Alliance for Lighting InformationAlliance for Lighting InformationThe Replacement of HPS with Metal Halide:
The research reference here, Berman 1991, justifying the suggestion that High Pressure Sodium (hps) produced illuminance levels could be reduced by 33% if the source was changed to Metal Halide (mh) is misapplied. The following is a quote from the referenced paper:
"Potential economic benefits of scotopically rich lighting
Because scotopically richer illumination appears be the preferred spectrum for smaller pupil size and greater brightness perception in interior lighting conditions, it is our proposition that lamps with high scotopic output for a given input power will be more cost-effective than lamps of low scotopic output for the same level of input power. "
Note the inclusion of the phrase 'in interior lighting conditions'. Dr. Berman is very explicit in his paper that the research was conducted under typical interior lighting conditions. The inference that this could be applied to much lower lighting levels, as are typical of exterior lighting, is not Dr. Berman's. The reduction of illuminance level from 30 footcandles to 20 footcandles is not the same as a reduction from 0.2 footcandle to 0.13 footcandle as perceived by our visual system.
The statements "Research has shown that a MH lighting design can satisfactorily replace HPS while offering lower overall lighting power density and illumination levels. This is achievable because MH light output includes more colors of the visible light spectrum than does HPS. As a result, the MH user perceives as much light as the HPS system user but at lower illumination amounts." shows a complete misunderstanding of the physical mechanics of the visual system and a complete misinterpretation of Dr. Berman's work. Dr. Berman explains in detail the mechanism for this phenomenon in his paper. It has nothing to do with "more colors of the visual spectrum" but instead with Metal Halide sources' ability to produce more radiant energy of short wavelengths (blue light). Blue light is largely responsible for pupillary contraction - smaller pupil size increases depth of field and hence increases visual acuity. A smaller pupil size also decreases retinal illuminance. When working with high illuminance levels the decrease in retinal illuminance is not much of a problem as there is still plenty of light to keep the visual system in the photopic adaptation range. When working with lower illuminance values such as are common in exterior lighting situation, the decrease in retinal illuminance may inhibit the visual system. Additionally, Dr. Berman's work focused on such visual tasks as reading which is a much less dangerous activity than is operating 4000 pounds of moving SUV.
Even if there was evidence to support a 33% reduction in illuminance by switching to metal halide, the numbers don't add up. If you consider that a 250-watt hps has 27000 mean lumens and you reduce this by 33% that means that a metal halide lamp would need to produce 18090 mean lumens to yield the 33% reduction in illuminance. A 250-watt metal halide lamp produces about 13000 mean lumens. How does this save energy when you would need more Metal Halide wattage than High Pressure Sodium?
The evaluation of the impact of replacing high pressure sodium lamps on page 14 of the report states that the state will realize savings of 28 GWh for security lighting, 23 GWh for storage lighting, 96 GWh for parking lot lighting, and 40 GWh for walkway lighting. These values must be considered along with their bound errors. The bound error from Table 5 for security lighting indicates a 101% margin of error, for storage 95% margin of error, for parking lots 61% margin of error, and for walkways an 85% margin of error. With these high margins of error, the stated values are not very credible. It is also of note that the total Error Bound shown in Tables 5 and 6 do not correspond with the total shown in Table 1, but the Original Energy Usage in all the tables is the same. This indicates a problem with the statistical methods applied. The total error bound after making a theoretical change cannot be less than the baseline's total error bound - if this were ever the case it would mean that by making theoretical changes to the data the resulting data is somehow better than the original data at representing the population.
The Replacement of Mercury Vapor with Metal Halide:
In this part of the scenario the report states that a ratio of 56:90 is applied to the energy consumption of the Mercury Vapor lighting systems to generate the savings impact. There is no explanation as to how this ratio was derived and the only reference is to a "Technical Advisory Committee Meeting, 3 April 2002". Without a technical or mathematical explanation of the derivation of this ratio, there is no way to verify if the results generated by applying this ratio are valid. This ratio could have been picked from thin air, and any results from applying are not technically reliable.
While many studies of the spectral effects of light sources have been conducted, including the information in this report, so far there is no conclusive scientific evidence that one spectral distribution is better than another for practical outdoor visual tasks.
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