Solid-state lighting (SSL) products are integrated systems that consist of a number of subcomponents. Generally, these subcomponents include the following:
- A light-emitting diode (LED), an LED array, an integral lamp, or an organic light-emitting diode (OLED) panel;
- Secondary optics to control the distribution of the light;
- Heat sink, thermal management components, or thermal interface material; and
- Driver and control devices.
A very thorough discussion of the subcomponents, their performance, and needed areas of improvements was given in the 2013 NRC report Assessment of Advanced Solid-State Lighting (NRC, 2013), and some of the salient descriptions are included in the Annex 4.A. Important improvements remain for these SSL systems, particularly in regard to thermal managements and lighting control. Nevertheless, since the 2013 report, significant progress has been made in the design and manufacture of these systems, in terms of cost, efficacy, and compatibility with lighting controls. The applications described in this chapter, both for retrofit and emerging applications, will continue to demand increased energy efficiency, light quality, controllability, and reliability of these SSL systems.
As discussed in Chapter 2, the so-called “overnight potential” energy savings from retrofitting incandescent and fluorescent lamps in residential and commercials buildings is approximately 5 quadrillion British thermal units (quads), roughly 40 percent of the (source) energy consumed by lighting in the United States today. However, energy savings is not the only source of motivation for changing from legacy to SSL products. Light quality and the suitability of products for different applications also drive lighting design decisions.
As discussed in Chapter 1, in 2015, 6.4 percent of the installed U.S. base of indoor lighting products were LED products, while outdoor lighting LED products accounted for 14 percent of the installed base. The first applications of LED products were for outdoor lighting because of the promise of a long life with subsequent reduced maintenance, and the total installed base of outdoor lighting represents less than 5 percent of the number of units in indoor lighting installations.
SSL is a growing technology and is now widely accepted by the design and commercial building industry, and is growing in popularity with the general public. During this relatively early stage of commercialization, most common SSL products are LED lamps and luminaires that replicate existing legacy form factors, such as medium screw-base lamps, recessed troffers, and cobra head-style1 luminaires for street and roadway lighting.2 These are used in similar applications as their legacy lamp predecessors, but with distinctly different appearances, such as heat sinking fins and multipoint light sources. Exceptions to this are retrofit LED lamps with medium screw-base that mimic the appearance of the incandescent filament of traditional lamps.3
Only in the last couple of years have new form factors been introduced to address lighting quality issues, such as reduced glare, diffuse distributions of emitted light, improved beam appearance, and color consistency. The use of light guides is gaining popularity, resulting in better optical control and reduced glare. Examples of LED products that use light
1 So named because of the resemblance with the head of a cobra snake.
2 See, for example, OVF LED Roadway Large Cobrahead website at http://www.cooperindustries.com/content/public/en/lighting/products/roadway_lighting/_182918.html.
3 See, for example, OSRAM, “LED Retrofit CLASSIC A,” https://www.osram.com/osram_com/products/lamps/led-lamps/consumer-led-lamps-with-filament-style-led-technology/led-retrofit-classic-a/index.jsp, accessed October 20, 2016.
OLED products have been introduced by a couple of manufacturers.5 Customers have appreciated the aesthetics of these products, especially in terms of shape, product sleekness, uniformity of light distribution, and smoothness of dimming performance. However, product efficacy is lower than LED products,6 and cost is an overwhelming barrier to widespread adoption of OLEDs for illumination applications (DOE, 2015).
The combination of LED and OLED technology into one luminaire leverages benefits from both of the technologies. An example of this (Figure 4.2) is a luminaire with both a direct and an indirect light distribution, with LEDs emitting light upward toward the ceiling and OLEDs emitting the light that is seen from below.
Color quality has increased with products offered in a wider range of correlated color temperatures (CCTs), and high color rendering index (CRI) Ra values. For example, products that are used for accent lighting in museums offer quality colors without harmful infrared and ultraviolet light that can degrade the artwork.7
In residential lighting, LED lamps for wirelessly connected homes are becoming common, and there are many brands that can be purchased in home improvement stores (Colon and Torres, 2017). These products are intended to replace the standard incandescent or compact fluorescent light (CFL) lamps in homes and be controlled by smartphones or tablets. Control interfaces allow users to switch, dim, and change the color of the lighting. Some of these systems can be as simple as a lamp or group of lamps together that are paired with a remote control device.8 Others include a bridge that is also connected to the internet via the local Wi-Fi network. These bigger systems can control lighting in an entire house and include other devices, such as window shades and thermostats. The use of a bridge enables connection to a smartphone or tablet, which gives the user remote control capabilities when away from home. Different manufacturers use different communication protocols, such
4 See, for example, Lumination™ LED Luminaire—EP Series website at http://www.gelighting.com/LightingWeb/na/solutions/indoor-lighting/suspended/lumination-ep-series.jsp.
7 National Gallery of Art, “Effects of Light Exposure,” http://www.nga.gov/content/ngaweb/conservation/preventive/preventive-light-exposure.html, accessed March 7, 2017.
8 Pairing is done either in the factory or during the installation.
as ZigBee,9 Wi-Fi,10 Bluetooth,11 or proprietary protocols. The lighting industry has formed a consortium called the Connected Lighting Alliance,12 which has endorsed the use of ZigBee 3.0 as the preferred open protocol for manufacturers to adopt. One of the main concerns for smart home systems with a hub is security. Hackers can break into these systems and gain access to personal information (Moore, 2016; Halper, 2016; Grau, 2015).
The use of controls in municipal applications (e.g., roadways) is discussed in the section “Retrofit Applications.” The use of controls in industrial applications is discussed in the section “Product Design and Specification.”
Although automotive lighting applications do not directly contribute to the Department of Energy’s (DOE’s) energy savings goals, they do provide market opportunities for new technologies at early stages. With time, performance improvements and cost reductions allow these devices to be used in general illumination applications. During the past two decades, LEDs have found applications in cars and other vehicles, both inside and outside. According to Strategies Unlimited, LEDs will enjoy the most growth in general lighting and in automotive applications during the next several years (Pruitt, 2015).
New applications include headlamps using LEDs and laser diodes, car-to-car communication using visual light communication (VLC), and the use of OLEDs for aesthetics. Automakers have demonstrated the use of semiconductor lasers in cars (Figure 4.3). These laser lighting systems use phosphor conversion of the blue laser light to create white light. Micromirrors break the beam into pixels that shine on the roadway and on road signs. This headlamp system can direct the light away from oncoming traffic to prevent blinding the drivers of other vehicles.
