Tungsten-halogen lamp

The halogen cycle is the key innovation. When tungsten evaporates from a hot filament, it diffuses toward the cooler bulb wall, where it combines with halogen atoms (iodine or bromine) to form a volatile tungsten halide. Convection currents carry this compound back toward the filament, where the high temperature dissociates it, depositing tungsten back onto the filament and freeing the halogen to repeat the cycle.¹ This regenerative process prevents bulb blackening and allows the filament to run hotter—up to about 3400 K—yielding a brighter, whiter light with a higher color temperature than standard incandescent lamps.

The concept was patented in 1959 by General Electric engineer Elmer Fridrich, and the first commercial halogen lamps appeared in the early 1960s, initially for slide projectors and studio lighting. The quartz envelope, necessary to withstand high temperatures, gave rise to the trade name "quartz-halogen" or "quartz-iodine" lamp.

Halogen lamps require a compact, heat-resistant envelope, usually made of fused quartz or high-melting-point glass like aluminosilicate, because the bulb wall must reach temperatures high enough to sustain the halogen cycle (typically above 250°C). The filament is often coiled-coil tungsten, and the fill gas is a mixture of an inert gas (argon, krypton, or xenon) with a small amount of a halogen—usually iodine or bromine compounds. The high gas pressure (several atmospheres) reduces tungsten evaporation but also increases the risk of explosion, so the envelope must be thick and robust.

Some lamps include a dichroic reflector to project light forward while allowing infrared radiation to pass out the back, reducing heat on illuminated objects—a feature common in MR16 spotlights. Others use a halogen infrared (HIR) coating that reflects infrared back to the filament, improving efficacy by up to 15%.

Halogen lamps are used where high color rendering and compact size are needed: automotive headlamps, studio and stage lighting, film projectors, fiber-optic illuminators, and scientific instruments such as microscopes and spectrophotometers. In the home, they appear as linear double-ended lamps (e.g., T3) and low-voltage MR16 and GU10 reflector bulbs.

Specialized variants include tungsten-halogen lamps for aerospace (e.g., aircraft landing lights) and medical endoscopy light sources. A notable niche is the halogen cycle in high-intensity discharge lamps, where metal halides are used, but that is a distinct technology. In the 1990s, halogen lamps with infrared-reflective coatings (dichroic or HIR) became popular for energy savings, though they still lag behind LEDs.

One obscure fact: the halogen cycle only works if the bulb wall is hot enough; if the lamp is dimmed too much, the wall cools and the cycle stops, leading to rapid blackening and shortened life.² Another is that iodine was the first halogen used, but bromine compounds are now more common because they allow a more compact envelope.

Halogen lamps played a role in the development of fiber-optic communication as early light sources for testing. Also, the quartz envelope transmits UV radiation, so many lamps include a UV-blocking filter to protect artwork and skin. Finally, the halogen lamp was the last major improvement to incandescent technology before LEDs, and its phase-out in many countries (e.g., EU and US) was driven by efficiency standards, yet it remains in niche applications where instant full brightness and high CRI are essential.

Halogen cycle

A regenerative chemical cycle that returns evaporated tungsten to the filament, preventing bulb blackening.

Quartz envelope

A bulb made of fused quartz, able to withstand high temperatures and pressures.

Dichroic reflector

A reflector that transmits infrared while reflecting visible light, reducing heat.

Color temperature

A measure of the whiteness of light, expressed in kelvins (K).