We all know mercury vapor lamps often shift to a greener hue, LED street lights shift to purple as the phosphor wears off, and HPS lamps often shift to a whiter light than before when closer to the end of life, but for some reason, MH lamps color shift to various colors, and I’m not sure why.

Some shift to green (like a mercury vapor bulb), some shift to almost an warm white color, some shift to pink, some shift to a bluish daylight color, and I’ve seen some even shift to an orange high pressure sodium type color (except much dimmer than an HPS lamp). Why is it so various?

Well, most metal halide lamps depend on the sum of light radiated by multiple elements in the fill to create the final color. There are exceptions like 8000K white lamps containing only dysprosium (and cesium usually, but it is not active) and  green lamps containing only thallium, and these mostly do not drift.

As the lamp ages, mainly two processes related to color shift happens.

First, light-emitting metals somehow leave the arctube cavity. Some drift through the walls to the outside. Some got trapped in the walls. As these atoms have different properties (atomic mass, atomic size, electrical charge) they do so at different speed, so final balance of metals changes and so does the color.

Next, usually arctube temperature increases. Walls blackening means less of applied energy escapes the arctube and more got trapped inside. Also, aging electrodes lose their properties and become less effective emitters, raising arctube voltage and so, power. As different halides have different evaporation temperatures, this causes a change of their parital pressures balance and so lamp color drift, too.