1

Introduction

CONTEXT

Illumination is one of a number of modern energy services provided by electricity, a premium energy carrier with the advantage that it can be transmitted over large distances and converted on-demand at the point-of-use. Annual energy consumption in the United States is roughly 100 quadrillion Btu1 (quads) (Figure 1.1) (NRC, 2010b). Of this, roughly 40 percent is used to generate electricity, the vast majority of which is harnessed and sold to end users (3,750 TWh in 2010) in the residential (38.5 percent), commercial (35.4 percent), industrial (25.9 percent), and transportation (0.2 percent) sectors (EIA, 2011). General lighting for illumination consumes approximately 20 percent of the electricity used in the United States, accounting for between 7 to 19 percent of all residential electricity use and 31 to 36 percent of all commercial electricity use (Azevedo et al., 2009). Reducing energy consumption through conservation (i.e., using less of an energy service), improved thermodynamic efficiency, or greater efficacy (i.e., using less fuel) in delivering energy services (e.g., miles per gallon) has been the focus of a number of federal and state government programs of tax incentives, grants and contracts for research and development, standards (e.g., for appliances and vehicles), and building codes.2 The extraction of energy resources and their processing, conversion, delivery, and use can have negative impacts on human health and the environment (NRC, 2010a). To the extent such impacts scale with (i.e., are proportional to) the quantity of energy consumed, improving the efficiency of end-use of electricity can mitigate them. This makes improvements in end-use technologies a critical aspect of U.S. energy policies.

The standard incandescent light bulb, in wide use in the residential sector, still works mainly as Thomas Edison invented it, with more than 90 percent of the electricity consumed being converted to heat. The ability to dramatically decrease the energy used for lighting requires new technologies that use less power but are also affordable and capable of producing high-quality light. Given the availability of newer lighting technologies that convert a greater percentage of electricity into useful light, there is a lot of potential to decrease energy used (i.e., the amount of electricity) for lighting. Although technologies such as compact fluorescent lamps (CFLs) have emerged in the past few decades that will help achieve the goal of increased energy efficiency, solid-state lighting (SSL) stands to play a large role in dramatically decreasing our energy consumption for lighting.

Electricity end-use is part of a larger system, and it is instructive to consider the electricity grid and its overall efficiency. The power plants by which electricity is generated in the United States operate such that roughly two-thirds of the primary energy in the fuel is lost in electricity generation.3 Losses also occur in electricity transmission and distribution (6.5 percent), in which electricity is converted to heat, and finally when converting electricity to energy services such as illumination, space conditioning (i.e., heating, ventilation, and cooling), and cooking services (Figure 1.2).

Since the advent of the incandescent bulb, a number of new lighting technologies, discussed in detail below, have been demonstrated and in some cases entered widespread deployment to provide general and specialized illumination. A recent entrant is SSL. At the epicenter of SSL sits the semiconductor. In addition to using the semiconductor in electronic devices, scientists have been able to make the semiconductor emit light (Holonyak and Bevacqua, 1962). The most common semiconductor-based light source is the light-emitting diode (LED). If organic materials are used to fabricate the LED, it is called an organic LED (OLED). These two technologies are capable of creating “light bulbs”

________________

1 Btu stands for British thermal unit and is a measure of energy. For instance, 1 gallon of gasoline would release approximately 124,000 Btu.

2 A review of such programs can be found in NRC (2010b, pp. 264-269).

3 The estimated two-thirds loss is based on using a single number for the so-called thermal efficiency of the fleet of power generators (called thermal because it is the amount of heat converted to useful work). There can be large variations among the different plants, however. Natural gas combined cycle plants can, for example, have efficiencies above 50 percent.



