This is a type of light. HMI stands for Halogen Metal Incandescence. HMIs are very bright, power efficient lights. They are balanced for the Color Temperature of Daylight, making them handy in mixed lighting situations. However, they are rather expensive, costing something in the few thousands of dollars, and are not very portable due to the large and heavy ballast that is attached. Also, and this is vitally important to keep in mind, they must be used with a Crystal Sync camera, otherwise they will flicker and throb.