Mercury Vapor Mercury Vapor was the first widely accepted HID lighting source. Advances in technology have lead to color corrected mercury vapor lamps, which cast a relatively clean white light. This is done by coating the outer glass globe of the lamp with phosphors. The lamps were about as efficient as fluorescents, the fixtures were smaller, and lasted much longer. The lamps could also operate in extreme cold.
Lumen depreciation is a drop in light output of the lamp over time.
Metal Halide Metal Halide Lamps are a distant cousin of mercury lights. The basic lamp is the same as a mercury lamp, but with other metallic elements added. The result is a good quality white light. Metal hallide has not gained wide acceptance as a source of street light. It is mostly found in parking lots and inside commercial and industrial buildings.