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1 Introduction CONTEXT decrease the energy used for lighting requires new technolo- gies that use less power but are also affordable and capable Illumination is one of a number of modern energy services of producing high-quality light. Given the availability of provided by electricity, a premium energy carrier with the newer lighting technologies that convert a greater percent- advantage that it can be transmitted over large distances and age of electricity into useful light, there is a lot of potential converted on-demand at the point-of-use. Annual energy to decrease energy used (i.e., the amount of electricity) for consumption in the United States is roughly 100 quadrillion lighting. Although technologies such as compact fluorescent Btu1 (quads) (Figure 1.1) (NRC, 2010b). Of this, roughly lamps (CFLs) have emerged in the past few decades that will 40 percent is used to generate electricity, the vast majority help achieve the goal of increased energy efficiency, solid- of which is harnessed and sold to end users (3,750 TWh in state lighting (SSL) stands to play a large role in dramatically 2010) in the residential (38.5 percent), commercial (35.4 per- decreasing our energy consumption for lighting. cent), industrial (25.9 percent), and transportation (0.2 per- Electricity end-use is part of a larger system, and it is cent) sectors (EIA, 2011). General lighting for illumination instructive to consider the electricity grid and its overall effi- consumes approximately 20 percent of the electricity used ciency. The power plants by which electricity is generated in in the United States, accounting for between 7 to 19 percent the United States operate such that roughly two-thirds of the of all residential electricity use and 31 to 36 percent of all primary energy in the fuel is lost in electricity generation.3 commercial electricity use (Azevedo et al., 2009). Reducing Losses also occur in electricity transmission and distribution energy consumption through conservation (i.e., using less of (6.5 percent), in which electricity is converted to heat, and an energy service), improved thermodynamic efficiency, or finally when converting electricity to energy services such greater efficacy (i.e., using less fuel) in delivering energy ser- as illumination, space conditioning (i.e., heating, ventilation, vices (e.g., miles per gallon) has been the focus of a number and cooling), and cooking services (Figure 1.2). of federal and state government programs of tax incentives, Since the advent of the incandescent bulb, a number of grants and contracts for research and development, standards new lighting technologies, discussed in detail below, have (e.g., for appliances and vehicles), and building codes.2 The been demonstrated and in some cases entered widespread extraction of energy resources and their processing, conver- deployment to provide general and specialized illumina- sion, delivery, and use can have negative impacts on human tion. A recent entrant is SSL. At the epicenter of SSL sits health and the environment (NRC, 2010a). To the extent such the semiconductor. In addition to using the semiconductor impacts scale with (i.e., are proportional to) the quantity of in electronic devices, scientists have been able to make the energy consumed, improving the efficiency of end-use of semiconductor emit light (Holonyak and Bevacqua, 1962). electricity can mitigate them. This makes improvements in The most common semiconductor-based light source is the end-use technologies a critical aspect of U.S. energy policies. light-emitting diode (LED). If organic materials are used The standard incandescent light bulb, in wide use in the to fabricate the LED, it is called an organic LED (OLED). residential sector, still works mainly as Thomas Edison These two technologies are capable of creating “light bulbs” invented it, with more than 90 percent of the electricity con- sumed being converted to heat. The ability to dramatically 3 The estimated two-thirds loss is based on using a single number for the so-called thermal efficiency of the fleet of power generators (called thermal 1 Btu stands for British thermal unit and is a measure of energy. For because it is the amount of heat converted to useful work). There can be large instance, 1 gallon of gasoline would release approximately 124,000 Btu. variations among the different plants, however. Natural gas combined cycle 2 A review of such programs can be found in NRC (2010b, pp. 264-269). plants can, for example, have efficiencies above 50 percent. 6

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INTRODUCTION 7 Industry Transportation STUDY ORIGIN 28% 31% Congress recognized the potential for energy savings in (28 quads) (30 quads) the lighting sector in the Energy Independence and Security Act of 2007. Congress requested that the Department of Energy (DOE) contract with the National Research Council (NRC) to conduct a study to assess the status of SSL as a technology. The statement of task is separated into three main sections (Box 1.1): a review of the development of SSL technology and products, a discussion of future impacts, and the implications of the study for decision-making. The main Commercial tasks for the study were to investigate the following: Residential Buildings Buildings 19% • Status of SSL research, development, demonstration, 23% (18 quads) and commercialization in the United States; (22 quads) • Timeline for commercialization of this technology as FIGURE 1.1 Total primary energy consumption in the United a replacement technology for current light sources; States, 2010 (in quadrillion Btu, or quads). Total U.S. primary • Past, current, and future cost trajectories for SSL; energy use in 2010 was 98.0 quads. • Consumer acceptance of and potential benefits from 1.01.eps SSL; • Potential barriers to success of the industry, both in research and development (R&D) and manufacturing or “lamps” that are much more efficient and have a much and commercialization; longer life span than either incandescent bulbs or compact • International aspects of SSL; fluorescent bulbs. LEDs and OLEDs alone cannot be used • Applications for the technology, both current and for illumination applications; additional electrical, thermal, future; structural, and optical components are necessary to create • Unintended consequences of SSL in different SSL products. Throughout the rest of the report, the term applications; “SSL products” will be used to describe integrated LED or • Application of lessons learned from the commercial- OLED lighting systems. In addition, LEDs and OLEDs are ization of CFLs to the roll out of SSL; and not limited to the current shape of existing lighting technolo- • Recommendations to DOE for research, develop- gies and, therefore, have the potential to dramatically alter ment, and deployment activities. how we integrate light into our buildings and how our future “light bulbs” and luminaires might look and behave. FIGURE 1.2  Example of how end-use efficiency influences overall fuel conversion efficiency. In this example, typical for residential use of electricity for illumination, the efficiency of converting the chemical energy stored in coal to the electricity entering a building is about 33 percent (0.35 × 0.94). But after accounting for the low efficiency of the incandescent light bulb, the efficiency of converting chemical energy to light energy is only 1.3 percent. All values are approximate. SOURCE: Updated and adapted from National Research Council (2010b).

