HID – The Burning Question … Base Up, Base Down or Horizontal: What’s the Deal with Orientation?
By Jerry Cassel, LC Senior Specification Engineer
How many times have you wondered why a certain lamp is vertical base up or vertical base down, universal or horizontal? What does it matter and why should I need to know?
The reason for the concern, or at least something to be aware of, has more to do with color consistency, lumen output and life. So, lets explore these issues.
Unlike incandescent and fluorescent lamps where color variations are usually imperceptible, metal halide lamps have a normal expected color variation lamp-to-lamp and over life. To the casual observer, this color difference may be judged as improper lamp operation. Though all manufacturers of Quartz Metal Halide lamps attempt to control this variation, current science of the technology makes the total elimination of color variation a physical impossibility. However, understanding and adjusting certain factors over which the user has control can achieve some additional reduction of color variation.
GE standard Multi-Vapor® lamps are multi-component lamps. The lamp contains various metals, which reside in the arc tube in gaseous form. Applying a voltage to the arc tube electrodes produces light. That voltage excites the gaseous metals to the point where they emit light — each metal emitting a different spectral color.
LAMP-TO-LAMP COLOR VARIATION Variations in the individual pressures of each of the gasses produce minute but nevertheless noticeable variations in lamp color appearance. This electrically produced light can be additionally affected by other variables as well. Variables such as arc tube dimensional variability, temperature effects, chemical mixture tolerances, input voltage variations, ballast wattage control and a host of other contributing factors all affect a lamps appearance.
VARIATION OVER OPERATING LIFE During the first 100 hours of lamp operation (referred to as lamp seasoning), the halides are blending together for the first time and will display even wider color variation than when they reach their stable operating point beyond 100 hours. At about the 100-hour point, the halides are sufficiently mixed so that infant variation is minimized.
However, as a population of lamps begins to age, another color shift becomes apparent. This phenomenon occurs as the chemistry changes within the arc tube as a result of sodium migration through the arc tube wall. This type of color shift is predictable in direction. The standard family of Multi-Vapor® lamps normally shifts to a warmer color (lower Kelvin temperature), while the family of lamps with an MXR prefix (Halarc® lamps), shifts to a cooler color (higher Kelvin temperature) as the lamps age. This color shift