Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
1 Chapter 1 â Introduction Research Objectives The lighting industry has changed dramatically over the past decade. The optical system design of legacy high-intensity discharge (HID) luminaires was restricted to the lamp, refractor, and reflector design, which had limits in the distribution of the light, controls, and adaptability. Roadway luminaires have moved beyond this design methodology to include the vast possibilities presented by solid-state lighting (SSL). At present, in the form of light emitting diodes (LED), SSL uses lower energy, reduces maintenance, improves color, and can be easily dimmed and controlled. The SSL roadway luminaire has developed very rapidly, and design standards and methods of applying this new technology has lagged in national recommendations. For example, American Association of State Highway Transportation Officials (AASHTO) target light levels are calculated over a grid limited to the traveled roadway. Any light that lands outside of the calculation grid is not considered although that surround light may provide a safety benefit. With greater and more precise control over the optical distribution, SSL luminaire light levels beyond the calculation grid may be dramatically reduced, and the roadway design will still meet AASHTO criteria, although an element of safety might be missing. Therefore, research is needed to investigate the application of AASHTO criteria to SSL roadway lighting and, if the results dictate, provide guidance for changes or additions to those criteria. Additional research is also needed to explore the benefits and challenges of adaptive lighting controls and provide further guidelines for its use, as well as on the environmental and health effects of SSL roadway lighting. The objectives of this project are to develop more comprehensive guidelines in AASHTO-standard format for the application of roadway lighting related to the widespread adoption of SSL, and to identify gaps in knowledge where possible future research will enhance these guidelines. This research complements and supplements the ongoing efforts of the AASHTO Roadway Lighting Committee on the usage of SSL systems. Objectives of this Guide This guide uses the SSL systems research from this project and offers guidance on implementing SSL technology, specifically as it relates to the current American Association of State Highway and Transportation Officials (2018). For further details on the recommendations in this guide, see the NCHRP 05-22 Final Report. Sample specifications for LED roadway lighting as well as tunnel lighting are included in this guide to offer suggestions relating to testing, product review, luminaire and electrical components, and performance expectations. Sample specification requirements can then be incorporated in whole, or in part, to standard specifications already developed by many Departments of Transportation (DOTs) and agencies. Differences in Solid State Technologies Until recently, most roadway lighting was HID, specifically high-pressure sodium (HPS). There are many differences between SSL and HID beyond the physical equipment differences: (1) light distribution, (2) lighting output control (dimmability), (3) spectral power distribution (SPD), and (4) efficiency of lighting production suitable for human perception (luminous efficacy). Research comparing SSL to HID for roadway lighting has identified some benefits in terms of energy consumption, luminous efficacy, color rendering, and adaptability.
2 SSL uses semiconductors such as LEDs as a source of illumination rather than filaments or gas plasmas. In the last decade or so, white SSL has exceeded the efficacy of other lighting which manifests in less energy lost in the form of heat. LEDs operate by applying voltage to the leads of a diode, which causes electrons to accelerate. These electrons move through the diode material and recombine with electron holes within the device, which causes a deceleration and in turn releases energy in the form of photons. In broad-spectrum white SSL, the emitted photons create short wavelength light (blue â typically about 450 to 480 nanometers [nm]), which is phosphor converted to other colors or shades resulting in a white- appearing light. This white-SSL process occurs at a lower temperature than other light sources allowing for a longer life for the light source compared to gas or filament lights as well as lower costs for dimming and color. LEDs also emit light from a very small area on the diode in a specific direction (as opposed to omnidirectional), which results in more flexibility in terms of optical control. HIDs and LEDs differ greatly in terms of luminous efficacy, or how well a light source produces visible light as a ratio of luminous flux to power. A metal halide HID lamp ranges from 65 to 116 lumens per watt, which equates to an efficacy of between 9 and 17% percent overall. HPS lamps are slightly better, with a range of 85 to150 lumens per watt or an efficacy of 12% to 