Light and solid-state : reducing the carbon dioxide footprint is not enough.

Salvador Bará*

Universidade de Santiago de Compostela, Optics Area, Applied Physics Dept, Faculty of Optics and Optometry, 15782 Santiago de Compostela, Galicia (Spain).

ABSTRACT

Public and private lighting account for a relevant share of the overall electric power consumption worldwide. The pressing need of reducing the carbon dioxide emissions as well as of lowering the lumen·hour price tag has fostered the search for alternative lighting technologies to substitute for the incandescent and gas-discharge based lamps. The most successful approach to date, solid-state lighting, is already finding its way into the public lighting market, very often helped by substantial public investments and support. LED-based sources have distinct advantages: under controlled coditions their efficacy equals or surpasses that of conventional solutions, their small source size allows for an efficient collimation of the lightbeam (delivering the photons where they are actually needed and reducing lightspill on the surrounding areas), and they can be switched and/or dimmed on demand at very high rates, thus allowing for a taylored schedule of lighting. However, energy savings and carbon dioxide reduction are not the only crucial issues faced by present day lighting. A growing body of research has shown the significance of the spectral composition of light when it comes to assess the detrimental effects of artificial light-at-night (ALAN). The potential ALAN blueshift associated to the deployment of LED-based lighting systems has raised sensible concerns about its scientific, cultural, ecological and public health consequences, which can be further amplified if an increased light consumption is produced due to the rebound effect. This contribution addresses some of the challenges that these issues pose to the Optics and Photonics community.

Keywords: , solid state lighting, LED, light-at-night, ALAN.

1. LIGHTING THE NIGHT

The development of artificial light sources is one of the most significant advances of humankind. It made possible extending the human activities throughout nighttime, improving visual performance and safety and providing a subjective feeling of security after dark. Electric light sources were a major breakthrough in lighting technology, with carbon arc and carbon filament lamps already available at the XIX century. The popularization in the first decades of the XX century of new families of electrically fed sources of increasing reliability and efficacy (tungsten-based incandescent light bulbs, fluorescent lights and gas discharge lamps) enabled the extension of artificial lighting to all sectors of public and private activity.

According to the International Energy Agency,1 lighting consumed in 2005 about 2651 TWh of electricity, accounting for 19% of the global electricity consumption. This was more than the energy produced at that time by either all hydro or nuclear generating plants, and almost as much as that provided by gas-fired generation. The overall production of artificial light (including indoors and outdoors) was estimated to be 134.7 petalumen-hours (Plmh), 99% of which corresponded to lighting connected to the electric grid. This amounted to an average consumption of 21 megalumen- hours (Mlmh) per person and year, with strong regional differences (up to 30 times) depending on the industrialization level of each world region. At that year, most artificial light was produced by fluorescent sources (82.3 Plmh, 61.8% share), followed by high intensity discharge (36.3 Plmh, 27.2%) and incandescent lamps (14.7 Plmh, 11.0%). The main consumers were the commercial- and public-building sectors (estimated at 44.6% of the overall Plmh), followed by the

* [email protected]

8th Iberoamerican Optics Meeting and 11th Latin American Meeting on Optics, Lasers, and Applications, edited by Manuel Filipe P. C. Martins Costa, Proc. of SPIE Vol. 8785, 87852G © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2025344

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industrial (28.9%), residential (14.4%), and outdoor lighting sectors (12.1%). Outdoor lighting comprises several subsectors, whose relative weight in terms of light consumption was as follows: street and road lighting (53%), car-park illumination (40%), signage (6%) and billboards and airport lighting (1%).

The emissions of artificial light have been continuously raising since ancient times. Improvements in lighting technology and decreasing energy prices led to a sustained increase in light consumption which was particularly accelerated during the last centuries. According to Fouquet and Pearson's estimates,2 the total lighting consumption in the United Kingdom was 25000 times higher in the year 2000 than in 1800, amounting to a rise of 6500 times in the light consumption per capita. In the period 1995-2005 it is estimated that the global demand for artificial light grew at a 2.4% annual rate.1 Estimates made by Hölker et al.3 on a different sample of world locations and time slots ranging from 1950 to 2008 suggest an average rate of about 6% (with a range of 0-20%, depending on the place of measurement).

