Metal-halide lamps are high-pressure lamps which are typically operated at high powers. Such lamps have applications in lighting of sports facilities and the like.
Metal-halide lamps are being actively studied, both on earth and in space. The basic idea of such lamps is quite old, however: Charles Steinmetz was granted an U.S. Patent 1,025,932 in 1912 regarding a metal-halide lamp.
In these lamps, light is generated by plasmas of complex chemical composition. The light emitting species are metals such as Na, Tl, Dy and Ce, introduced into the mixture as metal halide salts (for example NaI), which are excited in a medium predominantly consisting of mercury. An essential feature of the plasma in MH lamps is that a minority of species (i.e. the rare earths) is responsible for the majority of the plasma properties. For this reason, these plasmas are very sensitive to external conditions and may posses strongly non-linear aspects. An example of this is the sensitivity to the gravitational force: turning from a vertical to a horizonal burning position can change the color of a MH lamp substantially.
This is remarkable since the plasma is mainly ruled by the presence of charged particles, and the electrical force on the ions is more than a nine orders of magnitude larger than the gravitational force. This sensitivity of MH lamps to the gravitational force is caused by an interplay between convection and diffusion. It is the competition between these transport components that leads to a segregation of elements. The light emitting elements remain in the lower part of the lamp which manifests itself in segregation of color.
For this reason, simulating this lamp requires combining a convection flow model with elemental diffusion and a ray-tracing radiation transport module.