When lighting the same project to the same illumination levels, the differences between the principal light sources in common use for outdoor lighting are substantial. Metal Halide (MH) sources have specific characteristics that distinguish them from High Pressure Sodium (HPS) sources, most notably the blue-white color for MH that is quite distinct from the amber-white light of HPS. These sources also have performance characteristics that make MH a significantly more expensive choice for providing the same lighting levels.
The first difference is that, for the same electricity, MH lamps produce less rated lumens than HPS lamps. The difference depends on wattage, and is slowly diminishing as MH lamp technology is improving, but at this point the initial efficacy of MH is 78% to 95% that of HPS. This means that from the very beginning, HPS produces more light for the same amount of energy
However, the difference between MH and HPS performance gets much more significant as the lighting system ages from initial to maintained conditions. Lamp Lumen Depreciation (LLD) for MH is two to three times as much as for HPS. Therefore the light produced by MH at "mean" life is only about two-thirds to three-quarters of that produced by HPS for the same input electricity.
This is why "on average" a HPS system only needs two-thirds to three-quarters of most everything compared to a MH system. This includes electricity, so HPS systems will have around 65% to 75% of the operating costs, energy use and associated pollution compared to a MH system. This also means that for each luminaire HPS can cover a bigger area, so HPS systems can get by with less equipment throughout: less concrete for foundations, less steel for poles, less energy to truck it and to install it.
Maintenance will restore the Lamp Lumen Depreciation, but maintenance frequency is another characteristic that makes MH more expensive than HPS. The rated life of MH lamps range from one-half to four-fifths of HPS, but the situation is actually worse than that. With the current definitions of "mean" and the degradation in performance associated with the aging of MH lamps, the realistic life of MH lamps is only 80% of the rated life (this also corrects the "misinformation" in the two definitions of "mean".) Therefore, a MH system will require around twice the maintenance of a HPS system that produces the same illumination levels. This is true regardless of the maintenance pattern - group relamping or spot relamping - since the practical life of the lamps will determine the timing for both patterns. The one advantage for MH systems in this is that the luminaires may get cleaned twice as often. On the other hand HPS systems have around half the costs for regular maintenance compared to MH systems.
In non-economic terms, MH systems have higher "costs" as well. Since MH lamps last about half as long as HPS and we have to use 50% more to produce the same light levels, there are around three times as many used MH lamps to throw away compared to HPS. The mercury content in a MH lamp is at best equal to - and at worst 100 times as much - as a HPS lamp of the same wattage. With three times as many lamps being thrown away, MH systems produce at least 3 times the mercury of HPS systems.
The blue-white light of MH is the reason MH systems also produce more light pollution than HPS systems with the amber-white color. This is something we learn from the blue sky. Rayleigh scattering is the phenomenon that causes the sky to be blue and orders the colors of sunrises and sunsets. Rayleigh scattering is also the predominant cause of the redirection that turns light from the ground into skyglow coming back at us from the atmosphere under clear sky conditions. By evaluating the radiant energy from MH and HPS lamps, the effect of Rayleigh scattering can be evaluated for the corresponding contribution to skyglow under clear sky conditions. Using the same number of lumens of light directed into the sky, the blue-white of MH produces more skyglow than the amber-white of HPS. When the skyglow is evaluated for its effect on dark-adapted eyes - such as astronomers - the effect from MH is around three times as much as from HPS.
Finally, recent research has identified a fifth photosensitive system within the eye, associated with the production of melatonin and the body's day/night cycle. There is evidence that disruption of this day/night cycle is associated with increases in some forms of cancer. The peak sensitivity of this photoreceptor is apparently around 445 to 485 nanometers. This is the blue portion of the spectrum where MH sources produce three times the radiant energy of HPS sources, and the scattered radiation from MH is nearly three times are much as from HPS.
While these potential effects on mercury pollution, light pollution and health effects may not have monetary values associated with then, without a doubt they are significant "costs" to be considered when making decisions about lighting systems.
Compared to MH systems, HPS lighting systems cost about two-thirds as much to install, cost two-thirds to half as much to operate, need about one-third of the maintenance and have around one-third of the effect on mercury pollution, light pollution and possibly on health effects. That is why MH lighting systems are more expensive than HPS lighting systems.
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