In cars, VLC will likely use low data rates and inexpensive sensors to make the overall cost affordable (Lewin, 2014). Some applications for OLEDs in cars include dashboard displays, heads up displays, inside digital rear-view mirrors, interior lights, such as dome lights, and external lights, such as tail lights and turn signals.13
The majority of existing lighting applications use legacy products. Replacement of these products has slowly begun. The most common retrofit is a lamp replacement, with mixed results.14 For example, in commercial applications, linear fluorescent lamps can be replaced with tubular LED (TLED) lamps. The performance of TLEDs differs greatly from fluorescent tubes, including the spatial distribution of emitted light (Gavin, 2014). For example, GE Lighting’s TLED has a beam angle of 130 degrees instead of the 360 degrees of a fluorescent tube (GE Lighting, 2014). Reflectors in fluorescent fixtures are designed specifically for 360 degree emission. The function of the reflectors is considered when the locations of the fixtures are specified, to achieve a reasonably uniform distribution of light. The use of TLEDs with different distributions in existing fixture installations can result in overly bright and dark patches throughout spaces. Other concerns include the use of ballasts (and associated safety concerns about disconnecting them), power quality, dimmability, and increased weight on the sockets (LEDs
Magazine, 2014). The Underwriters Laboratory (UL) classifies TLEDs into three types: Type A TLEDs operate on existing fluorescent ballasts, so they serve as direct replacements for fluorescent tubes. Type B TLEDs connect directly to a building’s line power and require the removal of the ballast from the circuit. Type C TLEDs require a separate driver, requiring the existing ballast to be replaced by a driver in the luminaire. Type B TLEDs raise a specific concern, which is not shared by all in the lighting industry, because the lamp sockets for linear fluorescent lamps are typically not rated for line power connections, leading to potentially unsafe installations, unless lamp sockets are replaced.15 In addition, some Type B TLEDs have the same pin configurations as the fluorescent tubes they replace. This leads to concerns that, in the future, a TLED could be replaced with a fluorescent tube, which requires a ballast that would not otherwise be present. A final consideration for all types of TLEDs is that the energy and financial savings from these retrofits are very small because of the high efficacy and inexpensive prices of fluorescent lamps they are replacing.
In exterior lighting, early street lighting replacements have experienced some negative public feedback, especially due to increased glare and blue light appearance (Scigliano, 2013; Andrews, 2015). Dark Sky advocates also are encouraging either minimal or no short-wavelength (blue) light in exterior lighting to minimize skyglow (IDA, 2010). Exterior applications have greatly improved with more attention on warmer CCTs, reduced glare, and increased use of dimming controls to adjust light intensity during periods of low activity (GE Lighting, 2015; Hill, 2016). Municipalities are limiting CCT and allowing adaptive lighting controls to dim the LED street lighting late at night.16 The American Medical Association (AMA, 2016) has written a position statement—discussed below in the section “Lighting for Health”—on the effects of street lighting on the circadian rhythms of people, recommending that street lights have CCTs of 3,000 K or less.
In residential and commercial applications, smooth, flicker-free dimming is typically expected. This can be very difficult to achieve when LED lamps are operated with existing incandescent dimmers. Many lamps are either non-dimmable or need to operate on a dimmer designed specifically for LED loads. LED loads draw low power, and incandescent dimmers typically have a minimum load rating of 20-40 W, which the LED load often does not satisfy. In addition, so called “smart dimmers” (which continue to operate when the lamp light output is off) require the lamps to allow current to flow through, to allow the dimmer to function, without turning on the LEDs. Both of these situations are addressed by the National Electrical Manufacturers Association (NEMA) SSL 7A standard (NEMA, 2016), as discussed in the section, “Industry Standards,” in Chapter 2. The installation of replacement luminaires in existing ceilings has challenges beyond legacy dimming systems, including limited ceiling access and existing electrical distribution systems not being designed for nonlinear loads.17
Lighting designers still struggle with specifying SSL technology, especially as the technology continues to evolve. These issues were highlighted in a DOE Lighting Designer Roundtable report (DOE, 2016a):
- There is a need for a method to compare products easily, especially when there is a specification requirement to name a primary product plus two alternative products from different manufacturers.
- There is a lack of transparency with regard to warranty coverage as market and sourcing remains unsettled. Some users have suggested the LED Lighting Facts label include such warranty information.
- It is difficult to evaluate products from data. Designers want to physically see each product.
- Information on drivers is needed, since driver failures are a problem.
- There is a lack of information and protocols on compatibility with controls.
- Products change so rapidly (during the design process and construction process) that catalog numbers are no longer current or the products are discontinued.
The lighting specifiers also discussed product data they need in order to specify projects, which is sometimes difficult to obtain. Data includes specifications for drivers and controls, color properties, information about optics, and many general information items, including flicker rate, code compliance, chip/module type, and manufacturer information.
Users are comparing SSL retrofit products with legacy lamps and luminaires and expecting equal or better performance. For instance, users expect smooth, flicker-free dimming and, in some applications, a warmer color appearance as the lamps dim.18 There is also confusion over luminaire and lamp compatibility with control systems (DOE, 2016a). Designers still have little knowledge of and information about power supplies or drivers. They rely on the luminaire manufacturers for control system compatibility information.
15 Several manufacturers (e.g., Maxlite, Premier Lighting, LaMar Lighting) market Type B TLEDs listed to UL 1598C. For a contrary opinion, see, for example, GE Lighting, 2014, “Considering LED Tubes,” 16339 (Rev 07/28/14), http://www.gelighting.com/LightingWeb/na/images/16339GE-LED-Tube-Lighting-Refit-Solutions-Whitepaper_tcm201-69385.pdf. There is no consensus within NEMA on this issue, and therefore no white paper exists.
16 See Smalley (2013), Los Angeles Bureau of Street Lighting (2014), and San Francisco Water Power Sewer, “LED Street Light Wireless Control Pilot Project,” http://sfwater.org/index.aspx?page=746, accessed August 9, 2016.
17 National Electric Code (NEC) 310.15(B)(5)(c) and NEC 210.4 (A) Informational Note No. 1.
18 Ann Kale, Ann Kale Associates, Inc., presentation to the committee on February 23, 2016.
Frustration over the lack of driver standards and choices is evident (DOE, 2016a). As discussed in Chapter 2, the lighting industry has made some progress in these areas recently, and this issue is also addressed in the section “Retrofit Applications.”
