Figure 9.8 shows how Multi-Vapor metal halide lamps are similar to mercury lamps. The chief difference is that the arc tube contains metal halides in addition to mercury. The outer bulb is of the same materials and functions as in the mercury lamp. Both clear and phosphor coated versions are available. The main electrodes are similar to those of mercury lamps except there is no emission coating and they are larger. The starting electrode is the same except there is a bimetallic switch that shorts the starter circuit to the main electrode after the lamp starts. The arc tube is slightly smaller than in the mercury lamp. The ends have a white reflective coating to control the arc temperature and metal vaporisation. Also the ends are moulded to a precise parabolic shape. The arc tube support is "frameless" to prevent magnetic interference with the metal halides. Most Multi-Vapor lamps require special metal halide ballasts, and compatibility between lamp and control gear should always be checked.

Figure 9.8. Multi-Vapor lamp construction.

Multi-Vapor lamps operate on the same general principles as mercury lamps. See Figure 9.9. The addition of metal halides into the arc tube affects the light output and improves the colour characteristics. The metal halides vaporise into halogen and metal in the central core of the arc (hottest area). The additional metals radiate more light than mercury and at desirable colour wave-lengths. It is the combination of several metals that produces white light and the number and proportion of the metallic constituents can vary this colour. As the metals and halogen move out of the central arc towards the tube walls they recombine at the cooler temperature. This halogen cycle repeats.

Figure 9.9. Multi-Vapor lamp operation.