NLPIP Specifier Report questions LEDs in street-lighting applications (updated)
RPI’s Lighting Research Center reports that LED-based street lights can’t meet RP-8-00 requirements at the pole spacing that HPS street lights achieve, and therefore the SSL alternative isn’t viable from a cost perspective.
A new report released by the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute (RPI) questions the economic viability of LED and induction street lights compared to legacy high-pressure sodium (HPS) lights.
The LRC issued the Streetlights for Collector Roads report via its National Lighting Product Information Program (NLPIP) stating that in a collector-roadway installation you would need double the LED-based solid-state-lighting (SSL) luminaires compared to HPS to meet the ANSI/IESNA RP-8-00 standard.
The NLPIP report does recognize the energy savings attributable to LED lighting, but notes that the cost of installing double the number of light poles and luminaires makes SSL too costly. “The LED and induction streetlights we tested required narrower pole spacing,” said Leora Radetsky, LRC lead research specialist, principal investigator and author of the report. “As a result, the life cycle cost per mile was dominated by the installation cost of the poles, as opposed to the initial cost of the streetlights or any potential energy or maintenance cost savings, as one may assume.”
Specifically, the SSL luminaires that the LRC tested fell short in illuminance measurements. In particular, the SSL products failed to deliver the street-side lumens that HPS lights delivered. Street-side lumens are the portion of the light spread in the quarter sphere downward and on the street side of the pole.
The LRC tested eight LED fixtures, four HPS fixtures, one induction fixture, and one Metal Halide (MH) fixture. The test included an examination of supplied photometric data and actual 360° photometric measurements that the LRC made with a calibrated mirror goniometer.
The data from the photometric files and the lab measurements was analyzed using Lighting Analysts’ AGi32 illumination engineering software to determine the maximum pole spacing for each luminaire. And the report notes that the results were closely matched based on the measured and supplied photometric data. The report states, “On average, the LED streetlights and the induction streetlight could be spaced only about one half the distance of the HPS streetlights and still meet RP-8 illuminance method criteria.”
The report does note that the results apply only to collector roadways that connect residential streets to major highways. Residential streetlight standards specify lower light levels and the report acknowledges that LED fixtures might meet residential requirements. The report is also not applicable to lighting on major highways where the fixtures are mounted much higher.
Ironically, the research also examined the mesopic vision concept where both cones and rods in the retina work together in the visual system. The report concludes that white or broad-spectrum light such as that generated by LEDs would allow the fixtures to be dimmed slightly and still provide equial levels of visual performance based on mesopic photometry.
LED proponents have argued the mesopic vision angle attempting to further the energy savings that can be achieved with SSL. But the LRC report dismisses energy savings completely given the upfront installation cost of the additional poles.
It will be interesting to watch the fallout from the report. Most LED street light trials are reporting positive results although many are in residential areas. Some are also in collector-roadway areas. At this point it’s not clear how those collector-roadway trials have reconciled RP-8 requirements. It could be that pole spacing in retrofit areas is sufficiently close that LEDs meet the RP-8 standard. Or it’s also possible that the parties responsible for trial installations have measured data that does not correlate with the LRC report.
We asked several LED street-light luminaire makers for reactions to the report. BetaLED is clearly among the leaders in the LED street light segment, and the company issued the following statement: “We have reviewed the Streetlights for Collector Roads report from LRC. Since it is an NLPIP Specifier Report, manufacturers with products included in the study were not consulted to provide insight, assistance or technical information directly to LRC researchers and investigators. With the wide range of options available from BetaLED, the choice of luminaire was not optimized for the specific application referenced in the report. If BetaLED had been given the opportunity to review or comment from a technical perspective, we believe the report would have generated different conclusions in favor of LED luminaries.“