I found a box of 10-15 assorted lights at my friends auto shop. There kinda dirty but i cleaned em up and they seem pretty good. i havent tried them out yet cause i want to find more about them first. Right: Sylvania 400w metal-arc. Middle: Ushio 400wMetal Halide Ed28/E39. Left: GE 320w Muilti-Vapor so i guess my question is: What is a metal-arc bulb, why is the 400w Ushio bulb smaller, and what is a "Muilti-Vapor" bulb??????