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1 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 possi- bilities presented by solid-state lighting (SSL). At present, in the form of light-emitting diodes (LEDs), SSL uses less 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 have lagged in national recommendations. For example, AASHTOâs 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 optical distribution, the light levels of SSL luminaires 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 (a) the benefits and challenges of adaptive lighting controls, so as to provide further guidelines for its use, and (b) the environmental and health effects of SSL roadway lighting. The objectives of NCHRP Project 05-22, âGuidelines for Solid-State Roadway Lighting,â were (1) to develop more comprehensive guidelines in AASHTO standard format for the application of roadway lighting related to the widespread adoption of SSL and (2) 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 use of SSL systems. Objectives of This Guide NCHRP Research Report 940: Solid-State Roadway Lighting Design, Volume 1: Guidance uses the research on SSL systems from NCHRP 05-22 and offers guidance on implementing SSL technology, specifically as it relates to the current AASHTO Roadway Lighting Design Guide (2018). For further details on the recommendations in this volume, see NCHRP Research Report 940, Volume 2: Research Overview. This guide includes sample specifications for LED roadway lighting as well as tunnel lighting to offer suggestions relating to testing, product review, luminaire and electrical components, C H A P T E R 1 Introduction
2 Solid-State Roadway Lighting Design and performance expectations. Sample specification requirements can then be incorporated in whole or in part within 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 differences in physical equipment: â¢ Light distribution, â¢ Control of lighting output (dimmability), â¢ Spectral power distribution (SPD), and â¢ Efficiency of lighting production suitable for human perception (luminous efficacy). Research comparing SSL with HID for roadway lighting has identified some benefits in terms of energy consumption, luminous efficacy, color rendering, and adaptability. SSL uses semiconductors such as LEDs as a source of illumination, rather than filaments or gas plasmas. In the past 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, typically about 450 to 480 nm), which is phosphor converted to other colors or shades, which results in a white-appearing light. This white-SSL process occurs at a lower temperature than other light sources, which allows for a longer life for the light source as compared with 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 to 150 lumens per watt or an efficacy of 12% to 22% (Rodrigues et al. 2011; Stouch Lighting n.d.) and are generally regarded as being highly efficient compared with fluorescents, arc lamps, and incandescents. These qualities account for the prevalence of HPS lamps 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 Lighting n.d.). 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. 2011). For example, at very low light levels, a source with more blue content may provide greater eye response. Beyond efficacy and lumen output, as compared with HID lamps, LEDs have a much better coefficient of utilization; that is, LEDs distribute light where it needs to be as a 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: â¢ They can be acutely dimmed to specific levels (Dyble et al. 2005; Li et al. 2009; Gil-de-Castro et al. 2013; Jin et al. 2015). â¢ They can be turned on and off rapidly without a need for a warm-up period (Gaston et al. 2012; Wang and Liu 2007). â¢ They can be fine-tuned for color output during the manufacturing process to achieve a range of correlated color temperatures (CCTs) (Liu and Luo 2011).
Introduction 3 â¢ Precise cutoffs can be implemented to increase the control of the lightâs focus (Timinger and Ries 2008). In general, LEDs are said to be better than HPS light sources in terms of photometric and economic performance. As of 2012, many organizations focused on roadway lighting have directed their focus to developing guidelines for LED device performance and predictions. These organizations include the Illuminating Engineering Society (IES), the International Commission on Illumination (CIE), Industrial Lighting Products, the U.S. Department of Energy (DOE), the American National Standards Institute (ANSI), the Transportation Association of Canada (TAC), and the National Electrical Manufacturers Association (NEMA). LEDs are said to offer from 50,000 to 100,000 hours of use before substantial degradation from overheating or pro- longed 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 CCT measured in kelvins (K). The CCT values are related in appearance to the absolute temperature of a black body radiator (incandescent). Figure 1 shows the SPD for HID and LED luminaires of a variety of CCTs. Shown are an HPS luminaire with a 2100K CCT and 3500K and 6000K LED light sources. Note that the CCT of light sources cannot be compared across different technologies; that is, the CCT of an HPS cannot be compared with 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, whereas 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 for describing the light output and spectral content of LEDs. 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 et al. 2012) and based solely on the cone detectors (the cones constitute three of the photoreceptors in the eye), particularly at high speeds, the ability of an LED light source 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. Source: Gibbons et al. (2015b). Figure 1. Spectral power distribution of overhead lighting types.