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8 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING BOX 1.1 Statement of Task The National Research Council (NRC) will appoint a committee to carry out this study and provide a report on the status of advanced solid-state lighting (SSL), in particular light-emitting diodes and organic light-emitting diodes. The report will provide an assessment of the current status of development of SSL products, a discussion on the future impacts of SSL, and a consideration of the study’s implications for the U.S. Department of Energy (DOE) and other agencies. Specifically, the committee will focus on the following three overarching tasks. (1) Review the Development of SSL Technology and Products The committee will assess: •  ast and current cost evaluations for SSL in relation to traditional lighting technologies; P •  he status of SSL research, development, demonstration and commercialization in the U.S.; T •  otential barriers to development and the prospects for overcoming them; P •  he status of SSL activities internationally and their implications for the manufacturing of SSL technologies in the U.S.; T •  he cost, lifetime, reliability, and consumer satisfaction associated with SSL for both indoor and outdoor lighting applications and how these T factors compare to traditional lighting technologies (incandescent, fluorescent, and high intensity discharge); •  he market-based performance attributes necessary for SSL based on review of on-going activities. T (2) Discussion of SSL Future Impacts The committee will estimate: •  he time line for the commercialization of SSL (and other possible technologies) that could replace current incandescent and halogen incandescent T lamp technology and meet the minimum standards required in Section 321 of the Energy Independence and Security Act of 2007; •  he barriers to widespread adoption of SSL technologies and strategies needed to overcome these barriers; T •  he benefits for consumers if SSL development and deployment is successful and the impact if these barriers are not fully overcome, particularly T as it relates to the new minimum efficiency standard taking effect; •  otential unintended consequences of SSL deployment, such as presented by traffic lights using SSL lamps that did not generate enough heat P to melt ice that built up on them. (3) Study Implications The committee will analyze: •  essons from the experience with the commercialization of compact fluorescent lighting and how that may affect potential proactive initiatives L by the Department of Energy and other agencies (with legislative direction, such as the Federal Trade Commission [FTC]); and •  ecommendations to the Department of Energy on research, development, and deployment activities, and potential collaborations with market R participants, especially manufacturers. The committee will provide a report to the U.S. Department of Energy, the Committee on Energy and Commerce of the House of Representatives, and the Committee on Energy and Natural Resources of the Senate. As mandated by Energy Independence and Security Act of 2007, the NRC could also provide an updated report by July 31, 2015. With these tasks in mind, the NRC established the Com- INTRODUCTION TO LIGHTING mittee on Assessment of Solid State Lighting (Appendix A) Americans are used to purchasing their lamps (i.e., light composed of diverse experts in the fields of solid-state light- bulbs) as a function of the rating in watts (and “watt equiva- ing, lighting design, human perception of light, industry lents”), a unit denoting the rate at which energy is produced commercialization, and policy to address the statement of or consumed. Intuitively, most people understand how much task. In conducting this study, the committee members relied light a 40 W incandescent lamp provides compared to a 60 W on their own expertise as well as many interactions with or 75 W lamp. As the technological options for lighting shift experts in the field (Appendix B).

