The M400/U high performance metal halide lamps are among the most energy efficient sources of white light available today for many interior and exterior applications. Please note that MH400 metal halide lamps cannot be operated without a ballast. The M400/U lamp is suitable for open luminaires only if operated vertically +/- 15 degrees, provided the installation is not near people or flammable or combustible material; otherwise, this lamp must be operated in a suitably enclosed luminaire.