SSL products can fall short of promises made, which is a further source of dissatisfaction of some users. As some of the earliest commercialized products age, evidence of long-term effects, such as color shifts, power supply failure, and shorter-than-expected product lifetimes (Poplawski, 2013; Royer, 2013; Miller, 2013) is starting to appear. SSL products sold today have most certainly improved over those that were commercialized first, but the industry nevertheless has to contend with some of these negative impressions.
LED lighting product life is one of the least understood factors of system performance. As a result, any lifetime claim for a complete LED lighting system, such as a lamp or luminaire, is essentially a guess. According to current industry standards, LED system lifetime is defined based on LED package lifetime (L70) in hours. The LED used in the product is tested according to IES LM-80, and the operation time to reach the 70 percent of initial luminous flux is projected according to IES TM-21. However, there are more components in an LED product than just the LED package, and currently there are no agreed-upon tests for larger systems such as light engines or luminaires. The failure of any component will result in product (i.e., luminaire) failure (DOE, 2013; NGLIA, 2014), the import being that components have an influence on the lifetime of the luminaire.
In general, LED lighting system failure can be parametric or catastrophic. When an LED system functions in the intended manner but outside the normal operating limits, failure is referred to as parametric. Lumen depreciation and chromaticity shifts are examples of parametric failure. When an LED system fails to produce light, this is referred to as catastrophic failure. LED failures due to closed or open circuits that cause complete loss of light are examples of catastrophic failure. In an LED system with many components, thermal expansion mismatch produces mechanical stresses that can cause material fatigue and lead to failure.
In LED applications, typically a lighting system is turned on and off. Research studies have shown that on-off cycling of an LED system can cause increased catastrophic failure compared to continuous-on only operation (Narendran and Liu, 2015). Therefore, LED system life testing should include on-off cycling. A recent life-test study investigated the impact of environment temperature and system use (on-off) pattern on LED product lifetime.19 Contrary to the common belief that the operating life of an LED product is unaffected by switching, results show that life is impacted by the application environment and on-off switching pattern.20 The International Electrotechnical Commission (IEC) is currently developing an SSL reliability standard.21
FINDING: The lifetimes of LED lamps and luminaires are estimated to be the time at which the luminous flux of the lighting product is expected to be 70 percent of initial luminous flux, based on continuous operation tests of the LED packages. Since LED systems are switched on and off during operation and are composed of many components, a proper LED system life test method should test the entire system with on-off cycling. Furthermore, both lumen maintenance and catastrophic failures should be considered when reporting product lifetime.
RECOMMENDATION 4-1: The Department of Energy should support light-emitting diode system lifetime research and encourage the Illuminating Engineering Society to develop a standardized system lifetime test method.
SSL technologies offer advantages that legacy technologies lack, such as small size, spectral flexibility, increased controllability, and high product efficacy. SSL also provides opportunities to develop new feature-rich products that provide additional benefits to users, with functions beyond illumination. Products designers, as well as lighting designers, are exploring new ways of using SSL products in innovative, dynamic lighting designs (DOE, 2016a). Dynamic lighting includes features such as changeable spatial distributions of emitted light, spectral tuning, and schedule programming, in addition to intensity variation.
However, light can be used for other purposes, some of which are becoming more widespread. Strictly speaking, some of these applications, such as agricultural lighting, are unlikely to reduce energy consumption and have the potential to do the opposite. However, if growth of these applications is inevitable, DOE may wish to consider ways of maximizing efficiency. Some of these applications, such as visible light communication, have the potential to increase the functionality of lighting that is also used for illumination. Growth of these applications may present new business opportunities for lighting manufacturers. In the following sections, the committee reviews both technology-enabled illumination applications as well as several non-illumination applications and explains the value propositions for each. Where applicable, the status and appropriateness of energy efficiency standards tailored to these applications is discussed.
19 See Narendran et al. (2016) and Lighting Research Center, “Developing a Predictive Life Test for LED Systems,” http://www.lrc.rpi.edu/programs/solidstate/LEDSystemLife.asp, accessed March 7, 2017.
21 IEC 62861, private communication with Karen Willis, Senior Lighting Program Manager at NEMA, June 23, 2016.
Changeable spatial distributions of light can be achieved by single luminaires that adjust the pattern of emitted light through a control system. For instance, instead of having separate accent lights and wall washers, one luminaire could accomplish both of these lighting effects. A hand-held control device may have a photo of the interior space; a user just has to touch the area that requires lighting, and the luminaire responds. An example of this is OSRAM’s OmniPoint luminaire (OSRAM, 2015). In exterior lighting, a pole-mounted luminaire could illuminate the street with an asymmetric “pro-beam” distribution (Figure 4.4) where light only is distributed in the direction of traffic (extension of headlamps), similar to what is done in some tunnel lighting, and provide separate sidewalk illumination when a pedestrian is present.
OLEDs have the potential to facilitate unique applications of light. In order for OLED products to be widely adopted, several barriers, such as cost and efficacy, need to be overcome.22 In the future, OLED products may be transparent, be available in dynamic shapes, be available in larger size panels, and feature spectral tuning.23 In these ways, OLEDs have the potential to offer aesthetic possibilities that cannot be readily achieved with other technologies. Further developments in OLED luminaire technologies are uncertain, since OLEDs are being developed by the display industry and currently have very little presence in the lighting industry.24
Since the goal of the DOE SSL program is to reduce the energy consumed by lighting, the focus has primarily been on existing uses of light. These are predominantly illumination applications in which the intended function of the light is to enable people to visually perceive illuminated objects.
DOE has identified addressing physiological responses to light as one of the key issues and challenges for LEDs. In the core technology research and development (R&D), DOE (2016b) has identified blue light hazard, health, and productivity for humans as the particular challenges. The AMA (2016) outlines several health concerns associated with excessive short-wavelength (blue) light, including disability glare and possible retinal damage. In addition, they are concerned with environmental disruption for many nocturnal species, as well as human circadian rhythms disruption, by blue light at night. Circadian disruption from outdoor street lighting, indoor room lighting, and electronic devices (e.g., computer monitors and mobile devices such as cell phones) during the evening has been associated with sleep disruption, obesity, impaired daytime functions, and increased cancer risks. The AMA strongly recommends warm white light (3,000 K or less) for nighttime lighting, shielded lights to
23 Sebastian Suh, “LG Display,” presentation to the committee on February 24, 2016.
prevent light trespass into homes, and dimming or turning off outdoor lighting when not needed.
