Technical Summary of Research into
Unit Power Density and Unit Uplight Density:
Conclusions

Table of Contents

Conclusions

The conclusions from this research are many and varied, addressing the procedure used and the details of comparisons of the results. However there is one primary conclusion from all the parts of this research: the performance of lighting systems must be evaluated as complete systems, and not by evaluating individual components or their characteristics.

From the preliminary phase [3], conclusions include that the optimization routine works properly for finding the lowest UPD value for a given combination of roadway and photometry, and that this system corresponds to the lowest UUD value for that combination. This procedure works for the illuminance and luminance methods, but does not work reliably for the STV method. The preliminary phase conclusions also indicate substantial potential for reductions in equipment, costs, energy use, and uplight, when luminance or STV methods alone are used instead the combination of illuminance and luminance methods used for all the other runs. Under the criteria in [1], the STV method appears to offer the lowest UPD, UUD and associated costs among the three methods.

The conclusions from the intermediate phase [4] further support this. UPD values for the Illuminance criteria are equal to or slightly better than those for Both criteria, while the UPD values for the Luminance criteria are consistently lower than for the other sets of criteria. This trend is less significant for full cutoff distributions, which frequently indicate lower UPD for the Illuminance criteria than for the Luminance criteria, particularly on wider roadways as shown in Figure 4.

Among the conclusions of the intermediate phase are that the best UPD values for MH are around 1.5 to 1.8 times the best for HPS, when comparing similar wattage and roadways. Lower wattage luminaires consistently have lower UPD values than higher wattage luminaires. The lowest UPD values in any group are typically systems with non-cutoff or semi-cutoff distributions, although this trend becomes less significant as wattage decreases or roadways become wider. This trend is less evident in MH than in HPS, since the differences in UPD values between systems using distributions with different cutoff classifications is less for MH than for HPS systems. The reductions in UPD values when using the luminance method are proportionally greater for MH systems than for HPS systems, and allow some exceptional MH systems to approach the UPD values for comparable HPS systems. The increase in UPD values when full cutoff distributions are required is evident and relatively consistent over the range of wattages and roadways, but diminishes as roadways get wider and wattage increases.

From the advanced phase of the research [5], the conclusions include further confirmation of all the earlier conclusions, and more. The restriction of "overhang less than or equal to zero" makes little if any change to UPD values. As the width of the roadway increases, the change in UPD increases. When comparing differences in rated lamp lumens or LLF, the percentage decrease in UPD is only one-half to three-quarters of the increase in the "available lumens", the product of these two factors. This is significantly different from the widely held assumption that increases in maintained lumens produce "inversely equivalent" decreases in UPD. For example, a 10% increase (11/10) in maintained lumens has been expected to produce a -9% (10/11) change in UPD. The data indicates that this is not the case for narrower roads and may be the case only for wider roads and lower wattages.

The comparison for the change from Major Medium to Major High criteria shows that the conclusions about the rated lumens and LLF comparisons apply for even more aspects of lighting systems. The change in criteria has typically been assumed to correspond to a change in UPD of equal percentage, but instead the change in UPD is only one-half to three-quarters of the change in the criterion of average illuminance.

Finally, the results indicate a substantial potential for reductions in equipment, costs, energy use and uplight which correspond to lower Unit Power Density values for roadway lighting systems.