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INTRODUCTION 9 away from the incandescent lamp to more energy efficient alternatives such as CFLs and solid-state options (LEDs and OLEDs), the basic terms used for lighting discussions also need to change. Instead of thinking in terms of watts, con- sumers now need to learn a different measurement system, one that tells them how much light a product is going to emit (in absolute terms or per unit of power consumed) instead of the rate at which energy will be consumed. And this is just the beginning of the changes that consumers are likely to see if LED and OLED lighting continue to improve at their current rates. In this section, several key concepts and terms used in the lighting industry are introduced that will be used throughout the rest of the report. Lighting Equipment Lighting designers and engineers use different terms for LUMINOUS FLUX (lumens) lighting equipment than are used in the vernacular. In this FIGURE 1.3  Luminous flux (lumena). report, the engineering terms will be used. A luminaire is 1.03.eps the combination of light fixture hardware, a ballast or driver if applicable, and a light source, commonly called a lamp (i.e., a light bulb). Thus, the term lamp can refer to an incan- descent bulb, a CFL bulb, or an LED replacement “bulb.” This report will use the term lamp. A luminaire consists of, minimally, a lamp holder, commonly called a socket, and the way to connect the socket to the electrical supply. Most fixtures also contain optical elements that distribute the light as desired, such as a reflector, lens, shade, or globe. When needed, fixtures and luminaires contain a ballast or a driver. A ballast is an electronic device that converts incoming electricity to the proper voltage and current required to start and maintain the operation of a lamp. The term driver refers to the corresponding device used in an SSL luminaire. Lumi- naire examples include chandeliers, downlights, table lamps, LUMINOUS INTENSITY (candela) wall sconces, recessed or pendant mounted luminaires, and exterior streetlights. When equipped with lamps, they are 1.04.eps FIGURE 1.4  Luminous intensity (candela). called luminaires. The types of lamps typically encountered are discussed below in the section “Annex.” Metrics for Measuring Light Output the product. Watts describe the amount of electrical power The portion of the electromagnetic spectrum that can be consumed by the product, and lumens describe the rate at perceived by the human visual system is called the visible which it emits light. For example, most 60 W incandescent spectrum. The amount of light, weighted by the sensitivity lamps emit approximately 850 lumens. Similarly, many 13 W of the visual system, emitted by a source per unit time is its CFLs emit 850 lumens. luminous flux (Figure 1.3) and is measured in lumens (lm). Luminous intensity (Figure 1.4) is the luminous flux per This makes lumens one of the appropriate pieces of infor- unit solid angle, evaluated in terms of a standardized visual mation for lamp packaging to help consumers choose the response and expressed in candela. The magnitude of lumi- appropriate replacement lamps. Lumens provide a descrip- nous intensity results from luminous flux being redirected tion most closely related to brightness and should be referred by a reflector or magnified by a lens.4 This measurement to when choosing replacement lamps. A proliferation of is used primarily to describe the specific light intensity and fact sheets and labels has accompanied the recent introduc- tion of new lighting technologies, leaving some consumers 4 The concept of solid angle has a strict geometric definition but can be confused about the relationship between watts and lumens. thought of as a way to describe the focusing and redirecting of a light source That relationship is determined by the energy efficiency of by the lenses and reflectors in the luminaire.