The spectral power distribution of illumination can be adjusted, to minimize the potential negative effects of light, by tuning the spectral power distribution of light emitted by luminaires. Spectral tuning can be implemented as white tuning (blending warm white and cool white LEDs in different proportions) or as a variation on red-green-blue (RGB) color mixing. There is some research on the effects of light source spectrum on circadian rhythms25 and ecological consequences26 of certain wavelengths during periods of darkness. The effects of exposure duration, wavelength, and intensity are still being investigated, while broad assumptions about these effects are already being addressed by voluntary standards (IES, 2008; DIN, 2015).27 Given the current lack of consensus, further research is needed in order to provide product developers, lighting designers, and consumers with guidelines (Brainard, 2001, 2012; Figueiro et al., 2004, 2008, 2013, 2014, 2016; Figueiro and Rea, 2012; Figueiro, 2015, 2016; Rea et al., 2005; Rea and Freyssinier, 2013; Sahin and Figueiro, 2013; Thapan et al., 2001; Wood et al., 2013; Young et al., 2015).
An experiment is under way to regulate the circadian rhythms on astronauts on the International Space Station (ISS) with light. Since the cycle of sunlight and darkness occurs every hour on the ISS, it is difficult to use daylight for regulation of melatonin. A team of researchers (Brainard et al., 2012) have developed a dynamic spectrum lighting system that changes the color of light to help the astronauts fall sleep in their sleeping compartments. Instead of using medications to go to sleep, light that is void of short wavelengths is used.
FINDING: Although there is clear evidence that circadian rhythms are impacted by short-wavelength (blue) light, the consequences of these impacts are not fully understood. DOE has an R&D program that prioritizes investigations of physiological impacts of light.
Photosynthesis, the process by which plants convert carbon dioxide and water into carbohydrates (energy) and oxygen, requires chlorophyll pigments to absorb light. While this light traditionally comes from the Sun, there are numerous reasons to consider the use of electric lighting for horticulture. Food is transported an average of 4,200 miles throughout its life cycle, accounting for 11 percent of its carbon footprint (Weber and Matthews, 2008). Transportation from the food producer to the retailer is responsible for only 4 percent of the carbon emissions, however. Increased globalization of food production also makes countries vulnerable to food insecurity as a result of political conflict and natural disasters in other parts of the world (Weber and Matthews, 2008). The ability to grow crops outside of their natural climate zones and in spaces smaller than conventional farmland can reduce the energy consumed by transportation and reduce the risk of food shortages due to the global events.
Many crops appear green in color because they reflect more of the middle wavelengths of the visible spectrum (green light) and absorb more of the longer (red) and shorter (blue) wavelengths of light. Light throughout the visible spectrum can induce photosynthesis, but research has shown that crop yield is most heavily impacted by spectral power of relatively long wavelengths (red) and, to a lesser extent, spectral power of relatively short wavelengths (blue) (McCree, 1972).
Electric lighting technologies, particularly fluorescent and metal halide lamps, have successfully been used to grow plants indoors for decades (Helson, 1965; Duke et al., 1975). More recently, research has explored the use the LEDs for horticultural lighting. The study of one plant species showed that a combination of red and blue LEDs resulted in greater plant mass, leaf area, and chlorophyll content than a broadband fluorescent lamp or illumination by either single color (Kim et al., 2004).
Luminous efficacy, with the unit of lumens per watt, has meaning only in the context of human vision. The unit of luminous flux is the lumen, which is a function of the radiant flux of a light source, its spectral power distribution, and the visual system’s sensitivity to the different wavelengths in the visible spectrum. As such, it is not applicable to any other animal species (e.g., livestock) or to plant species. The need for different standards for the plants and livestock is not well understood by some in the building and lighting industries—Washington State has adopted a minimum luminous efficacy requirement for lighting used for plant growth.28 Unfortunately, this has the potential to unnecessarily increase energy use, with no benefit to the plants. There was also a proposal to the International Energy Conservation Code (IECC) 2018 to adopt a minimum luminous efficacy standard for lighting products used in plant growth. Fortunately, the code panel did not accept this proposal and a new, better-informed proposal will be developed. Lighting efficacy standards for such applications remain a work-in-progress.
The photosynthetic absorption spectrum of a typical plant is shown in Figure 4.5. The sensitivity of the human visual
25 G.C. Brainard, and J.P. Hanifin, Thomas Jefferson University Light Research Program, “Ecology, Physiology, Human Health and Light,” presentation to the committee on February 24, 2016.
26 Travis Longcore, “Ecology, Physiology, and Solid State Lighting,” presentation to the committee on February 24, 2016.
28 Private communication with Duane Jonlin, Energy Code and Energy Conservation Advisor to the City of Seattle, May 10, 2016. The 2015 WA State Code requires a luminous efficacy of at least 90 lm/W for lighting for plant growth or maintenance. Mr. Jonlin was able to change the requirement for the City of Seattle to a minimum requirement for the photosynthetic photon flux per watt of the light source, which makes more sense.
system, not shown, peaks at 555 nm. At this wavelength, plants have very little photosynthetic absorption. Optimizing for luminous efficacy clearly makes little sense when designing lighting products for plant growth.
Some plants thrive when lighted with blue and red grow lamps, such as shown in Figure 4.6.
The quality of light for plants is an active field of research among plant scientists. In addition to photosynthesis, researchers are interested in photomorphogenesis, which is a study of light-mediated development in plants, such as seed-formation, seedling development, and blooming. As is the case for lighting spaces used by people, it is important to strike the right balance between energy efficiency of the lighting system and the quality of light for the needs of plants. It is too early to develop standards for plant growth lighting in terms of some form of efficacy. Furthermore, such efficacy will most likely be species-dependent.
Lighting also has an impact on the development of animals used for food. For example, days with longer exposure periods to light (photoperiods) increase the amount of milk produced by dairy cows (Peters et al., 1978; Dahl et al., 2000). Cattle, sheep, and deer also show increased growth with longer photoperiods (Forbes, 1982). In many instances, longer photoperiods induce more food consumption, which accounts for the growth (Forbes, 1982). Some research has shown that, when the amount of food consumption was controlled for, animals with longer photoperiods were larger, but not heavier, than those with shorter photoperiods (Peters et al., 1978). However, other research has found that increased photoperiods increased the weight of cattle without increasing their food consumption (Peters et al., 1978).
Increased photoperiods have not been universally found to be beneficial. For instance, the sudden onset of excessively long photoperiods (23 hours per day) has been linked to increased incidences of growth abnormalities and mortality in broiler chickens (Classen and Riddell, 1989).
