A halogen lamp is a type of incandescent lamp. It also uses a tungsten filament like any other incandescent lamp but it is encased inside a much smaller quartz envelope instead of a glass envelope that is used in conventional incandescent lamp. Usually, glass envelope is either evacuated or filled with an inert gas or a mixture of gases such as nitrogen, argon and krypton. When electrical power is applied to the filament, it becomes hot enough to become incandescent i.e. the filament glows and emits light. During operation, the tungsten evaporating from the hot filament condenses on the cooler inside bulb wall, causing the bulb to blacken. This blackening process incessantly reduces the light out-put over the life of the lamp. To eliminate this blackening problem halogen lamp was designed. Halogen bulbs produce whiter and brighter light, use less energy, and last longer than standard incandescent bulbs of the same wattage. Majority of halogen lamps vary in power from 20-2,000 watts. Low voltage types range from 4-150 watts.
Halogen lamp is filled with the same inert gases as incandescent lamps mixed with small amounts of a halogen gas (generally less than 1% bromine). The halogen chemically reacts with the tungsten deposit to produce tungsten halides. 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 is known as the tungsten-halogen cycle and it maintains a constant light output over the life of the lamp. To make this halogen cycle to work following are the basic requirements:
- The bulb surface must be very hot otherwise the halogen may not effectively vaporize or fail to adequately react with condensed tungsten.
- The bulb needs to be smaller and made of either quartz or a high-strength, heat-resistant grade of glass known as aluminosilicate.
Since the bulb is small and reasonably strong due to its thicker walls, it can be filled with gas to a higher than normal pressure. This slows down the evaporation of the tungsten from the filament, increasing the life of the lamp. The higher pressure and better fill gases can extend the life of the bulb and permit a higher filament temperature which results in better efficiency.
Depending on the type of halogen lamp, the bulb material can be of two types:
- Quartz i.e. fused silica: Quartz glass has the appropriate temperature resistance for the tungsten-halogen cycle, which produces bulb temperatures of up to 1,652°F (900°C).
- Aluminosilicate glass: It is generally used for lamps of low wattage up to about 120 watts.
The electrical properties of the lamp are determined by the filament wire dimensions and shape or geometry. The higher the operating voltage, the longer the wire must be. For higher wattages, a thicker wire is required. The filament is wound into the shape of a coil of different configurations, based on the lamp application. The most common configurations are
- Round core
- Flat core
- Double filament
In special cases, other configurations are used, either modulated (for maximum efficacy of light generation) or segmented (for uniform distribution of light). Filaments are also oriented in two ways, axial or transverse. In double-ended cylindrical lamps, the orientation is always axial whereas in single-ended lamps the orientation is determined by the application.
Halogen HIR lamps are designed with a special infrared reflective coating on the outside of the bulb to ensure that the radiated heat, which otherwise is wasted, is reflected back to the lamp filament. The filament burns hotter so less power is needed to keep the filament hot. These lamps provide a small, white light source with excellent color rendering. HIR lamps consume up to 27 percent less energy than standard halogen lamps. This type of energy savings can significantly impact the environment by reducing CO2 emissions. Moreover, a few Halogen HIR lamps last up to 60 percent longer than standard halogen lamps, reducing maintenance and labor costs.
- Halogen bulbs are more expensive than standard incandescent lamps.
- Although more efficient than other large incandescent lamps, tungsten halogen lamps are inefficient relative to fluorescent and high intensity discharge (HID) lamp types.
- Halogen lamps can also pose a safety threat, as the heat generated can range from 250-900°F (121-482°C).