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10 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING distribution of a luminaire. Illuminance is the concentration illuminated variable-sized flat light sources forms, such as of luminous flux incident on a surface (Figure 1.5). The unit sheets or tapes, because the total luminous flux will depend of illuminance is lux (lx), and it indicates the number of on the surface area of the product. lumens per square meter. Lumens per square foot are called The luminous efficacy of a lighting product is the ratio of footcandles (ftc). Whereas luminous flux relates to the total the luminous flux to the total electrical power consumed and output of a lamp or lighting product, illuminance relates to has units of lumens per watt (lm/W). A perfect light source— the amount of light striking a surface or point. Illuminance that is, one that converts all the electricity into visible light— depends on the luminous flux of the light sources and their would have an efficacy of 408 lm/W for an assumed color distances from the illuminated surface. rendering index (CRI; a measure of color quality, discussed Luminance is a measure used for self-luminous or reflec- below) of 90 (Phillips et al., 2007).5 The luminous efficacy tive surfaces (Figure 1.6). It expresses the amount of light, of a typical 60 W incandescent lamp (luminous flux of weighted by the sensitivity of the visual system, per unit area 850 lumens) is such that only 14.2 lumens are emitted per of the surface that is travelling in a given direction and is watt of power drawn by the light bulb. As efficacies increase, expressed as candelas per square meter (cd/m2). When refer- more of the power is used to generate visible light, and this ring to illuminated surfaces, luminance is determined by the leads to a more efficient product. High color quality LEDs incident light (illuminance) and the reflectance characteris- currently are being manufactured with efficacies in the range tics of the surface. For instance, light- and dark-colored walls of 60 to 188 lm/W. It should be borne in mind that efficacy is will have different luminance values when they have the different from efficiency. The efficiency of a lighting system same illuminance. Luminance is a metric used for internally is the ratio between the obtained efficacy and the theoretical maximum efficacy of a light source (408 lm/W for a CRI of 90) and is always expressed as a percentage. Thus, it accounts for the ballast efficiency (if there is one), the light SOURCE OBSERVER source efficacy, and the luminaire efficiency (see Figure 1.7) in one lumped parameter. Thus, incandescent lamps with sys- tem efficacies ranging from 4 to 18 lm/W (depending largely on the wattage of the bulb) will have system efficiencies of only about 0.2 to 2.6 percent. Efficiency does not, however, account for the perceived quality of the light. Using the theo- retical maximum of 408 lm/W and the ranges of efficacies for different lighting technologies leads to the ranges of system efficiencies shown in Figures 1.7 and 1.8. VISIBLE SPECTRUM AND QUALITY OF LIGHT The human eye can generally detect light with wave- lengths between 380 nm (corresponding to blue/violet light) ILLUMINANCE (lux) and 750 nm (corresponding to red light). The spectral power FIGURE 1.5  Illuminance (lux). The amount of light striking a distribution (SPD) determines several important properties surface or point, measured in lux (lx). of a light source. The SPD describes the relative amount of 1.05.eps light per wavelength per unit time emitted by a light source and is often graphically represented, as shown in Figure 1.9. Figure 1.9 shows the SPDs of a halogen lamp, a red, green, SOURCE OBSERVER blue (RGB) LED (which produces white light by combin- ing red, green, and blue component LEDs), an OLED, and a combination of four colored lasers. LUMINANCE The color of emitted light as perceived by people, called chromaticity, is regulated by the spectral composition. The human visual system does not process light on a wavelength- by-wavelength basis. Instead, the brain receives signals from only three input channels, the different cone photopigments found in the eye. Because of this, countless different SPDs can produce light identical in chromaticity. To illustrate this, 5A different choice of color rendering index = 80 would lead to a maxi- FIGURE 1.6  Luminance of a luminaire. mum efficacy of 423 lm/W, and so forth. 1.06.eps

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INTRODUCTION 11 Luminaire Efficacy of Ballast efficiency Lamp efficacy efficiency luminaire 2 to 16 lm/W (no ballast) Incandescent 40% to 90% 4 to 18 lm/W 6 to 30 lm/W (no ballast) Halogen 15 to 33 lm/W 40% to 90% Fluorescent 16 to 90 lm/W 60Hz AC 65% to 95% tubes 40% to 90% 60 to 105 lm/W 9 to 68 lm/W 65% to 95% CFL 40% to 90% 35 to 80 lm/W 4 to 120 lm/W 70% to 95% HID 40% to 90% 14 to 140 lm/W 18 to 170 lm/W 75% to 95% 40% to 95% White LED 60 to 188 lm/W FIGURE 1.7  Efficacy of lamps and luminaires. Values in the left-most column report the range of efficiencies for ballasts and electronic 1.07.eps drivers. Values in the central column report efficacies for different lighting devices. The values on the third column report ranges of luminaire efficiencies. The values on the right-most column report the overall system efficacies of the luminaire. SOURCE: Adapted from Azevedo 7 bitmaps with vector type & rules et al. (2009), where the efficacies for white LEDs were updated to reflect currently commercialized warm and cool white LEDs. NOTE: AC = alternating current; HID = high-intensity discharge; Hz = Hertz; LED = light-emitting diode. FIGURE 1.8  Overall efficiencies of lighting systems (lower bounds) and devices (upper bounds) when assuming that the theoretical maximum lamp efficacy is 408 lm/W; LED = light-emitting diodes; HID = high-intensity-discharge lamps; CFL = compact fluorescent lamps. Lower and upper bounds correspond to the low- and high-efficacy values shown in Figure 1.7. SOURCE: Azevedo et al. (2009). the four widely varying SPDs shown in Figure 1.9 would all white light sources and refers to the temperature of a black- produce light that would appear indistinguishable. body radiator that produces a light perceived to be most On the correlated color temperature (CCT) scale, all similar in chromaticity to the white light source. A typi- four spectra lights in Figure 1.9 are approximately 3,000 K. cal incandescent lamp has a CCT of 2,500 kelvin (K) to C ­ orrelated color temperature is used to describe nominally 3,000 K, whereas office and school lighting is often 4,000 K