The effect of light intensity on animal growth has not been thoroughly studied. Broiler chickens exposed to higher intensities of light (150 lux [lx]) had lower body fat and higher body protein than those exposed to lower intensities (5 lx) (Charles et al., 1992). Illuminances greater than 5 lx during rearing do not appear to impact the rate of sexual maturation of broiler chickens raised for breeding (Lewis et al., 2008). Illuminance also does not have an impact on egg production, provided it is at least 10 lx (Lewis et al., 2008).
Similarly, limited research has investigated the impact of light color on animal behavior and growth. One study (Prayitno et al., 1997a) has found that chickens raised under white light demonstrated more walking than those reared under red, blue, or green light. Those raised under red light displayed more pecking at the floor, wing-stretching, and aggression than those illuminated by the other colors. However, growth and food consumption was not impacted by light color. Another study (Prayitno et al., 1997b) exposed adult chickens to white, red, blue, and green light in alternation. No significant difference in behavior or energy expenditure was found as a result of illumination by the different colors.
While most of the research on the impact of light on livestock focused on food production, animal preferences can inform the design of lighting systems that maximize animal welfare. A study of the illuminance preferences of pigs found
that they significantly preferred the lowest illuminance available to them (2.4 lx) to the other available illuminances (4 lx, 40 lx, and 400 lx) (Taylor et al., 2006). When cattle were taught to control electric lighting in their enclosure, they chose to be illuminated about half (54 percent) of the time (Phillips and Arab, 1998). In these cases, research on animal behavior suggests that less lighting consumption can be beneficial to animal health and have co-benefits for reduced energy consumption for illumination. DOE’s 2016 R&D Plan (DOE, 2016b) discusses livestock production briefly, and the impacts of lighting on livestock production are included in its prioritized investigation of physiological impacts of lighting. DOE speculates that LED lighting could have benefits for animal behavior and well-being because of the ability to tune the color of the light, in addition to reducing energy costs, compared to incandescent lighting.
FINDING: Some state and local building energy codes are starting to consider efficacy requirements for lighting in applications other than illumination, such as plant growth. DOE has an R&D program that prioritizes investigations of physiological impacts of light (DOE, 2016b), including plant and livestock responses to light.
RECOMMENDATION 4-2: The Department of Energy should consider initiating a broad stakeholder project to develop appropriate energy efficiency metrics for the most important emerging lighting applications, including horti-
culture and livestock, that are not for illumination of spaces used by people.
Smart lighting can deliver traditional illumination and provide new functionality. The Internet of Things (IoT), connected lighting, and smart lighting are terms commonly used in the lighting industry today. In the initial period of SSL advancement, LED light sources succeeded in saving energy in most lighting applications. Now, with better lighting controls and connectivity to a network, LED lighting is evolving toward the IoT to provide greater value to end users (O’Malley, 2015; Harbers and Manney, 2014).
To address security concerns and bandwidth limitations of communications systems, communication using visible light is being considered and studied. This is called Visual Light Communication (VLC) and has also been given the name Li-Fi (Light Fidelity) by the IEEE standardization committee,29 whose scope covers this technology.
Li-Fi systems are networked two-way data communication systems that use visible light by switching the current to the LEDs on and off at high temporal frequencies, beyond the flicker fusion frequency of the human visual system. Li-Fi has potential to be very high speed, perhaps as much as 100 times faster than Wi-Fi, with demonstrations claiming to have achieved data transmission rates of from 500 megabits per second (Grobe et al., 2013) to 9 gigabits per second (Gbps) (Chi et al., 2015) and on up to 200 Gbps (BBC News, 2015). Because communication uses visible light, it is limited to a line-of-sight, meaning that it cannot be used to communicate through walls or other such opaque obstacles. This limits the communication range compared to radio transmission, but has the benefit of not being detectable outside enclosed walls and is not easily subject to eavesdropping.
In addition to building systems communication,30 VLC also can be used to communicate with occupants via smart devices. Today several companies are developing VLC systems for shoppers at stores.31 In these systems, a shopper’s smartphone works with the LEDs in the light fixtures in the store. These sensors can detect the shopper’s location within the store, identify the displayed items being viewed by the shopper, and transmit promotional materials to the shopper’s smartphone with the hope of aiding businesses to increase sales.32
Commercial lighting IoT is promising to transform the way spaces are illuminated with some systems focusing on total energy reduction through a variety of strategies (such as occupancy sensing, daylight harvesting, dimming, etc.). Light fixtures, with sensors and a network connection, can sense the environment and send commands to the LEDs to change lighting characteristics to cater to the needs of the application. Sensors can detect not only that a room is occupied but also by how many people are in the room, so that the ventilation system can be adjusted accordingly, for instance. In outdoor applications, “smart cities” is a popular term these days.33 In this application, outdoor light poles and street and area light fixtures are outfitted with sensors and cameras that can be networked to make cities more energy efficient and safe (Murthy et al., 2015) by detecting crimes and reporting them automatically to the police. Likewise, for indoor applications, several companies have launched add-on systems to help existing building management systems conserve energy and service failed hardware. Despite the enthusiasm in the industry and potential benefits, IoT lighting has many challenges to overcome before gaining widespread use. These include privacy and security concerns, standards and interoperability of lighting products, and regulations. The concern about regulations is also discussed in Chapter 2.
Lighting companies have begun collaborating with information technology (IT) companies, including GE, Apple, and, in a joint venture, Acuity and Qualcomm.34 Since the United States is strong in IT, this is a potential area of lighting in which U.S. industry can excel.
FINDING: The number of applications in which SSL is being used has greatly increased since the National Research Council’s 2013 report Assessment of Advanced Solid-State Lighting was released. These new applications that go beyond illumination are attracting a diverse set of companies from adjacent markets.
Some impediments to innovation are simply the result of the relative immaturity of the SSL. For instance, the commercialization of OLED lighting products is limited by a lack of basic measurement methods. There are no industry standards for the measurement of luminous flux, luminous efficacy, chromaticity, spectral power distribution, color rendering, or lumen maintenance of OLED luminaires. This is obviously problematic for the specification of OLEDs, but it also makes it difficult to track technological developments of OLEDs and compare their performance to other lighting technologies. Consensus standards do not exist for the measurement of basic photometric and colorimetric quantities of
29 IEEE 802.15.7r1.
30 See, for example, Lux, 2016, “World’s First Li-Fi Office Opens in Paris,” June 27, http://luxreview.com/article/2016/06/world-s-first-li-fioffice-to-open-in-paris?cmpid=LUXproducts06302016.