Artificial light has changed the world. It has been -and continues to be- one of the key factors enabling the development of modern economies and lifestyles. The evolution of its technology was marked by punctuated equilibrium: long periods of stable development followed by short periods of rapid change where deep technological breakthroughs radically altered the lighting landscape. No wonder the witnesses of these periods of change were deeply moved when looking at the marvels of the novel light sources. In the night of April 2, 1851, what was probably the first public demonstration of carbon-arc electric lighting in the Iberian Peninsula was made at the old cloister of the Universidade de Santiago de Compostela, making a profound impression on all attendees. As a fictional librarian appearing in a little novel written seventy years later put it "The night was swept away from Earth".4

2. THE EXPLICIT COSTS OF LIGHTING: PRICE AND CARBON DIOXIDE EMISSIONS

The public debate about lighting costs is usually focused on two main factors: the direct expenses incurred by the users of lighting services as they appear in the lighting bill, and the greenhouse gas emissions (mainly CO2) associated to the production of the electric energy -and, in a smaller measure, to the direct burning of fosil fuels- required to produce light. At the time of writing, the overall amount of money spent in public and private lighting is a key factor of concern for policy makers worlwide, especially in those economies most heavily affected by the financial downturn. The need of reducing the global greenhouse gas emissions has a longer history and it constitutes today a relevant part of the mainstream environmental policy of many developed countries, even if the practical results of the strategies implemented so far still fall short from expectations.

The unit price of lighting, per lumen·hour, has dramatically decrased as new technological improvements allowed for a more efficient use of energy and the energy itself became more affordable. One Mlmh costs today 20000 times less than seven centuries ago,2 about 6000 times less than in the XVII century1 and 3000 times less than in year 1800.2 The 2005 world estimate for the overall cost of the electricity used for lighting amounted to USD 234 billion (109) with an average cost of USD 2.8 per Mlmh. Taking into account the equipment and labor costs the overall figure for electric lighting rises up to USD 356 billion. Outdoor lighting consumed about 8% of total lighting electricity, producing 16.1 Plmh with a source efficacy higher than the average, and an overall cost of USD 18.7 billion (USD 1.33 per Mlmh).1 Fuel-based and vehicle lighting contributed to the bill with additional USD 38 and 66 billion, respectively. Overall, lighting consumed the 1.2% of the world GDP for that year.

Global greenhouse gas emissions associated to lighting are far from being modest. According to the IAE report quoted 1 above, the global CO2 emissions per year amounted to 1900 Mt (1528 Mt due to electric grid lighting, 190 Mt from fuel lighting and 181 Mt from vehicle lighting). This figure is about the same as the combined CO2 emissions from France, Germany, Italy and UK, or the 70% of the emissions produced by all world’s light-duty vehicles.

3. ADDRESSING THE EXPLICIT COSTS: SOLID-STATE LIGHTING

The pressing need of reducing these explicit and easily perceived costs has fostered the search for practicable saving approaches. The main emphasis has been put in achieving more efficient lighting systems. The implicit assumption underlying this strategy is that by reducing the amount of energy needed to achieve a given illumination level the overall

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electricity bill and the greenhouse gas emissions will be significantly reduced, both for the individual consumer and for the world as a whole.

In 2005 the world's lighting system average efficacy was estimated to be 50 lm/W, a figure not particularly exciting but a significant improvement over the mere 18 lm/W achieved in 1960. It grew at a nearly constant rate of 2.8% per year from 1960 to 1985, slowing afterwards to an annual 1.3%.5 The overall efficacy of any lighting system depends on a combination of factors, as the source efficacy and the luminaire efficiency including power losses at the drivers. In an attempt to resume high growing rates, a major effort has been made in the last years to increase the overall system performance by increasing source efficacy.

The most succesfull approach so far for attaining this goal is LED-based solid state lighting. Although the market penetration of LED lighting is still very small, it is growing rapidly. According to reports quoted in the 2013 edition of the US Department of Energy Solid-State Lighting Research and Development Multi-Year Program Plan6 the overall 2012 worlwide LED sales (including, but not restricted to, illumination lamps) were estimated to be a total of USD 14.4 billion with a 25% grow rate. Long term predictions of the lighting market evolution are difficult to made, but several analysts give figures in the range 25-50% for the percent of LED-based lamps that will be sold worldwide in 2020. Lamp prices are still an issue that difficults their widespread acceptance: compared on a normalized output basis, the price of a LED-based lamp is still eight times higher than that of an halogen bulb and about twice the price of a dimmable compact . However, it is expected that technological improvements and the foreseeable market expansion will substantially lower these prices in the next few years.6

