The ambivalence remains, even when the supposedly more critical answers of light pollution experts (n
= 89) are separated from the supposedly less critical views of lighting professionals (n
= 67) in our sample. Among the light pollution experts, 90% (80) are critical of LEDs, whereas 43% (38) of the respondents in this group support the claim that LEDs can reduce light pollution (Figure 2
). Almost as many of them (36) are ambivalent towards LED lighting and agreed to statements both for and against LEDs. Among the lighting professionals, 42% (28) think that LEDs reduce light pollution, whereas a surprisingly larger share of 49% (33) responded that they cause light pollution. The overlap in the group of lighting professionals is only 19%.
Many of the respondents further explained their views on LED lighting and light pollution in open statements. In the following two sections, we summarize their statements against and in favor of LED lighting.
3.1. “Cheaper, More Blue and Brighter”: LEDs Contribute to Light Pollution
In total, 137 light pollution experts and lighting professionals explained why LEDs contribute to more light pollution and 92 explained why they can reduce light pollution. When asked why they thought that LED technology contributed to more light pollution, they mentioned LED-related problems and usage-related problems that are well known in expert communities but have not yet been solved. LEDs add to light pollution since they are “cheaper, more blue and brighter”, summarized one light pollution expert very pointedly. Other statements offered slightly more detail, but were generally short and concise. In our qualitative analysis, we categorized them into six different arguments plus one residual category. Table 1
lists the categories in order of their importance and illustrates them with exemplary survey statements.
It is obvious that the arguments operate on different levels and are not mutually exclusive. Some respondents address several LED-related physical aspects like light levels, glare and light spectrum in one sentence. Others explain why these physical aspects matter in the context of light pollution, for instance, by pointing to circadian disruption. Usage-related statements show that the physical aspects of LED lighting are not set in stone, but result from design choices and specific usages.
When comparing the arguments of light pollution experts and lighting professionals, we find interesting commonalities and differences (Figure 3
). Among the light pollution experts, almost two thirds (65%) highlight the problem of color temperature; that is, the blue-rich spectrum of LED lighting. Just over one third (36%) mention the issue of rebound effects. More than one in four light pollution experts (27%) point to generally brighter light levels and about 9% complain about glare, often combined with remarks about poor, unshielded luminaire designs (4%) and bad lighting designs and installations (12%). In comparison, the lighting professionals in our sample mostly highlight rebound effects (63%), followed by issues with blue-rich color temperatures (27%), increasing light levels, poor luminaire and lighting design and/or bad installations (all 17%), and glare (10%).
The differences between the two groups seem related to the different foci of the two groups. Lighting professionals experience the increasing use of LED lights, their clients’ demands, unsatisfying LED products and bad lighting design and planning on a daily basis, which can explain their greater awareness of these issues. This became particularly clear in our thematic discussion about the regulation of city lights, where lighting designer Cinzia Ferrara expressed her concern about “a world that doesn’t have any limits”, where people always desire more [18
] (p. 165). Moreover, many of the lighting professionals consider excessively bright light levels and glare to be effects of poor design and hence, a result of problematic planning and installations. In line with this feedback, Allan Howard, another discussion participant who works as a lighting and building consultant for an international engineering company, stressed the importance of professional lighting expertise in building projects, which also means ensuring “that the contractor doesn’t substitute the (lighting) designers” [18
] (p. 153).
In contrast, light pollution experts are more concerned about color temperature. This is not surprising, considering that the light spectrum plays a key role in ecological, medical, as well as astronomical research on artificial light at night [25
]. Astronomers know that blue-rich lighting scatters more in the atmosphere (Rayleigh scattering) and thus contributes to undesirable sky glow, which is also reflected in the scientific literature [26
]. Neuro-scientists and biologists show that blue-rich light at night has the greatest effects on the circadian rhythm of humans [29
] and on a number of wildlife species [9
]. They further recommend light sources with narrow spectral ranges, as ecologist Sybille Schroer explained in our thematic discussion on color temperature choices: “[T]he broader the spectrum…, the broader the group of organisms you affect. Because light is never neutral, it affects organisms in different ways and some are more sensitive to short wavelengths, others to longer wavelengths” [18
] (p. 80).