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12 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING Halogen RGB LED Lasers OLED FIGURE 1.9  Spectral power distribution from very different light sources that were chosen to produce identically appearing white light. The red, green, blue (RGB) light-emitting diode (LED) produces white light by combining red, green, and blue component LEDs, as does a combination of four colored lasers. 1.09.eps 4 bitmaps with vector labels to 5,000 K. Lower CCTs include more light nearer the red objects illuminated by the source, a property referred to as end of the visible spectrum and are perceived to be “warmer,” color rendering. Although color rendering is determined by the while higher CCTs tend toward the blue end and are per- spectral output of a light source, it cannot be predicted by a ceived to be “cool.” In somewhat of a misnomer, the label- cursory inspection of the shape of the spectral power distribu- ing is indicative of the feelings they evoke rather than their tion, and subtle differences in SPD can produce marked differ- actual temperatures. Although the color of daylight changes ences in the chromaticity of illuminated objects (Ohno, 2005). throughout the day and with location on Earth, it is com- The SPD also determines the LER (i.e., the luminous monly described as having a CCT of 6,500 K. Although CCT efficacy of radiation) of a light source. In technical terms, is widely used among lighting manufacturers and designers, LER is the ratio of luminous flux to radiant flux.6 In simple it only describes one dimension of light source chromaticity, terms, the LER is luminous efficacy that could be achieved in the blue-yellow direction. It does not consider pink-green if the light source was able to convert electricity to light per- shifts in white light color, although Duv is a measure increas- fectly with no losses. The final luminous efficacy of a light ingly used for that information. source is determined from both the LER and the efficiency The most common system for specifying and com- with which the technology converts electricity to light. The municating the precise chromaticity of light sources sensitivity of the human visual system differs for the various uses CIE 1931 (x,y) chromaticity coordinates (CIE, 2004). wavelengths in the visible range. The relationship between The CIE 1931 (x,y) chromaticity diagram is shown in wavelength and the relative sensitivity of the human visual Figure 1.10. The curved edge of the outer horseshoe shape system is described by the spectral luminous efficiency func- on the diagram is the spectrum locus and is comprised of the tion (Vλ) (CIE, 1926) which is shown by the dashed curves colors of monochromatic (only one wavelength) radiation. in Figure 1.11. This function peaks at 555 nm. Light of this The straight edge line is the purple line, and the colors are wavelength has a LER of 683 lm/W, setting the upper bound always a combination of red and blue (not monochromatic). Chromaticity does not provide all of the color information 6 Radiant flux is the amount of electromagnetic energy emitted per unit of interest for general illumination applications. The color time at all wavelengths including visible light and other spectral bands. As of the light itself does not predict the appearance of colored such it will exceed the luminous flux.

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INTRODUCTION 13 for luminous efficacy, as illustrated by the 555 nm laser in panel a. It is important to note that white light cannot achieve 683 lm/W, only light at 555 nm can. Visual sensitivity is markedly lower for light in the short- and long-wavelength regions of the visible spectrum. The other three panels of Fig- ure 1.11 show different SPDs and their corresponding LER. Panel b shows an RGB white LED, panel c shows a different type of white LED (called a phosphor LED, to be discussed later), and panel d shows the SPD of a typical incandescent lamp. As shown, the effect of spectral power distribution on luminous efficacy can be substantial. The incandescent SPD has a relatively low LER because it has a lot of energy in the very long visible and infrared wavelengths, to which the visual system is either minimally or completely insensitive. Although the wavelengths of light to which the eye is most sensitive lie in the middle of the spectrum, a light source composed of light only in the middle of the visible spectrum would not be useful for general illumination. To achieve desirable color characteristics, light of other wavelengths must be present. There is generally a trade-off between luminous efficacy and color quality (Ohno, 2005). Depend- FIGURE 1.10  CIE 1931 (x,y) chromaticity diagram. Numbers ing on the application and goals of a lighting product or lit indicate wavelength of light, in nanometers. SOURCE: Wikipedia environment, a luminaire manufacturer or lighting designer Commons. may choose to prioritize one trait over the other. For example, FIGURE 1.11  Spectral power distribution determines luminous efficacy of radiation (LER). The dashed green curves show the Spectral Luminous Efficiency Function and the black curves are light source’s spectral power distributions. NOTE: RGB = red, green, blue.

