Conference report: Light Pollution, Theory Modeling and Measurements 2024

Image credit: LPTMM organizers 
2270 words / 9-minute read

Summary: Every other year, experts in the science of light pollution gather to share research results and find about the latest advances in this distinctive field of study. Read about the outcomes of this year's Light Pollution, Theory Modeling and Measurements conference.​

Researchers from around the world met in Austria in mid-July for the 2024 edition of "Light Pollution, Theory Modeling and Measurements" (LPTMM). Dark Sky Consulting was there, and in this post we review highlights of the science results presented over four days.

LTPMM 2024 conference group photo

The LPTMM conference series, which began in 2013, focuses on the methods that scientists use to understand how artificial light at night (ALAN) is changing our world. Its theme this year was "the need of measurement standards in light pollution science". The conference organizers themed the proceedings around six broad topical areas:

• Topic A: Theoretical concepts of modeling light pollution
• Topic B: Numerical tools and simulation experiments
• Topic C: Effects of atmospheric aerosols, clouds, terrain, and obstacles on light pollution
• Topic D: Impact of spectral and angular characteristics of light sources and reflecting surfaces
• Topic E: Observational techniques, instrumentation, data, and products
• Topic F: Design and evaluation of dark sky friendly lighting technologies, regulations and outreach

​There are several key takeaways from LTPMM 2024. We step through them below and describe some of the specific studies presented at the conference.

We are still a long way from a detailed global view of the dynamics of ALAN seen from space

Scientists have had access to images of the Earth at night as seen from space for decades. But regular views of ALAN on our planet with instruments optimized for the purpose are still unavailable. Noam Levin (The University of Queensland & Hebrew University of Jerusalem) gave a plenary talk in which he summarized current satellite capabilities in this area. He also identified five key challenges that are holding back advance of remote sensing of ALAN:
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1. The need for strong and reliable identification of clouds in nighttime images
2. Monitoring hourly changes in nighttime lights over a single night
3. Different times of the night in which satellites pass over a given place on Earth
4. Better estimates of blue light emissions
5. Understanding how ALAN is distributed close to the ground to explain what we see from orbit

Of those, Levin identified the blue-light issue as the most pressing.

Cheap computing power puts the spotlight on specifying problems accurately

On the second day of the conference, Alexander Wilkie (Charles University, Prague) presented a plenary talk with the intriguing title "Skylight models in realistic computer graphics". He described creating models of the daytime sky that appear in popular media. Modelers work to get the physics of light scattering in those models right. What they have learned, and the methods they use, can inform our models of nighttime sky light.

​While making these computer models was once very resource-intensive, that's no longer the case. "You can go shopping," Wilkie said, referring to various hardware and software combinations. Computing power is no longer a limiting factor. Rather, Wilkie suggested, the challenge is to specify science questions accurately. This improves the efficiency of calculations and can lead researchers to answers more quickly.

Models of light pollution are more realistic than ever

Researchers spent considerable effort in the last decade on adding complexity to their models of light pollution. Newer modeling software takes into account real-world weather conditions. It follows ALAN further on its journey from source to target, through many orders of light scattering. And it now adds elements once unavailable. Stefan Wallner (University of Vienna) presented a new model including the polarization state of light. His group tested its output using a custom all-sky scanner that efficiently detects sky light polarization. This helps "ground truth" such models.

Brian Espey (Trinity College Dublin) showed a modified light pollution model that incorporates information about the size and height of obstacles in urban environments. Dense clusters of tall buildings, for example, create a "canyon" effect in satellite images of cities at night. Different measurements of light result depending on the angle of observation from orbit. He found that high-resolution obstruction modeling provides a way to calculate lighting conditions and estimate energy usage. This, he argued, can be a lever on decisions about how cities use outdoor lighting.
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Models are also beginning to inform other areas of investigation besides night-sky brightness. Alexandre Simoneau (Cégep de Sherbrooke) showed a new version of his group's ILLUMINA modeling software. Users can apply this version to problems involving light originating outdoors that enters indoor spaces through windows. Coupled with direct measurements of this light flux, it represents an important advance in understanding how much nighttime light exposure people receive indoors. That, in turn, addresses concerns about the extent to which that light may affect their health and wellbeing.

