continue to improve much as voltage is reduced to less than about 90 percent of the bulb’s voltage rating.
Dimmers can be used as soft-start devices to extend the life of any particular halogen bulbs that usually fail from “necking” of the ends of the filament. The bulb can be warmed up over a period of a couple of seconds to avoid overheating of the “necked” parts of the filament due to the current surge that occurs if full voltage is applied to a cold filament. Once the bulb survives starting, it is operated at full power or whatever power level optimizes the halogen cycle (usually near full power)
The dimmer may be both “soft-starting” the bulb and operating it at slightly reduced power, a combination that often improves the life of halogen bulbs. Many dimmers cause some reduction in power to the bulb even when they are set to maximum.
(A suggestion from someone who starts expensive medical lamps by turning up a dimmer and reports major success in extending the life of expensive special bulbs from doing this.)
Ultaviolet from Halogen Lamps
There is some common concern about the ultraviolet output of halogen bulbs, since they operate at high filament temperatures and the bulbs are made of quartz instead of glass. However, the filament temperature of halogen bulbs rated to last 2,000 hours or more is only slightly greater than that of standard incandescent lamps, and the UV output is only slightly higher. Halogen fixtures typically have a glass or plastic shield to confine any possible bulb explosions, and these shields absorb the small traces of shortwave and mediumwave UV that gets through the quartz bulb.
Higher temperature photographic and projection bulbs are different. The much higher filament temperature of shorter life bulbs results in possibly significant hazardous UV. For maximum safety, use these bulbs in fixtures or equipment designed to take these bulbs, and in a manner consistent with the fixture or equipment instructions.
For those who want to take special precautions against UV, a UV blocking clear filter gel such as the GamColor no. 1510 may be a practical solution. This filter gel withstands use moderately close to halogen lamps and withstands heat to maybe 100 to 150 Celsius or so. This filter gel can be placed immediately outside the glass shield of most fixtures, although the tubular shield in many popular 300 watt torchiere lamps gets too hot for the filter gel.
The GamColor 1510 is available at some theatrical supply shops.