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14 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING in a parking garage with lights on 24 hours a day, a speci- no detailed time-series data, and there is a large uncertainty fier may require excellent efficacy and accept subpar color regarding actual lighting electricity consumption. The recent q ­ uality. On the other hand, a museum may require superior lighting market characterization for 2010 from DOE (2012) color and be willing to sacrifice efficacy. estimates that electricity consumption for lighting in the Good color rendering can be achieved with such discon- residential, commercial, industrial, and outdoor stationary tinuous light spectra because of the properties of the other sectors is 175 terawatt hours (TWh), 349 TWh, 58 TWh, two elements in the process of perceiving object colors: the and 118 TWh, respectively, thus totaling 700 TWh for all reflectance of the objects and the absorption of the cone sectors. Another recent estimate, from the Energy Informa- photopigments in the human visual system. All objects, tion Administration (EIA, 2011), suggests that in 2010 the natural or artificial, reflect as a function of wavelength in a residential and commercial sectors used about 499 TWh very broad and continuous manner. The reflectance factors of of electricity for lighting, which corresponds to roughly these objects (the proportion of light reflected as a function 18 percent of the total electricity consumed by both of those of wavelength) do not show sudden spikes or isolated dips sectors.7 The most recent (2006) EIA data available for the in reflectivity across the visible spectrum. Because of this, manufacturing sector show 63 TWh consumed in lighting, the general shape of the reflectance factor can be interpolated which corresponds to 7 percent of all electricity consumed with fairly coarse wavelength sampling. The three cone by manufacturing and 2 percent of all electricity used by the photopigments responsible for color vision have absorption United States (EIA, 2009). functions that are very broad, continuous, and overlapping DOE (2012) reports a breakdown by technology type for in wavelength sensitivity. Each cone type responds to many each sector, estimating that in the commercial sector linear wavelengths, although sensitivity does change depending fluorescent lamps are responsible for 72 percent of light- on the wavelength. The outputs of these photoreceptors do ing electricity consumption, and that the residential sector not signal the wavelength composition of the stimulus to the is still dominated by incandescent lamps (accounting for brain. For instance, a certain level of activity from one cone 78 percent of residential lighting electricity consumption). type could result from a small amount of energy at every In 2010, incandescent lamps accounted for 45 percent of wavelength it is sensitive to or a lot of energy at only one lamps for all sectors in the United States. Linear fluorescent wavelength it is sensitive to. The visual system makes abso- lamps and CFLs together now account for a larger share in lutely no distinction between these two situations (Rushton, terms of number of lamps (48 percent), while LEDs account 1972). The perception of color arises from combining and for 0.8 percent. In terms of shipments, the Buildings Energy comparing the activity among the three cone types. There- Data Book (DOE, 2011) estimates that ENERGY STAR® fore, countless combinations of input wavelengths can lead lamps8 were 15 percent of total shipments of medium screw- to the exact same perception of color. These circumstances, based lamps in 2009. Overall, there is a lack of data on annual in which objects reflect in a fairly predictable manner and market characterization, which are crucial to understand the the visual system interprets incoming light in terms of three impact of current and future policies. broadly sensitive channels, allow a great deal of flexibility for the spectral content of light sources. A recent study dem- CONTENT OF THE REPORT onstrated an extreme case of this in which light sources were developed composed of only four lasers (i.e., sources with Chapter 2 provides an in-depth look at the suite of extremely narrow emission spectra) with high color render- instruments—R&D investments, standards, demonstra- ing quality (Neumann et al., 2011). tion projects, and so forth—by which governments have stimulated more efficient use of energy for illumination. The FINDING: A light source need not emit energy at every chapter also includes a case-study of early-generation CFLs visible wavelength in order to achieve high color quality in order to extract lessons applicable to the introduction of (Figure 1.9). An understanding of the spectral power distribu- SSL products in the market. Chapter 3 discusses the two tion’s effects on luminous efficacy and the color properties of candidate technologies for manufacture of SSL products— a light source will enable SSL developers to optimize energy LEDs and OLEDs—and evaluates the barriers remaining to efficiency while maintaining good color quality. widespread deployment in luminaires, including challenges in research, development, and manufacturing. Included as well is a primer on each technology. Chapter 4 focuses on the CURRENT LIGHTING CONSUMPTION luminaires themselves and the challenges to their assembly IN THE UNITED STATES At the beginning of this chapter, we briefly described the 7 EIA reports that it does not have an estimate for only public street and U.S. electricity use by sector. Concerning the contribution highway lighting, but these applications are considered part of the com- mercial sector in the EIA report and are thus included in the 499 TWh. of lighting to overall electricity consumption, it is generally 8 ENERGY STAR® is a voluntary program created by DOE and the agreed that nearly 20 percent of U.S electricity generation is Environmental Protection Agency to encourage energy efficient products used in lighting (Azevedo et al., 2009). However, there are and buildings through labeling. Discussed in Chapter 2.