33 Silver Spring Networks, “Silver Spring Networks for Smart City Street Lighting,” http://www.silverspringnet.com/article/silver-spring-networks-expands-smart-city-infrastructure-platform-through-new-collaboration-with-street-lighting-pioneer-selc/.
34 Marc Saes, Acuity Brands, and Aleksandar Jovicic, Qualcomm Inc., “LED Lighting as a Platform for Indoor Positioning for Mobile Devices,” presentation to the committee on February 24, 2016.
OLED lighting products. Such standards do exist for LEDs, including LM-80-15: IES Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Arrays and Modules; as well as TM-21-11: Projecting Long Term Lumen Maintenance of LED Light Sources. These apply to LED sources, only.
Though problematic, impediments like these are not insurmountable. Test methods can be developed if there is sufficient demand and commitment. Other challenges are more insidious, however. Current lighting design practices may be inhibiting the industry’s ability to develop truly innovative applications with SSL.
Many of the methods, measurements, and norms used in lighting design arose from the characteristics and limitations of old lighting technologies. For instance, the most commonly used measure of chromaticity for white light sources is CCT, which indicates the temperature of a blackbody radiator that is most similar in chromaticity to the light being described. The spectral power distributions (SPDs) of incandescent lamps approximate the spectra from blackbody radiators, but more modern lighting technologies are vastly spectrally different. There is no scientific evidence to support the premise that the chromaticities produced by incandescent sources are optimal for illumination. In fact, the limited research on this topic suggests that they are not (Rea and Freyssinier, 2013; Ohno and Fein, 2014). Nonetheless, product performance standards for chromaticity pressure manufactures to develop products with chromaticities most like incandescent technologies,35 although changes to the SSL chromaticity standard are currently being considered.36 Similarly, current color rendering metrics, including the CRI (CIE, 1995) and the new method described in IES TM-30 (IES, 2015), are rooted in the assumption that the appearances of colored objects are ideal when illuminated by an incandescent-like SPD for many chromaticities. Light sources that render color appearance differently are penalized, even though evidence suggests that these blackbody spectra do not lead to the most natural or attractive appearance of colored objects (Jost-Boissard et al., 2009; Ohno et al., 2015). Although color quality is widely believed to be an important aspect of consumer acceptance, the metrics used are quite rudimentary and based on the presumption that the first widely commercialized lighting technology was perfect.
Throughout the lighting design process, the intrinsic traits of incumbent technologies still dictate the shape and size of lighting products, patterns of emitted light, and ways in which lighting products are integrated into buildings. Even the role of lighting designers in design teams and the stage at which they are engaged with design projects are consequences of traditions that were shaped by the technologies available long ago. For instance, legacy products such as fluorescent luminaires and dimming ballasts come in standard options and are easy to specify and commission. Designers can select the same dimming ballast for many fluorescent luminaires. With SSL, each luminaire must be tested with a specific driver to obtain a UL certification, limiting the choices of equipment. As discussed in Chapter 2, UL has started a Class P LED driver program, which will harmonize the handling of these products with the way that the industry handles fluorescent lamp ballasts. The program is just starting, so it is premature to comment on results. Another example is the incompatibility between various light engines, drivers, and control systems. The interface between the light engine and the LED driver needs to be understood by the designer because there are two categories of LED drivers: constant current output and constant voltage output. When the former is used, it is possible to add LED modules in series up to the total power rating of the driver. But if constant voltage output is employed in the driver, each module must be rated for that voltage, and additional loads are added in parallel to each other up to the power rating of the driver. The interface between the control system and the driver is equally important for the designer to understand and specify. Common control inputs to the driver include phase-cut dimming (NEMA, 2016), DALI (digital) control,37 DMX control38 (another form of digital control commonly used in theatrical lighting), and 0-10 V analog control.39 As a result, designers must now act as control integrators in order to commission and troubleshoot problems in the field. Lighting control designs now need to include specifications of scenes, locations of automatic and dynamic controls, communication with fully networked devices, and the integration of Internet-connect and/or mobile apps (Weissman, 2014). This adds responsibilities that traditionally were not part of typical design scope and fees.40
FINDING: The lighting industry has developed standards for control interfaces for LED drivers, and there are industry conventions for driver output ratings. In addition, UL, in collaboration with the lighting industry, has started its Class P driver program to allow designers more flexibility with driver choices.
36 Yoshi Ohno, NIST, “Color Metrics: Where Are We? Where Are We Going To?,” presentation to the committee on February 24, 2016.
38 Entertainment Services and Technology Association, DMX512-A standard.
39 ANSI C137.1, in the process of being published.
40 Chip Israel, “Lighting Design Alliance,” presentation to the committee on February 23, 2016.
Similarly, approaches to the regulation of lighting products are outdated and, in some instances, hinder innovation. The development of connected (smart) lighting systems may provide additional functions that benefit users, such as lighting that aims to enhance the health of occupants. Some of these functions have little to do with providing illumination, but some of these operations have the potential to drastically reduce the energy consumed by lighting. These systems will consume a small amount of power, depending on the service that they provide, even when the lighting is off. At this time, regulators do not appear to understand these developments sufficiently.41 Instead, they are focusing on the luminous efficacy of the lighting system when illumination is provided and standby power consumption when the lighting is switched off. If the function of the standby mode is only to power the lighting equipment sufficiently to get input from sensors and other devices to turn lighting on when it’s needed, limiting standby power consumption to a reasonable level makes sense. But when regulators extend the same approach to connected lighting products, there is a risk that restrictive regulations will impede innovation of these types of products. The IEC has started to study these needs in their standards and is separating the needs of standby power consumption and power consumed by secondary devices or functions.42
Scientists and engineers, with support of DOE, have made remarkable progress increasing the luminous efficacy of lighting hardware (DOE, 2016b). DOE supports a goal of 200 lumens per watt (lm/W) luminaire efficacy by 2025, and this has been achieved in laboratory demonstrations. However, the energy consumed by lighting depends both on the luminous efficacy of the lighting technologies and the way in which those technologies are used to illuminate spaces. In illumination applications, the purpose of light is to enable users to see illuminated surfaces of objects, such as walls, floors, furniture, people, books, food, vehicles, etc. Only the light that reflects off illuminated objects and enters the eyes of viewers is useful—the rest of the light emitted is essentially wasted. From this perspective, the application efficacy of a lighting installation can be considered, both temporally and spatially. For example, if a room is lighted, but is unoccupied, energy is wasted, regardless of the luminous efficacy of the luminaires. Similarly, if a large space is illuminated, but occupants are not looking at all parts of it, energy is being consumed needlessly to light portions of the space that are not viewed.
