Licht-im-Terrarium: Literaturdatenbank

Discovered in 1802 by H. Davy, the phenomenon of incandescence is the oldest practical mean of light generation from electricity. In this process, optical emission arises from the constant energy change of electrons in hot solid materials, resulting in a continuous electromagnetic spectrum with a temperature-dependent Planckian wavelength distribution. Incandescence is implemented in lamps by driving an electric current through a thin filament made of tungsten, a refractory metal chosen for its high melting point (3695 K) and low vapor pressure (1 Pa at 3477 K). In order to limit thermal losses and material evaporation, lamps are in most cases filled with a protective gaseous atmosphere, and the tungsten wire is wound into a compact coil or coiled coil configuration. In order to ensure a stable filament structure at high temperature, the metal is doped with potassium or rhenium so as to promote the most favorable crystallographic structure.

When operated in a neutral Ar-N2 or Kr-N2 atmosphere, the filament temperature lies in the 2600–2800 K range, resulting in 5–8 % energy conversion into visible light. Better performances are obtained with a higher gas fill pressure combined with a tungsten-bromine cycle which prevents tungsten deposition onto the lamp wall. Due to a higher bulb temperature requirement (800–1000 K), halogen lamps are made with a refractory glass bulb in a very compact configuration. With a fill pressure reaching 3 bars, filaments can be operated in the 2800–3200 K range, resulting in 7–13 % energy efficiency.

Lamps for general lighting applications are both of the standard and halogen types and are made for an isotropic or a directed emission of light. Standard gas-filled lamps feature a 3.5–20 lm W−1 efficacy with a 1000 h average service life, optimized for the most economical lamp usage. Standard halogen lamps have a 9–25 lm W−1 efficacy with a mean service life reaching up to 10,000 h. Incandescent lamps are also made in a wide variety of configurations with different filament structures and temperatures so as to address specific lighting needs in traffic signals, automotive applications, on stages and studios, for infrared processing, and for instrument calibration. Finally, the most recent and refined lamp designs integrate an infrared mirror for energy conservation, resulting in compact general lighting sources with up to 35 lm W−1 efficacy, or feature a novel wafer-sealed bulb construction and a 5 bar xenon fill yielding 18.8 lm W−1 in a compact low-wattage package for automotive applications.

However, incandescent lamps are plagued by two intrinsic limitations, the first of which is a lumen efficacy constrained by the nature of the light emission mechanism and by the maximum filament temperature permitted by technology. The latter constitutes the second intrinsic limitation as the filament life is mostly limited by the formation and growth of local hot spots on the tungsten wire as a result of material evaporation and diffusion. These two limitations result in a relatively poor life-efficacy balance compared to other light source technologies. For this reason, incandescent lamps are being progressively phased out in a number of lighting applications as more efficient technological alternatives emerge.