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INTRODUCTION 15 and integration into buildings and electricity systems. Chap- either standard incandescent or halogen. The low-voltage ter 5 provides a perspective on the design and installation of MR-16 lamp (Figure 1.14) commonly used in accent, task, LED and OLED luminaires. Chapter 6 discusses the market and display lighting uses halogen technology. barriers to the adoption of SSL products. Fluorescent lamps are available in a range of shapes and sizes. Linear fluorescent lamps are frequently used in commercial spaces (offices, stores) and are typically long ANNEX 4-foot tubes. They are often installed in recessed luminaires There are many different kinds of lamps. Most of the in the ceiling or are pendant-mounted from the ceiling. All lamps used in residential applications are omnidirectional fluorescent lamps require a ballast. CFLs are available with (emit light in all directions) incandescent lamps, typically screw bases and an integral ballast (Figure 1.15) for use as with a medium screw base (Figure 1.12) that fits into most replacements for incandescent lamps or with pin bases for residential luminaires. In addition, there are candelabra and use with a separate ballast (Figure 1.16). Both CFLs and intermediate base lamps that are commonly used in residen- linear fluorescent lamps produce light by exciting phosphors, tial applications, especially in chandeliers and wall sconces. which then fluoresce, with ultraviolet energy. A small amount Incandescent lamps produce light by heating a tungsten fila- of mercury is added to the lamp to emit ultraviolet light at a ment to a temperature of approximately 2,500 K to 3,000 K suitable wavelength for exciting the phosphor. where the filament glows or incandesces. High-intensity-discharge (HID) lamps are electric lamps Halogen lamps are incandescent lamps in which the with tubes filled with gas and metal salts. The gas initiates an tungsten filament has been enclosed in a capsule containing arc, which evaporates the metal salts, forming a plasma. This a halogen gas, typically bromine, which allows the filament results in an efficient and high-intensity light source. These to operate at a slightly higher temperature without reduc- lamps are suitable for both indoor and outdoor applications ing the rated life and resulting in a somewhat higher light and are generally used to light large spaces or roadways. All output than the standard incandescent lamp. Halogen lamps HID lamps require a ballast. are available that emit light omnidirectionally, as well as Mercury vapor, metal halide (Figure 1.17), and high- directional varieties, often known as reflector lamps. Reflec- pressure sodium lamps are examples of specific types tor lamps are designated by the properties of their reflectors, of HID lamps. HID lamps require a warm-up period to such as PAR (parabolic aluminized reflector (Figure 1.13) or reach stable output as well as a cool-down period before MR (multifaceted mirror reflector), and are most commonly restarting. FIGURE 1.12  Incandescent with medium screwbase (A-19). FIGURE 1.13  PAR 20 lamp (tungsten halogen).

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16 ASSESSMENT OF ADVANCED SOLID-STATE LIGHTING FIGURE 1.14  MR 16 lamp (tungsten halogen). FIGURE 1.16  Fluorescent lamp (T5) without integral ballast. FIGURE 1.17  Metal halide lamp (an example of high-intensity discharge lamp). FIGURE 1.15  Compact fluorescent lamp (screw base with integral ballast). REFERENCES CIE. 2004. Colorimetry. Third edition. Technical Report CIE 15:2004. Vienna, Austria: CIE Central Bureau. Azevedo, I.L., M.G. Morgan, and F. Morgan. 2009. The transition to solid- DOE (U.S. Department of Energy). 2011. Buildings Energy Data Book. state lighting. Proceedings of the IEEE 97:481-510. Washington, D.C.: U.S. DOE. CIE (Commission Internationale de l’Eclairage). 1926. Commission DOE. 2012. 2010 U.S. Lighting Market Characterization Report. Washing- Internationale de l’Eclairage Proceedings, 1924. Cambridge, U.K.: ton, D.C.: U.S. DOE. Cambridge University Press.

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