Measuring and monitoring light pollution: Current approaches and challenges.

Understanding the causes and potential mitigations of light pollution requires measuring and monitoring artificial light at night (ALAN). We review how ALAN is measured, both from the ground and through remote sensing by satellites in Earth orbit. A variety of techniques are described, including single-channel photometers, all-sky cameras, and drones. Spectroscopic differences between light sources can be used to determine which are most responsible for light pollution, but they complicate the interpretation of photometric data. The variability of Earth's atmosphere leads to difficulty in comparisons between datasets. Theoretical models provide complementary information to calibrate experiments and interpret their results. Here, we identify several shortcomings and challenges in current approaches to measuring light pollution and suggest ways forward.

Aerosol impact on light pollution in cities and their environment.

Measurements of artificial light at night represent an incredible challenge as the optical state of the atmosphere is highly unstable thus making both long-term trend analyses and inter-comparison of multiple observations difficult. Variations of atmospheric parameters, caused by either natural or anthropogenic processes, can massively influence the level of resulting night sky brightness caused by light pollution. Focusing on six parameters, either from aerosol optics or emission properties of light sources, this work literarily and numerically examines defined variations in aerosol optical depth, asymmetry parameter, single scattering albedo, ground surface reflectance, direct uplight ratio, and aerosol scale height. For each individual element the effect size and angular reliance is investigated, with results indicating that besides the aerosol scale height all play non-negligible roles in forming skyglow and environmental impact. Especially variations in aerosol optical depth and city emission function displayed severe discrepancies in consequential light pollution level. Hence, future improvement on atmospheric condition, i.e., air quality, focusing particularly on discussed elements indicates to positively influence the level of environmental impact caused by artificial light at night. We underline the need of inclusion of our outcomes to urban development and civil engineering processes in order to create or protect habitable areas for humans, wildlife and nature.

Light pollution as a factor in breast and prostate cancer.

Light pollution is a global environmental issue that affects photosensitive organisms. For instance, several researchers have recognized melatonin suppression in humans as a direct cause of long-term exposure to high artificial light levels at night. Others have identified low melatonin levels as a risk factor for a higher prevalence of hormone-sensitive cancer. This paper analyzes the association between light pollution, estimated as the emission analysis of satellite worldwide nighttime light collections from 1999 to 2012, and 25,025 breast and 16,119 prostate cancer events from 2003 to 2012. Both types of cancer increased during the study period, but light pollution increased in urban and peri-urban areas and decreased in rural areas. Cumulative light pollution during 5 years showed a positive association with breast cancer but not with prostate cancer. The association between light pollution and breast cancer persisted when adjusted to age-standardized rates with a mean increase of 10.9 events per 100,000 population-year (95% confidence interval 7.0 to 14.8). We conclude that exposure to elevated light pollution levels could be a risk factor for breast cancer in Slovakia. This work can interest researchers who study relationships between atmospheric pollutants and the growing cancer epidemic. The results and the methodology can be extrapolated to any country in the world if data is available.

Air pollution mitigation can reduce the brightness of the night sky in and near cities.

Light pollution is a novel environmental problem whose extent and severity are rapidly increasing. Among other concerns, it threatens global biodiversity, nocturnal animal migration, and the integrity of the ground-based astronomy research enterprise. The most familiar manifestation of light pollution is skyglow, the result of the interplay of outdoor artificial light at night (ALAN) and atmospheric scattering that obscures views of naturally dark night skies. Interventions to reduce night sky brightness (NSB) involving the adoption of modern lighting technologies are expected to yield the greatest positive environmental consequences, but other aspects of the problem have not been fully explored as bases for public policies aimed at reducing light pollution. Here we show that reducing air pollution, specifically aerosols, decreases NSB by tens of percent at relatively small distances from light sources. Cleaner city air lowers aerosol optical depth and darkens night skies, particularly in directions toward light sources, due to relatively short path lengths traversed by photons from source to observer. A field experiment demonstrating the expected changes when transitioning from conditions of elevated turbidity to cleaner air validated our hypothesis. Our results suggest new policy actions to augment and enhance existing light pollution reduction techniques targeting lighting technology and design.

Night-sky radiometry can revolutionize the characterization of light-pollution sources globally.

