In recent years, initial studies have explored the general populations’ views on lighting and light pollution. For instance, Lyytimäki and Rinne [21
] carried out an online survey to understand how people in Finland perceive and respond to light trespass and other light nuisances (n=2053). In Germany, Besecke and Hänsch [22
] explored how residents of an inner-city street of Berlin and inhabitants of a nearby suburban community perceived light and darkness before and after street lighting refurbishments to LED lighting. Green et al. [23
] used ethnographic data, household survey and documentary sources to explore responses to street lighting reductions in eight areas of England and Wales. This study complements this strand of research by providing results on expert
perspectives on the topic. Expert perspectives are relevant as outdoor lighting has long been delegated to expert systems and is only just re-emerging as a public issue [24
]. In contrast to studies of the general public, which are methodologically challenging as they demand asking people about their implicit practical knowledge about lighting [25
], focusing on experts makes it possible to investigate the issue—including its technical aspects—in more depth and detail, given the respondents’ higher level of previous engagement with specificities of the topic. Other than most laypersons, they pay attention to light and darkness and also have a vocabulary to express their observations and feelings about lighting. Moreover, expert opinions are also particularly relevant as they shape realities of artificial light and natural darkness by planning, designing or contesting lighting.
Since we could not draw on existing expert surveys, we had to come up with our own conceptual framework for assessing the group-specific views. Social-scientific theory suggests that expert groups form “communities of practice” with specific understandings and shared views on their respective issues of concern [26
]. Recent discourses in the two stakeholder groups allowed us to develop the three thematic areas covered in the survey based on explicit assumptions, as outlined below (Table 2
). In line with our empirical observation, Challéat and colleagues [19
] have described two camps vis-à-vis lighting in France: lighting professionals who promote a technical view on “light nuisance” and astronomers, conservationists and citizens who take an environmental stance against “light pollution”.
3.1. Light Pollution Experts and the Negative Side-Effects of Artificial Light at Night (ALAN)
To conceptualize the views of the light pollution experts on a global scale, we can draw on the growing body of scientific literature on the effects of artificial light at night (ALAN), which is also the basis for social scientific and planning discourses on ALAN as well as for activists. This interdisciplinary and emerging field can be roughly divided into three areas: Research mainly by astronomers and astrophysicists on sky glow and light trespass as an impediment to the observation of the universe; biological research investigating the impact of ALAN on individual animal and plant species, and increasingly, on ecosystems; medical research exploring the chronobiological hormonal effects that are triggered by ALAN and are suspected to increase the risk of depression, cancer, cardio-vascular diseases and obesity.
Experts who work on these issues have significantly shaped the notion of light pollution. Astronomers, both professionals and amateurs, are a driving force behind initiatives for dark-sky protection. With the spread of electric lighting in the early 20th century, they were among the first to criticize and quantify the reduced visibility of celestial objects [27
]. Today, they explore and develop new instruments and methods for assessing the illumination of the night sky [6
]. They also warn that blue-rich LED light scatters more strongly in the atmosphere and will, in combination with rebound effects, increase not only sky glow but also glare [31
Biologists and ecologists have been studying the effects of artificial light at night on birds, insects, aquatic organisms, reptiles, mammals and plants to understand and assess its impact on these different species as well as entire ecosystems. In recent years, they have substantiated their suspicion that light affects animal behavior (e.g., through distraction) and disturbs the circadian rhythm of living organisms more generally, both with negative consequences for the finely orchestrated processes of all life that have evolved over millennia under planetary rhythms of light and darkness [1
]. All light spectra can be potentially harmful, as different species are sensitive to different types of light. Therefore, full-spectrum light sources and blue-rich light seem to be more problematic than light with a narrow spectrum and longer wavelengths, as these will probably affect more species [33
]. Since circadian processes also govern the human body, exposure to ALAN, and particularly to blue-rich light, has also become a public health concern [5
]. Medical studies suggest that ALAN is a stressor for people who work night shifts or are exposed to blue-rich light at night, such as that emitted by LED lighting [35
Although scientific evidence on the biological impacts of ALAN is still patchy, many biologists and physicians have come to take a precautionary stance and promote the protection or restoration of natural darkness or reduced light levels. In that and in their reservations regarding blue-rich lighting, they share views and goals with astronomers, as well as with actors that engage critically with the illumination of the night from other viewpoints, such as culture or aesthetics [3
]. In the latter respect, it is frequently highlighted that we are losing the experience of natural darkness and the visibility of the stars and planets, which has been a key to human civilization [37
Light pollution experts also actively recommend, develop and test counter-measures. They develop models to assess the scope and effects of the problem, as well as the viability of solutions [3
]. They criticize the fact that existing lighting technology, lighting standards and regulations are not sufficient and that they should be updated to acknowledge issues of light pollution [39
]. Advocacy organizations such as the International Dark-Sky Association (IDA), but also researchers in the ALAN community, address the wide-spread ignorance of the issue and actively engage in raising awareness for light pollution (e.g., ida.org, cost-lonne.eu, stars4all.eu). Finally, light pollution experts are actively involved in shaping lighting technology (e.g., shielded luminaires, PC amber LEDs) and the governance of lighting via tools that range from technical recommendations (e.g., avoidance of light above the horizontal and blue-rich lighting) to education (e.g., in observatories) and mandatory legislation [40
Based on these discourses and developments, we assumed the following:
Regarding the definition of light pollution, we expect that light pollution experts contend that all artificial light at night is pollution, because even small amounts of ALAN are an alteration of natural darkness and may affect living beings and the possibility to observe the night sky. In terms of the problem’s dimensions, we assume that they highlight potential non-visual effects of light on flora, fauna, humans and ecosystems, as well as the cultural loss of natural darkness and star-filled skies.