Measurements of light pollution in many different colors are now more accessible

Global outdoor lighting technology and practices have experienced a sea change in the past decade. The arrival of white LED lighting products on the market led to its current dominance. With this has come a shift in the colors of light that cities emit at night. White LEDs, rich in blue light, have displaced earlier technologies that emitted much less of it. As a consequence, some measurement methods assuming the color of the "old" light may not work as well as before.
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In past years at LPTMM and other conferences, we heard from researchers who urged the light pollution research community to take heed. Adapting sensors and cameras to this new reality became clear. And our colleagues in the biological sciences were quick to remind us that measurement devices designed for the human visual system do not work well when applied to other species. Our sensitivity to different colors of light is just not the same as that of other animals. Gradually, researchers have come to make measurements in many colors besides those relevant to human vision.

Camille Labrousse (The Hebrew University of Jerusalem) studied the ongoing transition from legacy lighting sources to white LED. For this she used a four-color version of the popular TESS photometer (“TESS-4C”). She compared the TESS data with measurements using the LANCube device made by the Cégep de Sherbrooke. In turn, she compared both sources with remote sensing data from the Chinese SDGSAT-1 satellite. This helped her determine which kinds of light sources contributed the most light pollution in her study areas.

Iván Kopaitic (Pontificia Universidad Católica de Valparaíso, Chile) introduced BLUBO, a new device for measurement of skyglow in three colors. Kopaitic's team designed BLUBO to help with enforcement of Chile's national light pollution laws. It uses a photodiode and custom electronics to measure light at night in the blue part of the spectrum. He reported that tests of BLUBO are just beginning, but the early results are promising.

And the need to distinguish colors is not limited to ground-based measurements of light pollution. Alejandro Sánchez de Miguel (Universidad Complutense Madrid and the University of Exeter) described a new method of categorizing light sources in satellite remote sensing data. The process involves making color ratios of different sources and applying land-use maps to the satellite images. The results again underscore the value of color information: there is “significance of spectral diversity and spatial information in enhancing our understanding of light pollution and its sources.”

Ground-based devices to sense night-sky brightness continue to be measurement workhorses

Over two decades of experience with portable, inexpensive measurement devices like the Sky Quality Meter (SQM) have transformed our understanding of light pollution. The SQM, and devices like it, provide long time series that make it easier to spot trends. As more such data become available, we gradually get a better handle on how light pollution is evolving around the world.

Christoph Goldmann (Natural History Museum Vienna) shared data from 15 years’ worth of SQM data from Upper Austria. Even up to distances of hundreds of kilometers, Vienna is the main source of skyglow in the region. Long-term monitoring efforts like this help direct resources toward mitigations that may improve conditions over time.

Li-Wei Hung (U.S. National Park Service) presented early results from a new all-sky camera system. This camera enables in a single shot what before took over an hour to produce with a more narrow-angle system. She explained how her team overcame limitations like optical distortion at the edge of the camera field of view. And as with the group's earlier system, she found they could calibrate their images by observing certain stars whose brightness is very well known.
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Lastly, Christof Reinarz (Pontificia Universidad Católica de Valparaíso) showed the design and early results of a new sky brightness measuring device that has only one sensing element. The design uses a micromirror array that scans the sky light across the single-channel sensor. From this they can reconstruct "images" of the night sky using computer algorithms. The benefits of their design includes low cost, few moving parts, light sensitivity in important color ranges, and the ability to operate even in high ambient light levels.