High-brightness white LED sources are an essential component of this strategy. White light can be obtained using monochromatic color-mixed LEDs, as well as by the now prevalent combination of a blue-emitting LED with phosphor layers (Ce:YAG and others). Under controlled environmental conditions LEDs are able to achieve high luminous efficacies. White light LED packages with efficacies of order 95-165 lm/W are commercially available as of 2012. It can be reasonably anticipated that in the next few years commercial LED packages can approach efficacies of about 260 lm/W using monochromatic color-mixed LEDs and about 200 lm/W using blue LEDs with phosphor conversion, a really significant improvement over conventional sources. The overall luminaire efficiencies (including the losses due to thermal mismatch and the efficiencies of the driver electronics and the lamp fixture itself) are expected to grow from the present 62% values up to a 79%, allowing to increase the overall luminaire efficacy from the present average of 80 lm/W up to about 210 lm/W.6

However, the actual performance and the social acceptation of LED-based sources have yet to be demonstrated in long- term indoor and outdoor lighting applications. Some key issues have to be addressed: expected LED lifetime, including that of the acompanying electronic modules, temperature sensitivity, color quality, uniformity of different production batches and overall compatibility with the existing lighting infrastructures are but a few of them. Notwithstanding that, and besides luminous efficacy, LED based sources have distinct advantages that make them an appealing option: their small source size allows for an efficient collimation of the lightbeam (delivering the photons where they are actually needed and reducing lightspill into surrounding areas), they can be switched and/or dimmed on demand at very high rates, thus allowing for a taylored time schedule of lighting, and if properly designed they have satisfactory color rendering indices. Overall, there is little doubt that LED sources will be the key elements of the next lighting paradigm.

4. LIGHT POLLUTION: THE HIDDEN COSTS OF LIGHT AT NIGHT

The explicit cost factors addressed above are the most easily recognizable costs associated to lighting, but they are by no means the only ones. Lighting the night has also important externalities that translate into additional and relevant hidden costs. Most of them are consequences of what is known as light pollution i.e., the detrimental alteration of the natural levels of darkness due to artificial emissions of light. Light pollution manifests itself in different forms, as increased levels of night sky brightness, light trespass or .

Some level of light pollution is arguably unavoidable if light is to be used at night, and very few would contend that the strict preservation of the nighttime environment must be traded off against some basic human needs. However, the inappropriate use of light at night can have harmful consequences for humans as well. A growing body of research

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carried out in the last years helped to unveil the negative externalities associated to light at night. They are commonly known as "the dark side of light". Sensible concerns arose among the scientific community in three main fields: ecology, public health, and science and culture.

4.1. Ecological consequences of light at night

Light and darkness are deeply rooted in life structure. Since the first living forms appeared on Earth about 3.8 billion years ago life has evolved under a natural day-night cycle. Most organisms -including humans- have developed circadian oscillators finely tuned by this alternating illumination levels. Many aspects of their physiology and behavior are controlled by this cycle. The night is in itself the realm of an important proportion of biodiversity: more than 60% of all invertebrate and a 30% of all vertebrate species are nocturnal. The evolution of vertebrates, in particular, has been marked by a progressive colonization of the night environment.7

The natural levels of nighttime darkness have been increasingly compromised in the last decades by artificial light sources. Today the skyglow produced by city lights can reach levels exceeding in many places those of the natural twilight.8 The rapid change experienced in these illumination levels has been shown to cause disruptive effects on organisms, populations and ecosystems, affecting among others to the metabolism, reproduction, foraging behavior, predator-prey balance, orientation and migration.9 These detrimental effects extend to an ample variety of species across the animal and vegetal kingdoms.10,11

4.2. Public health issues

The alteration of the nightime environment has raised also sensible concerns about its detrimental effects for humans. Human circadian rithms, like those of other mammalian species studied until now, are controlled by a set of physiological oscillators which are synchronized by light/dark signals. The information about the external light levels is translated to the circadian pacemaker by a specialized class of non-imaging photoreceptor cells known as intrinsically photosensitive retinal ganglion cells (ipRGC) whose existence and role were unveiled about ten years ago.12,13 Conventional retinal ganglion cells (RGC) are neurons which receive image information from the cones and rods through bipolar and amacrine cells and release it to the upper sections of the visual processing system. The ipRGC are a third class of photoreceptors, fundamentally different from rod and cones, and with different physiological functions: the ipRGC detect the incoming photons via the photopigment melanopsin whose sensitivity peaks in the blue region of the spectrum (around 460-484 nm), and send directly their signals to the central circadian pacemaker located in the suprachiasmatic nucleus of the hypothalamus via the retinohypothalamic tract.