Regarding the governance challenge, we expect that light pollution experts call for more political commitment and highlight the need to raise awareness for light pollution, to provide more guidance and information to decision makers.
In terms of the problem’s solutions, there seems to be a widely-shared consensus in the light pollution community that systemic change is necessary. We therefore assume that light pollution experts recommend more sustainable technology, better education and information, as well as better technical guidance, lighting standards and stricter regulations.
3.2. Lighting Professionals and the Art of Planning, Designing and Manufacturing Light
Lighting professionals’ perspectives are less obvious, as they often do not refer to light pollution when they write about potential negative side-effects of lighting. As a lighting designer remarked in response to our survey invitation, “the term ‘light pollution’ is an evocative phrase for many lighting designers, including me. Our stance is that light is a pure and natural phenomenon, and the ‘pollution’ angle comes from the misuse of light, or light in the wrong place. We feel that the terms ‘obtrusive light’ and/or ‘light trespass’ are more fitting.” In a commentary published in Nature, lighting designer Zielinska-Dabkowska outlines the potentially negative health effects of lighting without mentioning the term “light pollution” even once [43
Looking at lighting practices and projects, energy and cost efficiency constitute long-standing benchmarks that can be linked to light reduction. Accordingly, the British Institution of Lighting Professionals (ILP) argues in its Guidance Notes for the Reduction of Obtrusive Light: “Do not ‘over’ light. This is a major cause of obtrusive light and is a waste of energy. There are published standards for most lighting tasks, adherence to which will help minimize upward reflected light.” [44
] (p. 1111).
This recommendation also reflects the basic stance that ‘good’ lighting means providing appropriate lighting for a given time, place and task. Light engineering and illuminating societies develop technical standards to provide orientation towards achieving this complex goal (ies.org, theilp.org.uk and licht.de). Lighting design associations and expert networks provide information, education and exchange platforms that enable their members to plan and design light in situation-specific ways and according to their clients’ needs (e.g., iald.org, pld-c.com, luciassociation.org). Light manufacturers also subscribe to this goal which allows them to further develop and diversify their product lines, for instance with a focus on heath or enhanced work performance [45
Knowing how to accomplish ‘good’ lighting is considered a characteristic and distinctive skill of lighting professionals, which qualifies them more than electricians, civil engineers, architects or private home owners to illuminate the world at night. However, in reality, such explicit lighting expertise is often ignored or only invited in the final stages of building or design projects. Therefore, light planners, specifiers and lighting designers often describe light pollution as a problem of missed opportunities: Short-sighted cost-benefit calculations, lack of expertise and time pressure lead to suboptimal solutions that cause nuisances and unwanted side effects.
Professional experience and knowhow appear to be particularly relevant in light of two major developments: For one, climate change policies affect lighting practices in the form of economic incentives, but also product bans like the out-phasing of the incandescent light bulb [25
]. For another, light-emitting diodes (LEDs) constitute a disruptive technological innovation [47
]. LED technology is widely seen as an energy efficient means to provide “the right light at the right place at the right time”, since LEDs are highly directional and can be digitally controlled and adapted in brightness and color temperature. They thus open new business opportunities, which also relate to issues of light pollution. For instance, light manufacturers work on optical systems that reduce glare, conduct their own research on the non-visual effects of blue-rich LED light and offer new products, including PC amber LEDs, to meet the demands of dark-sky friendly lighting schemes.
Finally, new conceptual approaches to lighting are relevant to light pollution debates. First, LED lighting is promoted with visions of adaptive “smart” lighting that responds to lighting needs, thereby reducing excess light [47
]. The notion of “human-centric lighting” highlights the relationship between light and well-being, thereby widening the thus-far dominant focus on more functional aspects like visibility and safety [49
]. In this concept, lighting professionals show a preference for adaptable white light sources with a continuous spectrum, which provide better color rendering than the widely-used sodium vapor street lamps, and are thus assumed to enhance visual comfort in outdoor spaces. Last but not least, lighting designers also highlight the value of darkness, but more for aesthetic than for environmental reasons, which are less prominently voiced in lighting projects [13
] (pp. 182–187).
Based on these observations, we assumed the following:
Regarding the problem’s definition, we expect lighting professionals to argue that it depends on the situation whether light is pollution, or even to reject the notion that light can be pollution altogether. In terms of the problem’s dimensions, they will probably be more concerned with reducing energy consumption and improving humans’ visual comfort (full light spectrum, no glare) and well-being than with protecting natural darkness and star-filled skies or reducing potential negative effects on flora and fauna.
With regard to the governance challenge, we expect that lighting professionals take responsibility and make the mitigation of light pollution their own task, as it calls for professional skills and constitutes a potential business case. It seems likely that they will argue that light pollution would not be a problem if lighting were properly planned and designed by experts.
Regarding the problem’s solutions, we accordingly assume that lighting professionals blame procedural and project-related shortcomings like the lack of lighting expertise in building projects and call for an earlier and more consistent involvement of professionals. We further expect them to rely on the self-regulatory functions of their professional institutions and the state of the art in their professional domain (existing guidelines, best practices, innovative technological solutions and products) rather than calling for ‘external’ intervention via stricter rules and more regulation.