Balloon- and drone-borne instrumentation development is better connecting with ground-based sensors

As interest in light pollution research rose in the last decade, scientists developed a variety of new ways of seeing light pollution. Satellite remote sensing platforms have been available for several decades, and citizen-scientists have measured night sky brightness from the ground for almost as long. Creative thinking is now exploiting the altitudes between the ground and orbit. New sensing platforms in the space between these extremes give information that was once impossible to get.
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One application of new platforms is making more complete measurements of the light emitted by cities at night. Pietro Fiorentin (University of Padua) showed how drone-borne sensors moving between multiple points about an illuminated region in 3-D space can make such light maps. Measuring the light intensity at points on a hemispherical grid, his research group's method fully resolves the emission function of lighting installations. This “open[s] new perspectives for urban planners and local authorities in planning more sustainable and environmentally respectful urban lighting strategies.”

William Fateaux (Cégep de Sherbrooke) presented the Reusable Open Stratospheric Explorer (R-OSE), a simple, low-cost, reusable platform for high-altitude measurements of night lights. Working with team member Alex Mavrovic, the group is testing Flying Eye (FLeYe), a balloon-borne device equipped with 12 cameras. They plan to combine FLeYe data with a complete inventory of both public and private lighting in the northern Canadian town of Iqualkuuttiaq to form a complete picture of ALAN in the area.

Light pollution science now interacts with society more than ever

Studies of light pollution have advanced far beyond their initial interest only among astronomers. Research now more often influences policy making, lighting technology development, and decisions about where, when and how to use ALAN. This includes bringing ideas to market. Andreas Jechow (Brandenburg University of Applied Sciences, Germany) shared recently published results of an effort in which researchers partnered with a European lighting manufacturer. Their goal was to design and test prototype lighting products whose light distributions are chosen to reduce insect attraction at night. They found that while reducing the illumination level caused little change in attraction, controlling the light distribution made a big difference. They saw insect attraction drop by up to a factor of 6 at their dark test site compared to the attraction rate to more "standard" luminaires.
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In separate presentations, Andreas Hänel (Dark Sky Germany) and János Sztakovics (Eszterházy Károly Catholic University, Hungary) argued for the importance of characterizing naturally dark "reference sites" in Europe. This is important in the context of European laws and initiatives such as the EU Biodiversity Strategy for 2030 and EU Nature Restoration Law of 2024. As laws and norms increasingly call for not only the slowing of environmental pollution but also its restoration to unpolluted conditions, tactics are shifting. Dark "references" in Europe serve as important benchmarks to gauge whether policies are working or not.

The time may have arrived for this maturing field to organize itself

The last decade has seen the gradual recognition of "dark sky studies" as a field unto itself. While this kind of research is highly interdisciplinary, both researchers and institutions are beginning to see its distinct qualities. Many attendees at LPTMM 2024 were students, which suggests that junior researchers see a future for themselves in this field.

Besides lively debates about the meaning of the research presented at LPTMM 2024, there was earnest discussion about where we can go next. This theme emerged at the final day's roundtable event on the topic "Towards the standardization of Light Pollution Metrology". The need for standards in the way that we take and report data is becoming more and more obvious. Many participants in the roundtable agreed that the field needs such standardization to push the collective research agenda forward.
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There is an emerging consensus that the field of light pollution studies should be professionalized, perhaps to include establishment of a learned society or similar representative body. A benefit of this approach is that such an organization could set and promote light pollution metrology standards. That would keep the process in the hands of the same people who make the measurements. Many agreed that a new organization could serve in this capacity well, while knowing the effort to launch it may be considerable.

LPTMM comes to North America in 2026

Researchers have plenty of time to ponder the meaning of what they heard at this year's conference. On the last day of the event, organizers announced the venue for the next edition in the LPTMM series: Observatorio Astronómico Nacional San Pedro Mártir in Baja California, México. The conference, set to take place between 2-5 June 2026, will again draw the world's leading researchers to learn and forge new collaborations together.