Seattle LED study—comments by Adrian Melott and Reid Nelson
Scientific research is normally published in journals after peer review—it is evaluated by a few other people in the field for comment and criticism. I do not know why this study was never so published. I did spend some time looking it over and thought of a few comments I would make if I were reviewing it. Our boldface below.
1. It is not stated whether different lamps are being compared at the same power consumption, or the same luminance, or what. This is important since luminance is important. You can sort of figure this out from the appendix, but it is not in the main body.
2. Several of points in the “overview of studies” criticize using CCT as a measure; yet this study uses CCT.
3. The Seattle study and its antecedents did not mention doing any tests in times of fog, mist, or of people with vision problems like cataracts or floaters.
4. There were no measurements of glare.
5. The study apparently used a small number of persons. Based on statistics, one would expect the results to jump around based on chance with a small sample. This may be related to item 6
6. In the full study, all pages included, it is revealed that in Seattle and San Jose, the detection distance was greater for higher color temperature, but in San Diego, the lower color temperature performed better. I have attached the full document. Rather than conclude that their results were questionable, they concluded that high color temperature was better based on 2 out of 3. This variability regarding detection distance which I have observed in the literature is the reason that I did not stress this measure. Appended below for example, is the abstract of a study from around the same time.
7. The “Seattle Study” was put forward to support the 4,000k lighting. However, on one of the pages that was omitted in the attachment, the authors of this Seattle study actually found that, in two of its three studies, city respondents preferred the lowest color temperature: “However, a few trends do appear among the studies in terms of preference for color and light source. In both San Jose and San Diego, the participants preferred the 3500K LED luminaire when responding to the overall style of the lighting.” Therefore, the study, when considered in combination with the author’s prior studies and the actions of those other cities, more strongly supports the choice of 3,000k lighting. Also, there is large dissatisfaction currently in Seattle with the choice of 4,000k, as evidenced in part by a petition being circulated which protests the glare and color temperature of its 4,000k lights. Finally, the Seattle study predates the AMA’s strong health criticism of the 4,000k lighting.
8. We have attached the full document so that the reader can check our statements.
Comparison of LED Luminaires for Roadway Lighting Using Object Detection and Color Recognition Distances
This study compared two LED luminaires and their abilities to provide detection distance and color recognition distance of potential roadway hazards. Detection distance is regarded as a metric of visibility. Color recognition distance is a metric for comparing the impact of the CCT (Correlated Color Temperature) of each luminaire and their color contrast impact. Mesopic vision, the mode of vision most commonly used for night driving, was considered in this study. The impacts of luminance and color contrast were addressed in this study. The experiment was performed on the Virginia Smart Road where small wooden targets of different colors and pedestrians wearing different colors were detected by drivers of a moving vehicle in a controlled environment. The key difference between the two luminaires was their color temperatures (3500K versus 6000K). The results indicated that neither light source provided a significant benefit over the other although significant interactions were found among object color, age, and lighting level. The results indicate that the luminaires provide similar luminance contrast but their color contrasts depend heavily on the color temperature, the object, and the observer.
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