 There are several different types of HID lights including mercury vapor, metal halide, high pressure sodium and conversion bulbs.Metal halide and HPS lamps produce a color spectrum that is somewhat comparable to the sun and can be used to grow plants.Mercury vapor lamps were the first type of HIDs and were widely used for street lighting, but when it comes to indoor gardening they produce a relatively poor spectrum for plant growth so they have been mostly replaced by other types of HIDs for growing plants.