Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Halogen Lamp

  • Wout van Bommel
Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_126

Synonyms

Definition

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.

Halogen Lamps

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

One way to eliminate bulb blackening is to add a small quantity of halogen (bromide or iodine) to the fill gas. Under the influence of the hot filament, evaporated tungsten particles chemically combine with the halogen particles. If the temperature is sufficiently high (bulb temperature at least 250 °C), the resulting mixture will remain floating in a gaseous state, so preventing it from condensing on the coldest part of the lamp bulb to create bulb blackening. The extremely high temperature in the vicinity of the filament causes a chemical reaction that splits the mixture into its original components, tungsten and halogen particles, and the former return to the filament. This phenomenon is called the halogen cycle. It is illustrated in Fig. 1.
Halogen Lamp, Fig. 1

The halogen cycle [1]

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

Bulb

Since a halogen bulb is so small, it becomes so hot that normal glass, as used with incandescent lamps, cannot be used because it would melt. Halogen bulbs are therefore made out of quartz, which can withstand high temperatures and mechanical stress. In the manufacturing process it can be handled similarly to glass (Fig. 2).
Halogen Lamp, Fig. 2

Halogen lamp with its components. The bulb can be as small as approximately 10 × 15 mm

Filament

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.

Fill Gas

As in normal incandescent lamps, a gas filling of krypton or xenon is used to reduce filament evaporation.

Lamp Cap

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.

Properties

Energy Balance

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).

Luminous Efficacy

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.

Lumen-Package Range

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).

Color Characteristics

Halogen lamps have a continuous spectrum radiating more energy at long wavelengths than at short wavelengths (Fig. 3). Because of the somewhat higher temperature of the filament in halogen lamps compared to that of the filament in normal incandescent lamps, the spectral energy distribution shifts slightly toward shorter wavelengths. Depending on the version (lower efficacy/longer life or higher efficacy/shorter life), halogen lamps have a color temperature of between 2,800 and 3,000 K, respectively. Their color temperature is always slightly higher than that of normal incandescent lamps, thus giving them a somewhat cooler white light. The color rendering index is 100.
Halogen Lamp, Fig. 3

Relative spectral energy distribution of a halogen lamp [1]

Lamp Life

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.

Lamp-Lumen Depreciation

Thanks to the halogen regenerative cycle, lamp blackening is minimal. Consequently, lumen depreciation with halogen lamps is very small.

Burning Position

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.

Switching

The switching behavior of halogen lamps is the same as that of normal incandescent lamps.

Dimming

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.

Mains-Voltage Variations

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 %.

UV Component

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).

Product Range

Grouping of Halogen Lamps

As far as operating voltage is concerned, halogen lamps can be placed into two groups (Fig. 4).
  • 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

Halogen Lamp, Fig. 4

Range of halogen lamps

Low-voltage halogen lamps are usually single ended. Mains-voltage versions can be either single ended or double ended, according to whether the electrical connection is at one end of the lamp (Fig. 5) or on two separated ends of the lamp (double), as illustrated in Fig. 6.
Halogen Lamp, Fig. 5

Single-ended halogen reflectors lamps. From left to right: aluminum reflector with antiglare screen, glass mirror reflector, and cool beam dichroic glass reflector; under pressed-glass, high-voltage, halogen lamp (PAR)

Halogen Lamp, Fig. 6

Double-ended mains-voltage halogen lamp

Single-ended halogen lamps are available both as “naked” bulbs (capsules such as in Fig. 7) and as reflector lamps. The reflector versions exist in three different types: mirror-coated glass reflectors, aluminum reflectors, and pressed-glass PAR reflectors (Fig. 5). The mirror-coated glass reflectors are produced with plain mirror-coating versions and with a dichroic coating (so-called cool-beam type; see later in this chapter).
Halogen Lamp, Fig. 7

Low-voltage halogen capsules

Some mains-voltage halogen bulbs (both single-ended and double-ended) are housed in an extra outer glass bulb or glass tube with the familiar Edison or bayonet lamp cap. These are called double-envelope halogen lamps (Fig. 8). They are the more-energy-efficient replacements for normal incandescent lamps.
Halogen Lamp, Fig. 8

Double-envelope mains-voltage halogen lamps

Cool-Beam Halogen Lamps

A special version of halogen lamps, the so-called cool-beam lamps, makes use of the interference principle. They consist of a small halogen bulb (capsule) housed in a reflector with an interference metal coating, also known as a ¼ λ, or dichroic coating (Fig. 9). This coating splits the radiation coming out of the halogen capsule into an infrared part (heat), which is transmitted through the coating and not reflected by it, and a visible part that is reflected but not transmitted. The practical result is that the mirror coating reflects practically all of the light while allowing the infrared to pass through it to the back of the reflector, taking about two-thirds of the heat from the light beam. Thus, only one-third of the generated heat is contained in the light beam. This is why these lamps are referred to as “cool-beam” or “dichroic” halogen lamps. When using this type of lamp in an installation, it is important to ensure that the heat radiated from the rear of the lamp can be dissipated backward. The thickness of the coating layer is slightly different for radiation arriving at the coating from different directions. This also means that the wavelength of the radiation where it is split into reflected and transmitted components is slightly different for different directions.
Halogen Lamp, Fig. 9

Dichroic interference coating (shown in yellow) in a cool-beam halogen lamp [1]

Halogen Lamps with Infrared-Reflective Coating

Another special version of the halogen lamp also makes use of the interference coating principle but for a completely different reason. Here the reason is thermal recovery: the interference coating applied to the halogen bulb is now so dimensioned that it reflects the infrared radiated from the filament back to the filament while allowing visible light to pass through (Fig. 10). In this way less external energy is needed to keep the filament at the required temperature, thus improving the efficacy of the lamp (by up to 25–35 lm/W). Moreover, the heat contained in the light beam is reduced by approximately 40 %.
Halogen Lamp, Fig. 10

Infrared reflective coating based on the interference principle used to reflect the infrared radiation of the filament back to the filament while allowing visible light to pass through [1]

Cross-References

References

  1. 1.
    Van Bommel, W.J.M., Rouhana, A.: Lighting Hardware: Lamps, Gear, Luminaires, Controls. Course Book. Philips Lighting, Eindhoven (2012)Google Scholar
  2. 2.
    Coaton, J.R., Marsden, A.M.: Lamps and Lighting, 4th edn. Arnold, London (1997)Google Scholar
  3. 3.
    DiLaura, D.L., Houser, K., Mistrick, R., Steffy, G.: IES Handbook, 10th edn. Illuminating Engineering Society of North America, IESNA, New York (2011)Google Scholar

Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.NuenenThe Netherlands