The signals of the ipRGC help to synchronize the circadian oscillators with the natural light/dark period. This rithm regulates, among others, the daily production of (N-acetyl-5-methoxytryptamine), an hormone signalling the chemical night of the body which is at the same time a powerful antioxidant and oncostatic agent. Normal release of melatonin takes place at central nighttime hours and only under strict darkness conditions. If light with appropriate spectral components is detected by the ipRGC -independently from the wake/sleep state of the subject- the melatonin production is rapidly blocked.12,14 The chronic exposure to light at night with blockage of melatonin release and the subsequent disruption of circadian rithms has been shown to have detrimental consequences for health both in humans and in animal models. Particular concerns have been raised by their probable influence on the incidence of several forms of cancer in the developed world,15 of which ample evidence has been collected from animal models.16

After a comprehensive review of the available evidence the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) published a detailed report concluding that shiftwork that involves circadian disruption is probably carcinogenic to humans,17 falling into the risk group 2A in the IARC scale (1: definitely carcinogenic; 2A: probably carcinogenic; 2B possibly carcinogenic; 3: not classifiable; 4: probably not carcinogenic. For a detailed description of the meaning of each classification group see Ref [18)]. Although the evidence in humans is still limited, there is sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night).17 A review of the adverse health effects of nighttime lighting also led the American Medical Association (AMA) to adopt a relevant set of recommendations in its recent 2012 annual meeting.19 The European Union's Scientific Committee on Emerging and Newly Identified Health Risks (SCENIHR) made public that same year an opinion based on a comprehensive analysis of the health effects of artificial light addressing a wider range of

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pathologies. Among other conclusions regarding light at night it states that "besides the beneficial effect of light, e.g. through synchronising the day-night rhythm, there is mounting evidence suggesting that exposure to light at night while awake (especially during shiftwork), may be associated with an increased risk of breast cancer and also cause sleep, gastrointestinal, mood and cardiovascular disorders possibly through disruption. Importantly, these effects are associated with light, without any specific correlation to a given lighting technology." 20

Particular health concerns are being raised by the accelerated pace of deployment of white sources based on blue (GaN or InGaN) LEDs with phospor conversion, due to their high spectral radiance in the blue region, well within the melatonin action spectrum passband. These sources are also a matter of concern regarding potential photochemical damage in the eyes (mainly by cellular oxidative stress in the retina).21

4.3. The loss of the night sky

Besides their negative ecologic and public health consequences, the increasing levels of light at night also endanger the immaterial heritage of humankind. Skyglow due to city lights near astronomical observatories is jeopardizing their ability to gather science data. The astronomy and astrophysics scientific community, indeed, was the first to call public attention on the progressive degradation of the quality of the night sky.22 The loss of the starry sky has also deep consequences regarding the cutural and aesthetic values of the nightscape: As of 2001 it was estimated8 that about two- thirds of the World population (and 99% of European Union and US population excluding Alaska and Hawai'i) lived in areas with light polluted skies. The same study revealed that almost one-fifth of the World population, more than two- thirds of the US and more than one half of the European Union could not see naked-eye the Milky Way. Nor surprisingly several international agencies, not-for-profit organizations and other stakeholders have taken an active position for the preservation of the quality of the night skies. One example of interdisciplinary approach is the International Initiative in Defence of the Quality of the Night Sky as Mankind’s Scientific, Cultural and Environmental Heritage, also known as Initiative Starlight, which in 2007 promoted the La Palma Declaration "in defence of the night sky and the right to starlight"23 with the participation of representatives of the UNESCO, UNWTO, IAU and other international agencies.24

5. TOWARD A SUSTAINABLE APPROACH FOR LIGHT-AT-NIGHT MANAGEMENT

As shown in the section above, explicit budgetary costs and greenhouse gas emissions are by no means the only issues to solve in order to ensure a sustainable, healthy and satisfactory approach to lighting. The present emphasis on energy efficiency and the current trend toward the wide adoption of a particular technological solution for general lighting purposes (phosphor-coated blue LEDs) may worsen the problem, due to their limited scope and unwanted side-effects. Sustainable solutions to lighting require global approaches that critically assess the explicit gains and specifically address the hidden costs described above.