Some strategies to increase application efficacy are already widely used in lighting design. For example, lighting control systems that dim electric lights when daylight is present and systems that automatically turn lights off when spaces are unoccupied, are common in commercial spaces (Williams, 2012). However, current approaches to increasing application efficacy are predominantly the same as those used for legacy technologies. The unique characteristics of LEDs, including ease of digital control, optical properties, and spectral flexibility, enable more radical approaches to minimizing the energy consumed by lighting. For example, some manufacturers have capitalized on the small source size of the LED and developed optics with maximized efficiency to direct light only to the places where it is needed (Narendran et al., 2015).
Suggestions have been made that the computer graphics method of light-field mapping, in which the relationship between the lighting and the appearance of all illuminated objects within a viewer’s field-of-view is determined in real time (Chen et al., 2002), could be leveraged in illumination applications. If an advanced lighting system were able to determine the visual field of all occupants in a building and tailor the lighting so that only the viewed surfaces were illuminated, application efficacy could be drastically improved (Tsao et al., 2014). In current lighting design practice, spaces are fairly uniformly lighted, a convention that is largely an artifact of the limitations of earlier technologies. Since occupants rarely view all portions of a space at any given time, much light is unseen and, therefore, wasted. Another proposed approach to increasing application efficacy suggests that the amount of light absorbed by illuminated surfaces can be reduced. Light absorption has traditionally been thought to be unavoidable in lighting design—the color and lightness of a surface determines the relative amount of light reflected to the observer and the amount absorbed and converted to heat. However, if each surface in a space were illuminated by light with a customized spectral power distribution that maximizes the amount of light reflected, application efficacy could be significantly increased (Durmus and Davis, 2015). Other approaches to increasing application efficacy could be simpler to implement. For instance, since the trade-off between luminous efficacy and color quality is known (Ohno, 2005), lighting systems could be developed that change these characteristics based on occupancy (Thompson, 2007). For instance, in spaces in which it is inappropriate to switch off lights in unoccupied zones (e.g., retail, hospitality, stairwells), high-efficacy, low-color-quality lighting could illuminate unoccupied areas. When occupancy is detected, the lighting could switch to higher-color-quality, lower-efficacy illumination. These instances show the opportunity to increase so-called application efficacy, above and beyond the improvement achieved from the luminaire (i.e., the product) by itself. These approaches to lighting are a significant
41 California Energy Commission, “Notice of Commission adoption hearing, availability of revised 15-day language, and opportunity for comment,” January 7, 2016, http://docketpublic.energy.ca.gov/PublicDocuments/15AAER-06/TN207218_20160107T132138_Notice_and_Revised_15Day_Language.pdf.
42 IEC 60598-1 8th Edition 2nd Amendment.
departure from current lighting design practice. Before they could realistically be implemented, a better understanding of the impact of lighting on the appearance of illuminated spaces is needed. Ideas for increasing application efficacy raise questions about the perception of lighting in peripheral vision, the detectability of temporal changes of lighting, and the impact of SPD on light absorption and color appearance. To minimize the energy consumed by lighting, lighting applications research is needed to complement advances in technology efficacy.
FINDING: The energy consumed by lighting is a function of both the luminous efficacy of lighting products and application efficacy of installations. DOE does set targets for light utilization for advanced luminaire systems in its R&D program, but its approach is still product focused.
RECOMMENDATION 4-3: The Department of Energy should develop strategies for supporting broader research that enables more efficient use of light in such a way that the application efficacy is maximized, with attention to both the lighting design process and the design of lighting products.
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SSL products in the commercial market employ a variety of LED white light sources, including an array of phosphor-converted LEDs (blue LED chips covered by a coating of phosphor); an array of cool white (i.e., high color temperature) LEDs combined with red LEDs to create a warmer white and feedback control to maintain light output and color; and an LED array with a mixture of multicolored (red, green, blue, etc.) LEDs. These LEDs or LED arrays are mounted on a heat sink to minimize the heat at the LED junction(s) and are powered by an electronic driver that produces power of the form required by the LED. In some cases, secondary optics are used to direct the beam in a specific manner. If the LEDs are packaged as an integral lamp to replace a traditional light source, the lamp envelope (i.e., glass bulb) is designed to mimic the form of the traditional source and includes a specific connector (e.g., an American National Standards Institute [ANSI] standard base).
LED and LED Array
As described in Chapter 3, white LEDs are commonly made by dispersing phosphor(s) in the encapsulant surrounding the blue (or near-ultraviolet) LED chip. The process of combining phosphors with the LED chip has evolved over the years. Some packages still use the original method of mixing phosphor(s) into an epoxy or silicone medium. Other packages use a layer of phosphor coated on the chip, while newer LED packages and products consist of phosphor layer(s) separated from the LED chip(s), commonly referred to as a remote-phosphor LED or product (Hoelen et al., 2008; Narendran et al., 2005). Remote phosphor-type LEDs minimize heat-induced efficiency loss in phosphors (provided the phosphor conversion efficiency is not very low as well as the absorption of phosphor-converted photons by the blue LED chip). An LED array is created by mounting and interconnecting individual LED devices on a printed circuit board, which is then connected thermally to the heat sink.
A unique feature of OLED lighting is that the device itself can form the installable fixture, because of its ability to be fabricated on any particular substrate or shape. Indeed, OLEDs can be fabricated directly on plastic blocks, flexible metal or plastic foils, or glass. In its configuration as an area lighting source, as discussed in Chapter 3, the luminaire itself operates without a significant increase in temperature above the room ambient. That is, in appropriately packaged devices, at a high surface luminance of 3,000 cd/m2, the luminaire temperature rise can be only a few degrees centigrade, creating no local or distributed heat load on the room environment.
In an LED lighting product, secondary optics are needed to tailor the output beam of a lighting product. LED products commonly designed for illumination applications have LEDs arranged in several different ways together with secondary optics. These designs include an LED array placed inside reflector(s) and behind total internal reflection (TIR) lenses. These methods help the collection and distribution of light in a specific manner. Refractive optics, commonly referred to as lenses, reflective optics, or reflectors, are generally designed as non-imaging optics to be used in illumination products for beam shaping. Researchers have designed and used complex optics to achieve difficult beam shapes (Tsais and Hung, 2011). Typically, no secondary optics are required for OLED panels.