4 Solid-State Roadway Lighting Design (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 lightâemitting LEDs have been considered as direct replacements for HPS lighting on streets and roadways. Because nighttime driving inherently involves low-light vision, luminance contrast typically prevails; however, color contrast can vastly improve an objectâs visibility (Lutkevich et al. 2012), especially when a source such as an LED has strong color-rendering capabilities closer to that of natural light (Gibbons et al. 2015a). Figure 2 shows some of the variations that can occur in detection distances for various CCT source types (the x-axis shows source typeâthose shown in wattage are HPS (250W and 400W) and those shown by CCT in kelvins are LEDâand system light output of 25%, 50%, or 100%). Asymmetrical distribution (ASYM) was also assessed. 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 by 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. From the standpoint of lighting design and specification, SSL requires a much different approach from that of HID technology. The life of the source is rated differently. HID sources are typically rated by the time in operation at which 50% of the lamps have failed, whereas LED sources are rated by the point at which lumen depreciation declines to 70% of the initial output. Some of these factors are quantified and included in LED test data acquired in accordance with Projecting Long Term Lumen Maintenance of LED Light Sources (IES TM-21-11) (IES 2011). Other data are usually available from luminaire manufac- turers. Further use of these data is discussed in Chapter 11. Figure 3 shows test data used for predicting the life of an LED luminaire. The testing is done at three different temperatures, a cal- culated life is estimated, and a rated life (which can be no more than Error Bars Throughout this guide, the figures presented may have error bars associated with them. These bars represent the magnitude of the standard error associated with analysis results. The standard error represents the 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 with 25% light output for the 4100K LED) show statistical significance. Source: Clanton and Associates Inc. (2014). M ea n De te cti on D is ta nc e (ft ) Figure 2. Mean detection distance differences for various sources. Prior research, as well as the research conducted for this project, shows differences in driver detection distances, with 4000K sources showing some advantages.
Source: IES (2011). Figure 3. TM-21 inputs for predicting lumen maintenance.
6 Solid-State Roadway Lighting Design 6 times the test duration) is determined. This testing only predicts the life of the LED, however; 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 testing mean time between failures (MTBF) to deter- mine reliability and product failure rates. Operating LEDs and drivers at higher temperatures can significantly reduce expected life. Established methods of classifying photometric distribution (IES Types II, III, IV, and 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 the standpoint of optical distribution often do not work well as descriptors for LED products. Figure 5 shows an example of the distributions from an HPS luminaire and an LED luminaire, both classified as Type II. There is also great variation between available products as well as subjective acceptance of color and brightness, a variety of control options, and different failure modes resulting from temperature and voltage variations. With HID street- lights, one could simply define the wattage and optical distribution (e.g., Type II, cutoff) and obtain similar results from different luminaires. LED luminaires, however, require a closer review and assessment. Note: Tcase = temperature of the case of the driver. Figure 4. Example of driver life determined by MTBF. The life of an LED can be considered as the point at which the lumen output depreciates by 30% or one of the components fails. Because the standard distribution types are often quite different with LED sources, they should not be solely relied upon when replacements for HID sources are being selected. Note: FC = footcandles; Typ = typical. Figure 5. Comparisons of HPS and LED luminaires using standardized distribution ratings.
Introduction 7 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 vary greatly with HID sources as well as among LED products. â¢ The 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 that should be understood and mitigated.