5.1. Beware of the rebound effect (Jevon's paradox)

Improving the efficacy of light sources and increasing the overall energetic efficiency of lighting do not necesarily lead to a smaller global energy consumption (and CO2 emissions). As W.S. Jevons pointed out in a now classic sentence of his book The Coal Question, written at mid-XIX century: "It is wholly a confusion of ideas to suppose that the economical use of fuel is equivalent to a diminished consumption. The very contrary is the truth."25 The actual validity of this apparent paradox, also known as the rebound effect or the Khazoom-Brookes postulate,26 can be checked in many fields of economics, including lighting.27

Overall, as Fouquet and Pearson point out regarding the introduction of LEDs, there is the danger that "based on past evidence, new technologies and falling prices provide an opportunity for dramatically increased current uses of lighting, for finding creative new uses, for redefining the lighting experience (and standards), and for becoming increasingly wasteful. (...) policies that focus on improving energy efficiency are likely, in the absence of rising energy prices, to reduce energy use by less than the resulting efficiency improvements, a potentially significant issue in relation to climate change and greenhouse gas emissions." (Ref [2], pp. 171-2).

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The expected energy savings arising from an enhanced efficiency are counterbalanced by three types of rebound effect:28 The direct rebound, which is the increased level of light consumption due to the smaller price of light allowed by this improved efficiency; the indirect rebound, which consists in an increased consumption of other energy-hungry goods allowed by the additional money available to spend in products and services other than lighting; and a third rebound component due to the effects on energy consumption of all market adjustments derived from the new efficiency scheme. Greene et al28 estimated that the magnitude of the direct rebound effect is about 20%, but no reliable data are available on the magnitude of the other two dimensions. Several authors conclude that taking into account microeconomic and macroeconomic effects, technological progress that improves energy efficiency will tend to increase overall energy use.26

5.2. Promoting a new culture of light

Light consumption is strongly influenced by cultural values and not only determined by functional needs. After a long history of photon-starved societies, artificial lighting became ubiquitous and light at night was closely associated to the ideas of progress, welfare, safety and social status.29 Increasing endlessly the illumination levels was taken as a synonym of better visibility, ignoring the basic findings of visual science. Light in itself was regarded merely as a harmless information carrier, and its consumption was limited almost exclusively by economical considerations. Nowadays the prevalent approach among wide sectors of our population and policy makers still seems to be "the more light the better" (everything else being equal), to such an extent that would make Goethe's Mehr Licht! pale in comparison.

This approach unavoidably led to a culture of light waste, contributing to the high levels of energy consumption, greenhouse gas emissions and light pollution we are experiencing today. Addressing this issue is not just a matter of raising the luminous efficacy of sources: what we need is a new culture of light, characterized by the consideration of the night as an essential part of the environmental and human natural cycles. Light at night should be managed with a careful assessment of its influence on individuals and ecosystems, taking into account its known side-effects and optimizing its use in function of thoughtfully oriented goals.

Relevant gains can be achieved in the short term by making only minor changes in the current lighting paradigm:30 optimizing what and when has to be illuminated, avoiding overillumination, blocking light emissions close to the horizontal or the upper hemisphere, preventing light spill specially in ecologically sensitive areas, carefully avoiding light trespass into residential buildings and judiciously choosing light sources by paying attention not only to their luminous efficacy and color rendering properties but also, and more importantly, to their detailed spectral composition as this is a key factor in the hazards they can pose to the environment and public health. These easy-to-adopt measures, complementing the comprehensive standards already used in lighting engineering and with an adequate enegy policy, will allow for a healthier night environment.

5.3. What can the Optics and Photonics community contribute to this collective effort?

Lighting is a technical and socially complex issue involving researchers, developers, manufacturers, distributors, regulating bodies and consumers. Any sustainable approach to lighting must address a variety of interests and achieve a sensible trade-off among often conflicting aims, putting at the forefront the citizens' needs including environmental and health issues as an essential part of the overall welfare. The Optics and Photonics community by its deep knowlegde of light, its propagation and its interactions with matter, is particularly well located to play a central role in the collective effort to achieve a healthier and more environmentally friendly lighting.