The city emission function (CEF), describing the angular emission from an entire city as a light source, is one of the key elements in night-sky radiance models. The CEF describes the rate at which skyglow depends on distance and is indispensable in any prediction of light-pollution propagation into nocturnal environments. Nevertheless, the CEF remains virtually unexplored because appropriate retrieval tools have been unavailable until very recently. A CEF has now been obtained from ground-based night-sky observations and establishes an experiment successfully conducted in the field to retrieve the angular emission function for an urban area. The field campaign was conducted near the city of Los Mochis, Mexico, which is well isolated from other cities and thus dominates all light emissions in its vicinity. The experiment has proven that radiometry of a night sky can provide information on the light output pattern of a distant city and allows for systematic, full-area, and cost-efficient CEF monitoring worldwide. A database of CEFs could initiate a completely new phase in light-pollution research, with significant economy and advanced accuracy of night-sky brightness predictions. The experiment and its interpretation represent unique progress in the field and contribute to our fundamental understanding of the mechanism by which direct and reflected uplight interact while forming the CEF.

Submicrometer-sized nonspherical particle separation by laser beam.

The radiation pressure exerted on sub-micrometer-size particles is shown to be an important factor predetermining the impact coordinates of the particles after being illuminated by a laser beam. Unlike spherical particles, the nonspherical ones can be deflected perpendicularly to the beam direction if the momentum transfer from the laser beam to a particle is large enough. Such an optical sorting is a useful technology, which can be used to isolate spherules of a specific size from a population of particles of random sizes and shapes. The system of ideal spheres has a wide range of applications in industry and also in the development of targeted optical devices, and so the methods for fast contact-less particle separation are expected to lead to considerable progress in the field. The theoretical model we have developed is demonstrated in a set of numerical experiments on metallic and nonmetallic particles.

Generalization of electromagnetic scattering by charged grains through incorporation of interband and intraband effects.

Scattering of electromagnetic radiation by electrically charged spherical particles is treated theoretically. A generalization of the approach is performed by incorporating both intraband and interband effects, while a new oscillatory term corresponding to the classical dispersion theory and the semi-quantum approach is considered. It is shown through a set of numerical experiments that interband effects may reduce the amplitude of resonant peaks for scattering, Q(sca), and absorption, Q(abs), and cause a shift of peak positions to longer wavelengths. In general, the resonant features due to interband and intraband effects can occur at different frequencies; thus, both together may result in qualitatively and quantitatively new optical signatures of electrically charged particles. This is a motivating factor for experimentalists who can use the particles as targeted probes, for example, in mapping the electric fields in different media based on scattering and/or absorption properties of electrified particulate systems.

Effect of charged-particle surface excitations on near-field optics.

The mechanism of charge on the near-field intensity distribution is revealed for metallic and dielectric particles with sizes ranging from 10 nm to 10 µm. The theoretical foundation of near-field intensity perturbations is in the discontinuity of the tangential components of the magnetic fields on either side of the interface between the particle and its surrounding medium, since excess electrons form a thin metal-like layer with elevated conductivity. We have shown that the local fields alter marginally if charges are imposed on a surface of a metallic particle. But an intensity amplification is identified in the vicinity of charged dielectric particles with sizes smaller than the wavelength. Specifically, we have demonstrated that the electromagnetic field is amplified near the poles of the particle as a result of the oriented electric and incident fields. In contrast, a dielectric particle that is large compared to the wavelength becomes opaque with a deep shadow at the side opposite to the beam incidence. As a result, intensity damping is identified near a charged sphere in the geometric optics regime. At significant charge densities, the physical properties of a conductive layer play a dominant role in forming the 3D intensity distribution independent of conductivity or permittivity of the particle core. These findings suggest that some electrically chargeable particles have the potential to be used as optical devices with properties tunable through their net surface charge.

Evaluating potential spectral impacts of various artificial lights on melatonin suppression, photosynthesis, and star visibility.

