As an additive of metal halide lamp, metal halide lamp is a kind of gas discharge lamp. It is developed on the basis of high-pressure mercury lamp. It is characterized by filling various rare earth halides in the lamp. At present, rare earth iodides are mainly used, which emit different spectral colors in gas discharge. Holmium iodide is the working substance used in the holmium lamp. High metal atom concentration can be obtained in the arc region, which greatly improves the radiation efficiency. Holmium can be used as an additive of yttrium iron or yttrium aluminium garnet.