22% (Rodrigues, Almeida, Soares, Jorge, Pinto, & Braga, 2010; Stouch, 2016) and are generally regarded to be highly efficient compared to fluorescents, arc lamps, and incandescents resulting in the prevalence in existing roadway lighting. However, modern LED lamps have been found to produce from 37 to as high as 303 lumens per watt, or a range of 0.66 to 43%(Cree, 2014; Stouch, 2016). Luminous efficacy can also be weighted by photopic and scotopic response curves because the eye is more sensitive to certain wavelengths depending on light level(Rodrigues et al., 2010). For example, at very low light levels a source with more âblueâ content may provide greater eye response. Beyond efficacy and lumen output when compared to HID lamps, LEDs have a much better coefficient of utilization distributing light where it needs to be as result of the small point sources (LED chips) and compact optical system. Most LED optical systems use very effective computer aided design and manufacturing which adds to their effectiveness. LEDs can be customized for their intended use. LEDs can be acutely dimmed to specific levels(Dyble, Narendran, Bierman, & Klein, 2005; Fusheng Li, 2009; Gil-de-Castro, Moreno-Munoz, & Rosa, 2013; Jin, Jin, Chen, Cen, & Yuan, 2015), be turned on and off rapidly without a need for a warm-up period(Gaston, Davis, Bennie, & Hopkins, 2012; Wang & Liu, 2007), be fine-tuned for color output during the manufacturing process to achieve a range of CCTs(Liu & Luo, 2011), and precise cutoffs can be implemented to increase the control of the lightâs focus(Timinger & Ries, 2008). In general, they are said to be better in terms of photometric and economic performance compared to HPS light sources. As of 2012, many roadway-lighting-focused organizations such as Illuminating Engineering Society (IES), International Committee on Illumination (CIE), Industrial Lighting Products, the U.S. Department of Energy (DOE), the American National Standards Institute (ANSI), Transportation Association of Canada, and the National Electrical Manufacturers Association (NEMA) have directed their focus to developing guidelines for LED device performance and predictions. LEDs are said to offer from 50,000 to 100,000 hours of use before substantial degradation due to overheating or prolonged use(Evans, 1997; Tetra Tech EM Inc., 2010), although the accuracy of these claims is still undetermined. In terms of life expectancy, LEDs far outperform HID lamps(Josefowicz, 2012). The spectral outputs of HIDs and LEDs differ greatly in terms of wavelength content but are often identified by the correlated color temperatures (CCT) measured in Kelvin (K). The CCT values are related in appearance to the absolute temperature of a black body radiator (incandescent). Figure 1 shows the SPDs for HID and LED luminaires of a variety of CCTs. Shown are an HPS luminaires with a 2100 K CCT, 3500 K LED, and 6000 K LED light sources. Note that the CCT of light sources cannot be
3 compared across different technologies; as such, the CCT of the HPS cannot be compared to that of an LED. The critical aspect of the two methods of generating light is that the phosphor-converted LED generally has a broad-spectrum output, and the HPS relies on the energy band conversion of electrons and has significant spectral gaps in the light output. Moreover, the SPD of LEDs can be tailored to achieve specific CCTs. One of the difficulties with the CCT is that it has become (wrongly) a de facto industry standard to describe the light output and spectral content of the LED. Figure 1. Spectral Power Distribution of Overhead-Lighting Types (Gibbons, Meyer, Terry, Bhagavathula, Lewis, Flanagan, & Connell, 2015) The color rendering capabilities and luminous efficacy of LEDs give them an edge in visibility performance. Because most of the detections in a roadway are foveal (Gibbons, Edwards, Bhagavathula, Carlson, & Owens, 2012) and based on the cone detectors only (cones make up three of the photoreceptors in the eye), particularly at high speeds, the ability of an LED light source, which is a much broader spectrum source than an HPS source, to render colors provides color contrast. This additional color contrast has been shown to provide detection benefits(Terry, 2011). Because LEDs render color so well, white-light-emitting LEDs have been considered as direct replacements for HPS lighting on streets and roadways. Because nighttime driving inherently involves low-light vision, typically luminance contrast prevails; however, color contrast can vastly improve an objectâs visibility(Lutkevich, McLean, & Cheung, 2012), especially when a source such as an LED has strong color-rendering capabilities closer to that of natural light(Gibbons, Li, & Meyer, 2015). Figure 2 shows some of the variations that can occur in detection distances for various CCT source types (the x-axis shows source type, those shown in wattage The spectral content of a light source has an impact on how well a driver can detect objects. Broader spectral content may result in better visibility.