Artificial light at night can be harmful to the environment, and interferes with fauna and flora, star visibility, and human health. To estimate the relative impact of a lighting device, its radiant power, angular photometry and detailed spectral power distribution have to be considered. In this paper we focus on the spectral power distribution. While specific spectral characteristics can be considered harmful during the night, they can be considered advantageous during the day. As an example, while blue-rich Metal Halide lamps can be problematic for human health, star visibility and vegetation photosynthesis during the night, they can be highly appropriate during the day for plant growth and light therapy. In this paper we propose three new indices to characterize lamp spectra. These indices have been designed to allow a quick estimation of the potential impact of a lamp spectrum on melatonin suppression, photosynthesis, and star visibility. We used these new indices to compare various lighting technologies objectively. We also considered the transformation of such indices according to the propagation of light into the atmosphere as a function of distance to the observer. Among other results, we found that low pressure sodium, phosphor-converted amber light emitting diodes (LED) and LED 2700 K lamps filtered with the new Ledtech's Equilib filter showed a lower or equivalent potential impact on melatonin suppression and star visibility in comparison to high pressure sodium lamps. Low pressure sodium, LED 5000 K-filtered and LED 2700 K-filtered lamps had a lower impact on photosynthesis than did high pressure sodium lamps. Finally, we propose these indices as new standards for the lighting industry to be used in characterizing their lighting technologies. We hope that their use will favor the design of new environmentally and health-friendly lighting technologies.

Skyglow effects in UV and visible spectra: radiative fluxes.

Several studies have tried to understand the mechanisms and effects of radiative transfer under different night-sky conditions. However, most of these studies are limited to the various effects of visible spectra. Nevertheless, the invisible parts of the electromagnetic spectrum can pose a more profound threat to nature. One visible threat is from what is popularly termed skyglow. Such skyglow is caused by injudiciously situated or designed artificial night lighting systems which degrade desired sky viewing. Therefore, since lamp emissions are not limited to visible electromagnetic spectra, it is necessary to consider the complete spectrum of such lamps in order to understand the physical behaviour of diffuse radiation at terrain level. In this paper, the downward diffuse radiative flux is computed in a two-stream approximation and obtained ultraviolet spectral radiative fluxes are inter-related with luminous fluxes. Such a method then permits an estimate of ultraviolet radiation if the traditionally measured illuminance on a horizontal plane is available. The utility of such a comparison of two spectral bands is shown, using the different lamp types employed in street lighting. The data demonstrate that it is insufficient to specify lamp type and its visible flux production independently of each other. Also the UV emissions have to be treated by modellers and environmental scientists because some light sources can be fairly important pollutants in the near ultraviolet. Such light sources can affect both the living organisms and ambient environment.

Optics of hemispherical top dome and its effect on tubular light guide efficiency: diffuse light case.

To predict the energetic effectiveness of a tubular light guide accurately, a theoretically founded approach has to be used rather than any empirical approximation. The computed illuminance below a light guide can become inaccurate if neither Fresnel's equations nor realistic optical path lengths in a cupola are taken into consideration. It is shown that incorporation of both of them into a theoretical model results in lowered luminous flux below the light guide. Assumption of directionally independent transmission coefficient leads to average errors in luminous fluxes of about 10%. The peak errors are typically higher and correspond to lightbeams crossing a hemispherical top dome near its circular base. The solution concept used in this paper can improve predictions of energetic effectiveness for tubular light guides under diffuse daylight conditions.

Light pollution in ultraviolet and visible spectrum: effect on different visual perceptions.

In general terms, lighting research has been focused in the development of artificial light with the purpose of saving energy and having more durable lamps. However, the consequences that artificial night lighting could bring to the human being and living organisms have become an important issue recently. Light pollution represents a significant problem to both the environment and human health causing a disruption of biological rhythms related not only to the visible spectrum, but also to other parts of the electromagnetic spectrum. Since the lamps emit across a wide range of the electromagnetic spectrum, all photobiological species may be exposed to another type of light pollution. By comparing five different lamps, the present study attempts to evaluate UV radiative fluxes relative to what humans and two species of insects perceive as sky glow level. We have analyzed three atmospheric situations: clear sky, overcast sky and evolving precipitable water content. One important finding suggests that when a constant illuminance of urban spaces has to be guaranteed the sky glow from the low pressure sodium lamps has the most significant effect to the visual perception of the insects tested. But having the fixed number of luminaires the situation changes and the low pressure sodium lamp would be the best choice for all three species. The sky glow effects can be interpreted correctly only if the lamp types and the required amount of scotopic luxes at the ground are taken into account simultaneously. If these two factors are combined properly, then the ecological consequences of sky glow can be partly reduced. The results of this research may be equally useful for lighting engineers, architects, biologists and researchers who are studying the effects of sky glow on humans and biodiversity.

