Outdoor High Intensity Discharge Lamps
Mercury vapor are known by the bluish cast to their light. They are the least efficient of the group and should be avoided for that reason. Metal halide lamps produce a bright, crisp, white light that is more aesthetically pleasing than the others, and is much more efficient than mercury lamps. Metal halide lamps are often seen lighting ball parks and are becoming more popular inside grocery and discount stores where they need lots of high quality light at the lowest operating cost.
High and low pressure sodium lamps are more energy efficient that the previous two and have a yellow cast to their light. High pressure sodium lamps are popular for roadway lighting, but low pressure sodium is very limited in its application because its light is monochromatic yellow, and therefore under it, you can not discriminate colors.
One characteristic to note about HID sources is that each requires a few minutes to come up to full output. If power to the lamp is lost or turned off, the arc tube must cool to a given temperature before the arc can be re-struck and light produced. Mercury vapor lamps require the longest re-strike time. Up to seven minutes may be required.