Reliability of Optics
Lens materials are usually made from glass, polymers, epoxies, or silicones. Material selection is very important, especially when designing long-life products. Some optical materials degrade when exposed to radiation (more specifically, short wavelengths like ultraviolet and “blue” radiation) and heat. This spectrally dependent light output deterioration is one of the main ways that LEDs degrade.
Thermal management is very important to enable reliable, long-life LED products, and the thermal management components in an LED product constitute a large fraction of product cost. A high-temperature LED junction can negatively impact LED life and optical performance, and as discussed in Chapter 3 in Annex 3.A, “An LED Primer,” this places considerable demands on the plastic lens and encapsulant material. At higher p-n junction temperatures, the amount of photons emitted decreases and the spectral power distribution shifts to longer wavelengths. Furthermore, the degradation of the encapsulant and the LED chip, over time, decreases the luminous flux. Electrical energy not converted to light contributes to the heat at the p-n junction. To keep the LED junction temperature low, all heat transfer methods, including conduction, convection, and radiation, must be considered. Heat conducted to the environment from the p-n junction encounters several interfaces and layers. Therefore, to keep the junction temperature low, the thermal resistance of every layer and interface must be very low.
Thermal Management Component and Strategies
An LED chip is typically encapsulated in a transparent material, such as epoxy, polymer, or silicone. These materials have very low thermal conductivities. As a result, the majority of the heat produced at the p-n junction is conducted
through the metal substrate below the chip and not through the transparent encapsulant. Usually, a high-power LED is mounted on a metal-core printed circuit board (MCPCB). When creating a product, an LED (or an array of LEDs) mounted on an MCPCB is attached to a metal heat sink using a TIM. Usually these heat sinks have extended surfaces, such as fins, which dissipate the heat to the environment by convection and radiation. Currently, a few manufacturers have started to mount the LED directly onto the heat sink to further reduce the thermal resistance from the junction to the environment and also to reduce the overall cost.
Common thermal interface materials are solder, epoxy, thermal grease, and pressure sensitive adhesive. Parameters that can influence thermal resistance include surface flatness and quality of each component, the applied mounting pressure, the contact area, and the type of interface material and its thickness. Adding conducting particles and carbon nanotubes (CNTs) to TIM to reduce thermal resistance has been studied (Fabris et al., 2011).
Most manufacturers exploit both conduction and convection methods to reduce LED junction temperature. Usually the heat sinks have a very large metal surface area, and, as a result, the integral lamp or the entire luminaire is much heavier than its traditional counterpart. Figure 4.A.1 shows typical weights for incandescent, CFL, and LED lamps of different types.
To make the weight of LED products comparable to traditional lamps, lightweight materials, like polymers and composites, with very high thermal conductivity are needed. The thermal conductivity of plastic materials can be increased by using fillers such as ceramics, aluminum, graphite, and so on. Injection-molded polymer parts of high thermal conductivity are an economical approach for cooling high-power LED products. Some also have investigated techniques such as heat pipes, like those used in computers, to keep LED junctions cooler.
While these passive cooling methods work well for certain types of SSL products, higher power LED lighting products (1,500 lumens and above) pose significant thermal management challenges. Passive heat sinks are not sufficient to keep the LED junction sufficiently cool. Therefore, to achieve desired lumen values in a small form factor (e.g., A-lamp, PAR lamp, MR16, etc.), active cooling may be required to dissipate the heat. Even though mechanical fans have been used in some high-power LED lighting products,1 they are not desirable for many reasons, including short life, acoustic noise, attraction of dust, and increased energy use. Over the past several years, other active cooling techniques have been investigated for managing the heat in high-power electronics, including synthetic jet and piezoelectric fan technologies. Synthetic jet technology uses a moving diaphragm that produces air movement by suction and ejection of air. Rapidly fired pulses of air are directed to where cooling is needed, such as heat sink fins, to improve cooling efficiency. Piezoelectric fans have several advantages, including longer life, lower acoustic noise, and lower power demand (Zhang et al., 2011). These techniques have shown promise and are worthwhile for further development for high-power LED cooling (Acikalin et al., 2007). Even though active cooling may be necessary for some products in some applications, for the majority of the applications, passive cooling is more desirable.
There is a strong interaction among LED device efficacy, the requirement placed on the thermal management system, and the cost of SSL. Increased efficacy reduces the heat generated per lumen, allowing either a shrinking of the necessary heat sink, and thus a reduction in cost and weight, or an increase in lumen output for the same physical luminaire.
References for Annex 4.A
Acikalin, T., S.V. Garimella, A. Raman, and J. Petrosk. 2007. Characterization and optimization of the thermal performance of miniature piezoelectric fans. International Journal of Heat and Fluid Flow 28(4):806-820.
Fabris, D., M. Rosshirt, C. Cardenas, P. Wilhite, T. Yamada, and C.Y. Yang. 2011. Application of carbon nanotubes to thermal interface materials. Journal of Electronic Packaging 133:020902.
Hoelen, C., H. Borel, J. de Graaf, M. Keuper, M. Lankhorst, C. Mutter, L. Waumans, and R. Wegh. 2008. Remote Phosphor LED Modules for General Illumination: Toward 200 lm/W General Lighting LED Light Sources. Paper presented at the Eighth International Conference on Solid State Lighting. Bellingham, Wash.: SPIE.
Narendran, N., Y. Gu, J.P. Freyssinier-Nova, and Y. Zhu. 2005. Extracting phosphor-scattered photons to improve white LED efficiency. Physica Status Solidi A 202(6):R60-R62.
1 See Peters (2012) and Ecomaa Lighting, Inc., “New 80W LED PAR Lamp with Fan Inside,” http://ecomaa.en.ecplaza.net/ecomaa-new-80wledpar--258939-1872324.html, accessed September 5, 2012.
Peters, L. 2012. Are MR16 LED lamps ready for the 50W-halogen switch? LEDs Magazine. June.
Tsais, Y.P., and C. Hung. 2011. Design of optical components for a light emitting diode zoom illumination system. Optical Review 18(2):224-230.
Zhang, K., D.G.W. Xiao, X. Zhang, H. Fan, Z. Gao, and M.M.F. Yuen. 2011. Thermal performance of LED packages for solid state lighting with novel cooling solutions, pp.1/7-7/7 in 2011 12th International Conference on Thermal, Mechanical and Multi-Physics Simulation and Experiments in Microelectronics and Microsystems (EuroSimE). Piscataway, N.J.: Institute of Electrical and Electronics Engineers.