FAQ
Answers to the questions we hear most about incandescent and halogen lamps. Don't see yours? Contact us — we've been answering bulb questions since 1992.
What is an incandescent light source?
Incandescent light sources are electric bulbs with filaments. When electric energy is transformed into heat energy, the temperature of the solid body is raised. The heated body gives off different levels of light according to the degree to which it has been heated. Halogen bulbs fall under this category.
What materials are used in halogen lamps?
There are two kinds of bulbs used in halogen lamps: transparent and translucent quartz bulbs. Materials inside the halogen lamp include a tungsten filament, molybdenum foil, and filling gases consisting of a combination of nitrogen (N₂), argon (Ar), krypton (Kr) and a small amount of halogen gas made up of iodine (I), bromine (Br), chlorine (Cl), and fluorine (F). The base of the lamp is usually made of steatite or heat-resistant metal.
Why is the halogen bulb made of quartz?
Quartz is used in halogen bulbs because it can withstand high temperatures. Quartz also ensures against heat shock.
Why is tungsten used as the filament?
Tungsten is used because of its high flexibility and low rate of evaporation at high temperatures. Tungsten also has a very high melting point of 3400°C, which lends to its slow evaporation.
Why is molybdenum foil used inside the halogen lamp?
Molybdenum foil is used as a conductor through the seal part of the lamp. It ensures the hermetic sealing of the lamp. Molybdenum has almost the same thermal expansion properties as quartz.
How does a halogen lamp work?
It works through the halogen cycle. As tungsten evaporates from the filament, it combines with halogen gas and becomes tungsten halide. This tungsten halide state is maintained at temperatures between 250°C and 1400°C. The temperature of the bulb is kept above 250°C in order to maintain this state and prevent the tungsten from sticking to the bulb wall and darkening it. When the tungsten halide nears the filament, the heat of the filament separates the tungsten from the halogen gas, redepositing the tungsten on the filament. The freed halogen gas then repeats the same process again — thus, the halogen cycle.
How do halogen lamps give constant light output?
The halogen cycle effectively prevents blackening of the bulb wall, so there is no reduction of light output or color temperature over the lamp's life.
How does a halogen bulb give a longer lamp life?
Halogen bulbs are more efficient than ordinary incandescent bulbs. They can give a longer lamp life expectancy because of the halogen cycle.
What is color temperature?
Color temperature is the temperature at which a black body emits radiant energy competent to evoke a color of the same hue and saturation as that evoked by radiant energy from a given source. In practical terms: it's the warm-to-cool color of the light, measured in Kelvin. For the full explanation, see our Characteristics of Light guide.
Why does a light bulb burn out?
Due to the high temperature the tungsten filament operates at, some of the tungsten evaporates during use. This evaporation leaves the filament thinner in some areas. The weakened area of the filament becomes hotter, which leads to even faster evaporation, causing the filament to burn back and the lamp to burn out.
Why do some bulbs burn out the instant they are turned on?
The thinned filament in a cold state cannot withstand the inrush surge of energy, and so the bulb burns out at the moment of switch-on.
Why does a bubble sometimes form on the quartz wall of a halogen bulb?
This is due to the quartz halogen lamp being overheated. Overheating causes the filament inside the bulb to sag and come too close to the quartz envelope, creating a bubble.
What are the common causes of lamp failure?
The most common causes are overheating and over-voltage.