4 are HPS (250 watt (W) and 400W) and those shown by CCT in K are LED, and system light output of 25%, 50%, or 100%, and ASYM shown was for and asymmetrical distribution which was also assessed). Throughout this document, the figures presented may have error bars associated with them. These bars represent the magnitude of the standard error associate with analysis results. The standard error represents uncertainty that is associated with any data collection effort. When the error bars overlap, meaning that they are in the same range (e.g., the 3500K LED in Figure 2), there is no statistical significance; those that are separated (100% light output compared to 25% light output for the 4100K LED) show statistical significance. Figure 2. Mean Detection Distance Differences for Various Sources (Clanton & Associates Inc., 2014) LEDs have some drawbacks. For example, the advent of LED lighting invites the use of more white or blue CCT lighting on roadways. The color-rendering abilities of these broader spectrum sources is much greater than the yellow associated with HPS or Low-Pressure Sodium (LPS) light sources. However, some research shows that driver vision may be negatively affected due to the adaptation of the eye when moving from darkness into an area lit by a white or blue light source(Boyce, 2009; Goldstein, 2010). Other concerns include performance in adverse weather conditions, sky glow, and health.
5 From a lighting design and specification standpoint, SSL requires a much different approach than HID technology. The life of the source is rated differently. HID sources are typically rated by the time in operation when 50% of the lamps had failed where LED sources are rated by when the lumen depreciation declines to 70% of the initial output. Figure 3 shows test data used for predicting the life of an LED luminaire. The testing is done for three different temperatures, a calculated life is estimated, and a rated life determined (which can be no more than six times the test duration). This testing however only predicts the life of the LED, and the lifetime of the driver is related to the case temperature. Figure 4 shows an example of driver lifetime as it relates to case temperature. Because the driver is an electronic device, it can be evaluated by mean time between failure (MTBF) testing to determine reliability and product failure rates. Operating LEDs and drivers at higher temperatures can significantly reduce expected life. Figure 3. TM-21 Inputs for Predicting Lumen Maintenance Prior research, as well as research for this project, show differences in driver detection distances, with 4000K sources showing some advantages.
6 Figure 4. Example of Driver Life Determined by MTBF Analysis (Tcase is the temperature of the case of the driver) The rated life of an LED luminaire is based on the point in time when the lumen output of the LED has reduced to 70% of its initial lumen rating. Some of these factors are quantified and included in the LED test data performed in accordance with (IES, 2011) TM-21 Projecting Long Term Lumen Maintenance of LED Light Sources. Other data are usually available from the luminaire manufacturers. Further use of these data is discussed in Chapter 11. Established methods of classifying photometric distribution (IES Type II, III, IV, V and short, medium, and long) are not as relevant with LED luminaires. Distribution classification methods that were often used for grouping similar HID products from an optical distribution standpoint often do not work well as descriptors for LED products. An example of an HPS luminaire and an LED luminaire distribution, both classified as Type II, and their differences is shown in Figure 5. There is also great variation between available products, as well as subjective acceptance of color and brightness, variety of control options, and different failure modes due to temperature and voltage variations. With HID streetlights, one could simply define the wattage and optical distribution (e.g., Type II, cutoff) and would get similar results from different luminaires. LED luminaires, however, require a closer review and assessment. Life of an LED can be considered as the point when the lumen output depreciates by 30% or when one of the components fails. Because the standard distribution types are often quite different with LED sources, they should not be solely relied upon to select replacements for HID sources.
7 Figure 5. Comparisons of HPS and LED luminaires using Standardized Distribution Ratings Key Differences Between Solid State and Traditional Light Sources â¢ Spectral content and effectiveness vary. CCT only partially describes LED source spectral content. â¢ Photometric distribution can greatly vary with HID sources as well as among LED products. â¢ Rated life of luminaires is based on different performance requirements. â¢ Solid state components require different considerations relating to electrical components and operations. â¢ Solid state luminaires allow for much greater flexibility regarding control, output, and optical distribution. These flexibilities offer potential benefits over HID and HPS. â¢ Solid state luminaires can create impacts relating to glare, environmental impacts, and subjective preferences which should be understood and mitigated.