Lamps that produce light as a result of an electrical current through a metal wire, contained in a transparent small bulb that heats the metal to incandescence. The bulb also contains halogen that reacts with the vaporized metal so that bulb blackening by vaporized metal is minimized.
During the operation of an incandescent lamp, tungsten evaporates from the filament and settles on the coldest place inside the lamp (the bulb wall), causing lamp blackening, which leads to a considerable depreciation of the light output during lamp life. In order to keep lamp blackening, and the corresponding light loss, within acceptable limits, bulbs of normal incandescent lamps are relatively large. But in order to be able to operate small-bulb high-light-output incandescent lamps, special measures have to be taken to prevent bulb blackening, which in these small bulbs would quickly lead to unacceptable light losses. The solution to this problem is to introduce halogen into the bulbs. These lamps are called halogen incandescent or, in short, halogen lamps [2, 3]. Thanks to their compactness they are extremely suitable for use in small reflectors to create well-defined light beams. They are widely used for accent lighting and in car headlamps. However, because of the relatively low efficacy and short lifetime of halogen lamps, the more efficient and longer-life gas discharge and solid-state lamp alternatives have become increasingly more important in all these segments.
Halogen Cycle Principle
A tungsten particle that escapes from one spot of the filament does not return to exactly the same spot. As a result, the lamp will eventually burn out, because there will always be some part of the filament that will become weak over the course of time, but clearly later than in a normal incandescent lamp. This means that the filament can be heated up to a higher temperature (up to 3,000 K instead of 2,750 K as in the case of a normal incandescent lamp) while having a longer life. Halogen lamps offer two to five times the life of normal incandescent lamps, rising from 1,000 to 2,000–5,000 h. The higher filament temperature also increases both the luminous efficacy by 10–50 % relative to normal incandescent lamps and the color temperature up to 3,000 K.
Materials and Construction
The single- or double-coiled tungsten filament can be placed axially or transversely in the halogen capsule. The placement has consequences for both the efficacy and the light distribution of the lamp.
As in normal incandescent lamps, a gas filling of krypton or xenon is used to reduce filament evaporation.
Halogen lamps are available with a large variety of lamp caps and corresponding bases: two-pin caps and twist caps that ensure that the optical center of the lamp is always in the correct position, ceramic lamp caps for high-voltage, high-lumen-output lamps that become very hot, and “normal” Edison and bayonet caps for high-voltage halogen lamps that can be used just as normal incandescent lamps.
Thanks to the higher working temperature of the halogen incandescent lamp, it is more efficient than a normal incandescent lamp. Some 12 % of the input power of a halogen lamp is radiated as visible light. Compare this with the 8 % of a normal incandescent lamp. The remaining power is lost as heat (through conduction, convection, and infrared radiation).
At 15–25 lm/W, the luminous efficacy of a halogen lamp is a factor 2–2,5 higher than that of an incandescent lamp. With the infrared-reflecting coating technology (see next section “Product Range”), the efficacy increases to some 35 lm/W. As with normal incandescent lamps, the lower the wattage, the lower the efficacy. There are halogen lamps where priority has been given to a long life at the expense of a somewhat lower efficacy (e.g., 5,000 h and 15 lm/W), and conversely, there are those where the emphasis is on efficacy and not so much on lifetime (e.g., 2,000 h and 25 lm/W).
Although the efficacy of halogen lamps is clearly higher than that of normal incandescent lamps, it is relatively low compared with that of gas discharge and solid-state light lamps.
Common types of halogen lamps are available in the range from some 50 to 2,000 lm (corresponding wattage range approximately 5–100 W). Double-ended mains-voltage halogen lamps are available in versions up to some 25,000 lm (1,000 W version).
Normal halogen lamps have a lifetime that is at least twice as long as that of normal incandescent lamps, thus at least 2,000 h. As explained above, with some halogen lamps priority is given to efficacy at the expense of lifetime: viz., the 2,000 h versions. Versions where the priority is given to lifetime can have a life of up to 5,000 h.
Thanks to the halogen regenerative cycle, lamp blackening is minimal. Consequently, lumen depreciation with halogen lamps is very small.
With certain exceptions, halogen lamps have a universal burning position. The exceptions are the high-voltage, high-wattage (750 W or more) double-ended types. Here the coiled tungsten filament is so long that with a position that is not near horizontal, the coil would sag so much that the individual coil windings would touch each other, leading to a short circuit.
Run-Up and Reignition
Like normal incandescent lamps, halogen lamps give their full light output immediately after switch on and after reignition.
The switching behavior of halogen lamps is the same as that of normal incandescent lamps.
Halogen lamps can be dimmed in the same way as normal incandescent lamps. Just as with normal incandescent lamps, the phase-cutting system is the more efficient system and is the usually employed. Below a certain dimming point the lamps cool down so much that the halogen cycle will no longer function. From this point on the filament starts evaporating, just as with a normal, dimmed incandescent lamp. The smaller bulb size of the halogen lamp leads to more blackening in this situation than is the case with normal incandescent lamps. However, since the filament in the dimmed situation is cooler than in the undimmed situation, dimming does not have a real negative effect on the lamp life.
The behavior of halogen lamps as a result of an overvoltage is the same as with normal incandescent lamps. Only a few percent of overvoltage results in a drastically reduced lamp life. For example, a permanent overvoltage of 5 % reduces the lamp life by 50 %.
Normal incandescent lamps radiate little UV. This is because the spectrum quickly falls off at the short-wavelength part of the spectrum. What little UV that remains is reduced to near zero because normal glass (the bulb material of incandescent lamps) is a very good absorber of UV radiation. Halogen lamps radiate more UV because of the higher operating temperature of the filament and because of the fact that normal quartz bulbs, unlike glass bulbs, do not absorb UV radiation. In order to limit harmful or damaging UV radiation from halogen lamps, the quartz used today for most halogen lamps is doped with UV-absorbing material (UV-blocking quartz).
Grouping of Halogen Lamps
Lamps operating on low voltage (6 V, 12 V, and 24 V, where the 12 V versions are most common)
Lamps operating on mains voltage
Cool-Beam Halogen Lamps
Halogen Lamps with Infrared-Reflective Coating
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