Some lights are bright white and others have more of a yellow glow. Why the difference?
The lights that appear to be a white light are called metal halide. They contain a vapor of mercury with some materials called halides that contain thallium, indium and sodium. When these materials are excited by the presence of a voltage, their normal state is raised to the point of luminescence and light is produced. Due to the composition of the elements contained within, the light contains most of the elements of the light spectrum and a white light is produced. Metal halide lighting is the best choice for pedestrian areas, as it provides a truer picture of colors and surroundings. This is especially true of sports lighting. Those lights that have a more yellow glow contain a vapor of sodium, which does not emit the entire light spectrum in luminescence. The High Pressure Sodium light source is much more economical and provides high light output for power input and thus is used in parking lots and roadway lights.
