Jun 9, 2019

More research on streetlights is just down the pike
By Norma Frank

The Pacific Northwest National Laboratory (PNNL) is planning to conduct a study that will generate understanding of dirt depreciation and other optical changes occurring in LED street and area lighting over their life. PNNL is undertaking this research on behalf of the U.S. Department of Energy’s Solid-State Lighting Program. The results will help inform industry, designers and end users about the effects of depreciation during street and area lighting service life.

This application is very important for several reasons:

Enormous number of installations. According to the Municipal Solid-State Lighting Consortium, there are more than 25 million streetlights operating in the U.S., in addition to numerous area luminaires.

Prime real estate for LED adoption. In its 2014 energy savings report, DOE estimated the total installed base of U.S. outdoor street and area LED luminaires at more than 3.3 million, and, citing rapid expected growth, forecasted LEDs would achieve nearly total conversion of this market by 2025.

Safety and security. Street and area lighting’s primary purpose is to provide functional illumination for nighttime activity, with safety and security being a critical part of the job description. Therefore, it is essential that the luminaire produce appropriate maintained light levels and maintain its distribution.

Demanding application. As outdoor lighting, LED street and area lighting operates in diverse weather, dirt and other environmental conditions. As such, it is more susceptible to maintenance factors such as lumen and dirt depreciation.

LED has changed perception of maintenance. Many end users consider LEDs as “install and forget” light sources, accepting significant risk. While some LED luminaires have features that make them prone to dirt accumulation, such as a sealed optical cavity, and low radiated heat, which makes them less likely to bake in substances such as particulates, they should be periodically cleaned if operating in dirty environments. However, luminaire cleaning typically occurred during relamping; if there is no relamping, the luminaire is often not cleaned. As a result, if the LED luminaire lasts as long as claimed, luminaire dirt depreciation could be a more significant light loss factor than typically accounted for in lighting design.

The problem is we know very little about how LED luminaires perform over their entire life, as the first generation of installed luminaires is just now approaching initially projected end of life, and documented field experiences over a full life cycle are slim. What we have learned is troubling.

Let’s look at two projects, both DOE Gateway projects. In the first, DOE tested LED lighting installed in September 2008 on the I-35W Bridge in Minneapolis. DOE found dirt depreciation averaged 4% after 5,000 hours (about 1.1 years) and 12% after 20,300 hours (about 4.6 years). This is similar to what we’d find with an enclosed and gasketed outdoor luminaire operating in a “clean” outdoor environment as published in IES-RP-36, though again, cleaning is less likely to occur with LED than with HID. If the dirt depreciation continued to track in line with RP-36, dirt depreciation would reach 20% after eight years—two-thirds of the depreciation indicating replacement is required, if L70 is used as the determinant for end of useful life.

In the second project, which evaluated street lighting in Kansas City, MO, DOE found that seasonable variables such as temperature and foliage produced a variation in light level on the ground by as much as 20 percent during the year. That means it is possible an outdoor luminaire could reach its L70 point, at least during certain times of the year, far earlier than believed.

This depreciation is not troubling in itself. Dirt accumulates on luminaires over time in outdoor environments. UV radiation, precipitation, humidity, particulates and fluctuations in temperature all act together to degrade lenses and other luminaire components. What is troubling is that there is a likelihood the luminaires are no longer cleaned or inspected, creating a risk to light levels and uniformity.

Designers need to account for actual dirt depreciation in their designs and advise the clients about the risks of failing to inspect and clean their luminaires on a periodic scheduled basis. What designers need is good information to be able to predict long-term effects. As a result, as chair of the joint IES/NALMCO Maintenance Committee, I called on the industry on behalf of the committee to conduct research to help quantify depreciation in LED outdoor luminaires.

The first effort was undertaken by IES, which engaged Virginia Tech Transportation Institute (VTTI) to study and quantify luminaire dirt depreciation among a range of LED roadway luminaire types. The resulting report, IES-RES-1-16, was an excellent start to understanding the issue and developing recommendations. The study generally characterized luminaires and assessed typical manufacturer recommendations for cleaning (if provided at all). VTTI concluded that dirt depreciation is different for LED luminaires than HID, issued cleaning recommendations, and estimated minimum potential dirt depreciation rates while also warning of potential changes in light distribution and uniformity. Specifically, VTTI presented a light loss factor of +1% dirt depreciation and +1% uniformity change per year for luminaires with flat glass luminaire optics, and +3% per year dirt depreciation and no uniformity change for luminaires with no luminaire optics. These general values may then be tweaked based on the type of optics used. For more information, visit http://bit.ly/2oNGeM2.

With this background, we can now talk about the study PNNL will be conducting for DOE. According to the DOE’s website, “This study aims to help identify different contributors to lumen depreciation and chromaticity shift, while also adding to the evidence of dirt depreciation over time for a wide variety of materials and luminaire designs. The ultimate goal is to provide public guidance on material selection and luminaire design, to better inform the next generation of LED luminaires.”

To complete the study, PNNL is seeking LED street and area lights. (The call went out in February, but as of mid-April, the webpage was still asking for equipment.) Though any luminaire with 10,000-plus hours of operation will be considered, ideally luminaires will be identified that are nearing end of useful life. Any type of street or area luminaire will be considered, as long as a performance baseline for it is available, such as a preinstallation photometric test or an equivalent new luminaire that can be evaluated for comparison. PNNL must also be provided total hours of operation within +/- 500 hours, location, and number of luminaire cleanings, if any. The luminaire ideally is still functional. Multiple luminaires can be submitted, though only one used and one unused luminaire is satisfactory. PNNL will compensate for this equipment by providing similar make and model luminaires as replacement, as well as sharing specific results.

I applaud the DOE for prioritizing maintenance by conducting research into factors resulting in depreciation and optical changes over luminaire life. I am hoping the results will facilitate industry best practices, better-informed design, and more knowledgeable end users.


Norma Frank

Norma J. Frank

Norma J. Frank, LC, CLMC, is CEO of Colorado Lighting (www.coloradolighting.com), a lighting management company providing energy management solutions for clients across the U.S., and chair of the IES/NALMCO Maintenance... More info »