In our survey, the number of lighting professionals who found blue-rich LED light problematic was much smaller (27%) than the number of light pollution experts (65%). The thematic discussion on color temperature revealed that there is a deep-seated, underlying conflict that cannot be easily solved. As opposed to the eco-friendly solution described above, lighting professionals prefer full-spectrum light sources and white light, which includes the blue part of the spectrum. The reason for this choice is twofold, as lighting engineer Tran-Quoc Khanh pointed out: “With blue-rich white light, we have both—more brightness and a better contrast on the street with the same amount of wattage. This means that we could reduce our wattages and energy use in cities and get the same amount of visibility that we have today” [18
] (p. 87). This is because the human eye is best adapted to full-spectrum daylight. Under twilight conditions, it is even more sensitive to shorter wavelengths [35
]. In addition to this physiological advantage, there is also a positive physical effect. Cooler LEDs are more energy efficient. Since the color conversion of blue LED light into white light via phosphor coatings absorbs energy, the energy loss is smaller in blue-rich LEDs.
In line with these photometric findings, lighting designer Nancy Clanton shared the results of a field experiment where people drove a car at 35 miles per hour (50 km/hour) and pressed a button as soon as they noticed an object on the street. Clanton reports that the 4000 Kelvin color temperature “significantly increased the reaction time” and “white LED light and a better color rendering helps to increase the visual performance of drivers compared to high pressure sodium” [18
] (p. 90). At the same time, Clanton and her discussion partners from India and Germany observed that there are places where citizens object to cool-white street lighting. Venkatsh Dwivedi, who is overseeing the large-scale LED refurbishment of public streetlights in India [4
], reported that they chose cool-white LEDs (5000 and 5700 Kelvin) “for efficiency reasons and … because the color looked much better”. Yet, when they changed the lighting around iconic places and heritage sites, citizens complained until a court decided that the light should be made warmer [18
] (p. 97). Citizen protests against cool LED light have also taken place in other places like Montreal, New York, Rome and Berlin [18
] (pp. 185–186) [36
Coming back to the survey results, half of the respondents mention not just one, but two to four aspects in their short statements. For instance, one in six respondents problematizes the blue-rich color temperature of LEDs and rebound effects in one breath: “… [because] most LED have a large blue light output and are used more excessively [because] of reduced energy cost.” Similarly, almost one fifth of the light pollution experts refer to color temperature in combination with increased brightness. Yet, while these physical aspects might contribute to light pollution independently of one another, other statements point to causes and effects and show clearly that physical LED-related aspects like blue-rich color temperature, glare and excessive light levels are often inseparable from (if not the result of) usage-related design choices.
Such problematic aspects arise during all stages of the innovation process, starting with the development of new LED products, the choice and procurement of new luminaires, the infrastructure planning and the installation on site. Experts are well aware of the pitfalls of professional lighting practices. As one light pollution expert points out, “lighting installations are never tested or checked at night.” Another suggests that light pollution increases because of “bad retrofits” and the existence of “many unshielded LEDs.” Retrofits are causally linked to higher light levels “when people do one-for-one swaps without considering higher visibility under LEDs, which could allow for overall lower light levels.”
Meanwhile, light pollution experts also criticize the fact that municipal LED users “do not care about the real possibilities (of LED technology) by dimming them.” In the same vein, a lighting professional criticizes “excessive spectrum at damaging wavelengths and poor glare control.” Another points out: “Poorly designed installations, and especially those that do not dim LED installations (relative to earlier technologies), are shown to result in greater glare and sky glow.” Yet another addresses project-level design decisions, market-level LED product designs and worldwide rebound effects in one sentence: “Horrible ‘white’ spectrums. Planar wavefront formation, concentrated source surfaces. High efficiency leads to over-illumination.” The comment also illustrates that rebound effects can be both quantitative (more light points) and qualitative (brighter light points).
Last but not least, several respondents criticize a general lack of consideration in the lighting field: “There is no holistic design intention”, complains a lighting professional, while others complain about the “specifiers” who bring LED technology into the real world through their planning and product choices. One lighting professional says specifiers do not do enough research and adds “cheapest is not best”. Similarly, another argues that “specifiers consider lumens per dollar, but seem to go no further on actual outcome or effect.” When considering poor design as part of the problem, it seems that unsustainable LED lighting does not result from the technology itself, but rather from unsustainable usage and design practices that do not adequately capitalize on the novel characteristics of the innovation. This impression is reinforced by our analysis of the arguments that suggest LED lighting can reduce light pollution.
3.2. “If Correct CCT and Good Design”: LEDs Reduce Light Pollution
For many respondents from both groups, and in line with the previous findings, the innovation’s potential is contingent on “correct CCT and good design”. For example, one expert states: “LEDs can be shielded to reduce light pollution. They can be switched on by demand and have several control options. They come in low CCTs for healthier light.” CCT stands for correlated color temperature, which can be adapted and even dynamically controlled in LED luminaires. “Good design” can thus refer to spectrum control, but also light levels, light distribution and even dynamic control.