As of today several relevant gaps in knowledge have been identified, particularly in the field of health sciences.19-21 An example of research area where data are lacking is the assessment of the nightly exposure patterns of the population at large to different light sources. Specific attention deserves the long term exposure to small amounts of blue and UV radiation in outdoors, workplace and home, due not only to ambient lighting but also to the use of electronic displays. Health concerns regarding these exposures are sound, especially those affecting the circadian system, but up to now they have not been satisfactorily addressed in safety standards (the current IEC/EN 62471 standard on "Photobiological safety of lamps and lamp systems"31 deals mainly with the effects of lighting on eyes and skin). Collecting the required exposure data needs the development of practicable measurement devices and protocols to be used in large scale epidemiological studies. The spectral composition of the radiation to which the population is exposed is a key factor to

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be measured. Other areas where data are scarce or still not enough comprehensive are the determination of the action spectra of several circadian mechanisms and the characterizaction of the short- and long-term evolution of the lamps available in the market (spectral time evolution after switch-on, they way they age, variablity among samples, etc).

The improvement of existing light sources and luminaires and the design of new ones are specific research topics where our community has a well established position. Probably one of the best contributions we can make is incorporating into the new source designs the recent findings about the environmental and public health effects of light at night. These effects shall be included as an essential part of the cost function, and the designs optimized to achieve optimum spectral compositions, in addition to the conventional parameters used today.

Finally, although the Optics and Photonics community in itself does not have any specific role as a regulator some of its members and societies do have, and the community as a whole is a respected voice in this field. Close cooperation with health science researchers to assess the risks of artificial light at night and to establish appropriate limiting exposure values will be instrumental for drafting and adopting new and more comprehensive safety standards that will improve the welfare of our societies.

ACKNOWLEDGEMENTS

This work was funded in part by the Galician Government, Programa de Consolidación e Estruturación de Unidades de Investigación Competitivas, grant CN 2012/156, and was developed within the framework of the Spanish Network for Light Pollution Studies (Ministerio de Economía y Competitividad, Acción Complementaria AYA2011-15808-E) whose support is also acknowledged.

REFERENCES

[1] Organisation for Economic Co-operation and Development (OECD)/International Energy Agency (IEA), [Light’s labour’s lost - policies for energy-efficient lighting], OECD/IEA, Paris, France (2006). [2] Fouquet, R. and Pearson, P.J.G., "Seven Centuries of Energy Services: The Price and Use of Light in the United Kingdom (1300-2000) ," The Energy Journal 27(1), 139-177 (2006). [3] Hölker, F., Moss, T., Griefahn, B., Kloas, W., Voigt, C. C., Henckel, D. , Hänel, A., Kappeler, P. M., Völker, S., Schwope, A., Franke, S., Uhrlandt, D., Fischer, J., Klenke, R., Wolter, C. and Tockner, K., "The dark side of light: a transdisciplinary research agenda for light pollution policy," Ecology and Society 15(4): 13 (2010). [online] URL: http://www.ecologyandsociety.org/vol15/iss4/art13/ [4] Cotarelo-Valledor, A., [La chispa mágica], Tip. El Eco de Santiago, Santiago de Compostela, Galicia (1923). [5] McGowan, T., “Energy-Efficient Lighting”, in T.B. Johansson, B. Bodlund and R.H. Williams (eds), [Electricity: Efficient End-Use and New Generation Technologies and their Planning Implications], Lund University Press, Lund, Denmark (1989). Quoted in ref 1. [6] Office of Energy Efficiency and Renewable Energy, [Solid-State Lighting Research and Development Multi-Year Program Plan], U.S. Department of Energy (April 2013). URL: http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/ssl_mypp2013_web.pdf [7] Hölker, F., C. Wolter, E. K. Perkin, and K. Tockner, "Light pollution as a biodiversity threat," Trends in Ecology and Evolution 25, 681-682 (2010). [8] Cinzano P., Falchi F. and Elvidge C., "The first world atlas of the artificial night sky brightness," Mon. Not. R. Astron. Soc. 328, 689–707 (2001). [9] Navara, K.J. and Nelson R.J., "The dark side of light at night: physiological, epidemiological, and ecological consequences", J. Pineal Res. 43, 215-224 (2007). doi:10.1111/j.1600-079X.2007.00473.x [10] Longcore, T. and Rich, C., "Ecological light pollution. Frontiers in Ecology and the Environment" 2(4), 191-198 (2004). [11] Rich, C., and Longcore, T. (editors), [Ecological consequences of artificial night lighting], Island Press, Washington, D.C., USA (2006). [12] Thapan, K., Arendt, J. and Skene, D.J., "An action spectrum for melatonin suppression: evidence for a novel non- rod, non-cone photoreceptor system in humans," Journal of Physiology 535.1, 261–267 (2001).

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