Scattering of electromagnetic waves by charged spheres: near-field external intensity distribution.

This Letter treats the scattering of electromagnetic waves by an electrically charged spherical particle in near-field approximation. Particular attention is paid to the external intensity distribution at the outer edges of the particle. The difference between scattering by a charged sphere and an electrically neutral sphere is significant only when size parameters exceed unity.

Approximate analytical scattering phase function dependent on microphysical characteristics of dust particles.

The approximate bulk-scattering phase function of a polydisperse system of dust particles is derived in an analytical form. In the theoretical solution, the particle size distribution is modeled by a modified gamma function that can satisfy various media differing in modal radii. Unlike the frequently applied power law, the modified gamma distribution shows no singularity when the particle radius approaches zero. The approximate scattering phase function is related to the parameters of the size distribution function. This is an important advantage compared to the empirical Henyey-Greenstein (HG) approximation, which is a simple function of the average cosine. However, any optimized value of average cosine of the HG function cannot provide the information on particle microphysical characteristics, such as the size distribution function. In this paper, the mapping between average cosine and the parameters of size distribution function is given by a semianalytical expression that is applicable in rapid numerical simulations on various dust populations. In particular, the modal radius and half-width can be quickly estimated using the presented formulas.

Light pollution simulations for planar ground-based light sources.

The light pollution model is employed to analyze spatial behavior of luminance at the night sky under cloudless and overcast conditions. Enhanced light excess is particularly identified at cloudy skies, because the clouds efficiently contribute to the downward luminous flux. It is evident that size of ground-based light sources can play an important role in the case of overcast sky conditions. Nevertheless, the realistically sized light sources are rarely embedded into light pollution modeling, and rather they are replaced by simple point sources. We discuss the discrepancies between sky luminance distributions when at first the planar light sources are considered and at second the point-source approximation is accepted. The found differences are noticeable if the size of the light source, distance to the observer, and altitude of a cloudy layer are comparable one to the other. Compared with point-source approximation, an inclusion of the size factor into modeling the light sources leads to partial elimination of the steep changes of sky luminance (typical for point sources of light). The narrow and sharp light pillars normally presented on the sky illuminated by point light sources can disappear or fuse together when two or more nearby light sources are considered with their real sizes. Sky elements situated close to the horizon will glow efficiently if luminous flux originates from two-dimensional ground-based entities (such as cities or villages).

Light-pollution model for cloudy and cloudless night skies with ground-based light sources.

The scalable theoretical model of light pollution for ground sources is presented. The model is successfully employed for simulation of angular behavior of the spectral and integral sky radiance and/or luminance during nighttime. There is no restriction on the number of ground-based light sources or on the spatial distribution of these sources in the vicinity of the measuring point (i.e., both distances and azimuth angles of the light sources are configurable). The model is applicable for real finite-dimensional surface sources with defined spectral and angular radiating properties contrary to frequently used point-source approximations. The influence of the atmosphere on the transmitted radiation is formulated in terms of aerosol and molecular optical properties. Altitude and spectral reflectance of a cloud layer are the main factors introduced for simulation of cloudy and/or overcast conditions. The derived equations are translated into numerically fast code, and it is possible to repeat the entire set of calculations in real time. The parametric character of the model enables its efficient usage by illuminating engineers and/or astronomers in the study of various light-pollution situations. Some examples of numerical runs in the form of graphical results are presented.

Reevaluation of the quondam dust trend in the middle atmosphere.

Quondam lunar eclipse photometry data offered valuable information on the optical properties of the middle atmosphere, including dust particles. However, in comparison with nonspherical grains, the simple model of spherical particles has a different effect on solar radiation penetrating horizontally through the atmosphere. It is shown that the systems, in which the smallest size fraction of dust particles dominates, reduce irradiation of the Earth's shadow more efficiently if the grains are of irregular shape. In contrast, the populations contaminated by a certain amount of large particles cause an opposite effect. Depending on the actual form of the size distribution function of the irregular grains, the irradiance within the center of the Earth's shadow may change by 2 orders of magnitude in the visible spectrum. It is therefore evident that dust properties retrieved in the past are eligible candidates for reevaluation to correct a view on the dust trend in the middle atmosphere. Sample calculations are presented for the lunar eclipse observed on 19 January 1954.