To look more closely at why the survey participants thought LED lighting can reduce light pollution, we identified four arguments plus one residual category. Table 2
offers an overview of these arguments in order of importance, with exemplary statements from the survey.
This time, we do not distinguish between luminaire and lighting design as they are not clearly discernable in the statements. For instance, the frequently and positively mentioned directed LED light can be the result of a well-shielded luminaire with customized optics and/or the outcome of good light planning and design. We do differentiate between “dimmed light levels” and “dynamic control”, even though the aim of sensor-controlled installations is to dim lights or allow temporal darkness—an approach some respondents referred to as “light when needed”. Technologically speaking, there is a difference between just dimming or switching off lights and dynamically controlled lights, since the latter requires some form of advanced, dynamic or responsive control system that is either sensor-based or digitally programmed. What is known as “smart” or “intelligent” lighting therefore marks the next step in terms of innovation. Smart lighting is more radical than retrofitting existing infrastructures with LEDs, even if such retrofits can also be dimmed and switched off with non-dynamic technology [21
]. This difference is also reflected by the fact that most of the survey respondents feel affected by LED lighting, while the introduction of dynamic, digitally controlled lighting systems plays a less important role in their light-related activities (the item ranked sixth in a list of ten potential trends, see Appendix A
In contrast to the arguments against LEDs, the arguments in favor of LEDs are quite homogeneously distributed in the two groups (Figure 4
). The majority of light pollution experts (84%) and lighting professionals (81%) argue that well-considered and appropriate LED fixtures and lighting designs can reduce light pollution. A light pollution expert acknowledges that there are generally “more standardly shielded fixtures” and that it is “quite easily possible to reduce the brightness.” Several lighting professionals highlight that beam control, meaning controlled light distribution, is easier with laser-like LED light sources: “LEDs are more flexible than other light sources and can bring light only to areas where it is really needed.” Another explains in more detail that better beam control means “less uplight and light trespass” and adds that LEDs also show “better dimming capabilities and better spectral control for ecologically sensitive areas (where spectral sensitivity is established, e.g., turtles, migratory birds)”, which is a color temperature argument.
In both groups, 41% of the respondents find that the customizable color temperature or “spectral control” of LED luminaires provides an opportunity to reduce the spectral range and make artificial light at night more ecologically friendly and healthy. Some light pollution experts explicitly promote the use of phosphor-coated amber colored LEDs: “The option of low-blue-light LEDs like PC-amber can offer a great combination of the pros of LEDs while reducing the amount of blue to amounts that are less than HPS.” On the issue of increasing brightness, 41% of the light pollution experts and 33% of the lighting professionals point out that LED technology facilitates dimming and better light distribution. A lighting professional explains “it can be dimmed easily, it can be easily directed because of the optical possibilities, and the spectrum light emissions can be regulated and customized.” Interestingly, only 25% of the survey participants mobilize the marketing argument that LED technology is perfectly suited for “smart” or “intelligent” sensor-controlled lighting systems (Figure 4
As with the pro LED statements, lighting design is causally linked to technological facts, as outlined by a light pollution expert: “Well-designed installations, in which light spill is well controlled, CCT is low and illuminance levels are appropriate, could well have the effect of reducing light pollution worldwide.” Another describes a trade-off but is still in favor of LEDs: “A recent change from old HPS lighting to LED street lighting showed an increase in sky quality. Although the penetration of higher color temperatures is greater, the directed output meant better results for dark skies.”
After having read very strong arguments against LED lighting it might be surprising that the same light pollution experts now highlight the great advantages of LED light: “LEDs are key to reducing light pollution: directionality, dimmable, tailored spectrum.” On the other hand, the expert statements make it clear that the advantages of the innovation depend on how it is used. The experts not only link the potential reduction of light pollution to design choices (>80%), but also stress the conditionality of their LED appraisal in the language they use.
More than half the statements in favor of LEDs contain an “if”, “could” or some other conditional grammatical form. To give just a few examples, one lighting professional argues: “LED lighting is a big chance to use the appropriate amount, the perfect light and spectral distribution for the intended purpose.” Another explains: “If used correctly, LEDs can reduce light pollution. It all depends on the light distribution and of course dimming.” Some conditional statements show that the reverse is happening: “When correctly fitted they are good. But because they are cheaper, people tend to overlight areas,” or “If it is very red and very dim, these could reduce light pollution—they seem never to be red nor dim.” The conditionality of positive observations resonates with Matthew Gandy’s diagnosis: “The transition to LED technologies”, said the geographer and urbanist, “illustrates a divergence between technologically infused environmental rhetoric and science-based ecological discourse, because the emerging political momentum to reduce light pollution is being offset by a new generation of energy-efficient technologies…” [37
] (p. 1103).