Lumen depreciation, color shift over life and cost reduction are the main reasons that a user should consider a group relamping program. When a lighting installation is new, the light level is high and the quality of the light is excellent. Over life, lamps suffer a decrease in efficacy (lumens per watt) caused by both the loss of the light producing chemicals within the arc tube and darkening of the inner walls of standard arc tubes. Most metal halide lamps will decrease to around 60% of initial rated light output at about half of rated life. The Uni-Form formed body arc tube lamp provides greater efficacy and better lumen maintenance than the older, standard pinch body arc tube technology.
In standard metal halide technology, not only is there a decrease in the amount of light generated over life, but the quality of the light also changes. The color of a metal halide lamp is made up of hundreds of spectral line emissions from the chemicals within the arc tube. This plot of the lines of color the lamp emits is called its spectral distribution. It is this collection of colored lines that our eye interprets as a "white" light.
You might remember holding up a prism to the sunlight and seeing the rainbow of colors produced. The prism is able to break down the sunlight into all of its many spectral components. Even though sunlight has all of these component spectral lines, our eyes, together with our brains, interpret the combination of all these colors as white sunlight. The same phenomenon is happening when you look at the light produced by a metal halide lamp.
One difference between sunlight and the light from a metal halide lamp is that the sun produces the same quality of light at all times. The differences we see in sunlight are caused by the effects of earths atmosphere, not by changes in the sunlight. The metal halide spectral distribution, however, is not as stable as the suns. Its spectral output comes from mercury and metal iodides present in the arc. As the chemicals are lost to the arc through reactions, the proportions of their contribution to the spectrum change. Our eye interprets these changes in spectral distribution as changes in lamp color.
Generally, a metal halide lamp will shift in color by about 200 to 300K during its economic life. If all the lamps in an installation are operated in the same fixture type and burning orientation, they will generally change color in the same direction and amount during economic life. Therefore, the color shift of the lamps will not be very noticeable. The problem arises when you spot relamp after economic life. By then a lamp may have shifted 600K or more causing lamps to appear bluish, pinkish, or greenish.
With spot relamping, a new, bright white, initial lumen output lamp is installed in a field of lumen depreciated and slightly color shifted lamps. The human eye is a wonderful device for comparing color and it, of course, is able to easily discern that the new lamp is both brighter and a different color than the old lamps. If you multiply this scenario over a series of spot relampings, you can see why eventually a metal halide lighting installation can have quite a non-uniform look.
The answer to this problem is a group relamping program in which the user relamps the entire installation at one time, usually sometime between 60% and 70% of rated life. This plan has a number of advantages:
Group relamping greatly reduces spot relamping labor costs because all lamps are replaced before they reach the point in life where failures are accelerating. In addition, the cost of energy is by far the largest portion of maintaining a lighting system. If lamps are not group relamped, almost half of this energy will be wasted on lamps that are performing below their designed level. As an example, an average 400 watt metal halide lamp consumes $800 of energy over its rated life. About $320 of these dollars are wasted on lamps not providing optimum performance. With group relamping, a new lamp and the labor to install it is generally less than 5% of the total energy cost. Group relamping also saves downtime because it can be scheduled over a weekend or shutdown period.
When lamps are of similar light output and color, a lighting installation looks better. Shopping centers, retail stores and even gas stations with canopies are retail spaces that are trying to attract customers. The quality of the lighting says much about the store and the merchandise. Group relamping maintains the quality of light in a space infinitely better than spot relamping.
Improved Light Levels
An installation that is properly group relamped will have better maintained light levels than one which is not. Higher light levels have been associated with increased safety, higher productivity and an increased sense of security. Since it costs the same amount of energy to run a fully lumen depreciated lamp as a new lamp of the same wattage, doesnt it make sense to install the higher lumen output lamp in the socket?