Aside from the method of coupling energy into the mercury vapor, these lamps are very similar to conventional fluorescent lamps. Mercury vapor in the discharge vessel is electrically excited to produce short-wave ultraviolet light, which then excites internal phosphors to produce visible light. While still relatively unknown to the public, these lamps have been available since 1990. Unlike an incandescent lamp or conventional fluorescent lamps, there is no electrical connection going inside the glass bulb; the energy is transferred "through" the glass envelope solely by electromagnetic induction. There are two main types of magnetic induction lamps: external core lamps and internal core lamps. The first commercially available and still widely used form of induction lamp is the internal core type. The external core type, which was commercialized later, has a wider range of applications and is available in round, rectangular and "olive" shaped form factors.