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Salvador Bará

Publications and source records attributed to Salvador Bará.

At least 19 recordsLinked to original sources
arXiv2026

Retinal hazards and glare from space mirrors

Projects of orbital mirrors reflecting sunlight towards Earth raise reasonable concerns regarding photochemical and thermal retinal hazards and detrimental effects on visual performance (disability glare). This paper addresses these issues based on some current eye safety and glare standards. Telescopic observations of single mirrors appear to pose a significant risk of photochemical retinal hazard. The risk is higher for vulnerable populations. Unaided eye viewing of a mirror keeps the retinal risks low, both photochemical and thermal; however, disability glare may be unacceptably high, affecting awareness and response in critical visual operations. The simultaneous presence of several mirrors in the visual field of the observer, seen with the unaided eye or through optical instruments, will significantly increase all types of hazards. The kind and severity of their combined effects will depend on the angular distance between them. Additional information is needed to evaluate the effects of the transient flashes of light during mirror slewing operations.

physics.med-ph↗
arXiv2026

Variations on a classical theme: On the formal relationship between magnitudes per square arcsecond and luminance

The formal link between magnitudes per square arcsecond and luminance is discussed in this paper. Directly related to the human visual system, luminance is defined in terms of the spectral radiance of the source, weighted by the CIE V($λ$) luminous efficiency function, and scaled by the 683 lm/W luminous efficacy constant. In consequence, any exact and spectrum-independent relationship between luminance and magnitudes per square arcsecond requires that the latter be measured precisely in the CIE V($λ$) band. The luminance value corresponding to m_VC=0 (zero-point of the CIE V($λ$) magnitude scale) depends on the reference source chosen for the definition of the magnitude system. Using absolute AB magnitudes, the zero-point luminance of the CIE V($λ$) photometric band is 10.96 x 10^4 cd/m^2.

astro-ph.IM↗
arXiv2026

Direct assessment of the sensitivity drift of SQM sensors installed outdoors

Long-term monitoring of the evolution of the artificial night sky brightness is a key tool for developing science-informed public policies and assessing the efficacy of light pollution mitigation measures. Detecting the underlying artificial brightness trend is a challenging task, since the typical night sky brightness signal shows a large variability with characteristic time scales ranging from seconds to years. In order to effectively isolate the weak signature of the effect of interest, determining the potential long term drifts of the radiance sensing systems is crucial. If these drifts can be adequately characterized, the raw measurements could be easily corrected for them and transformed to a consistent scale. In this short note we report on the progressive darkening of the signal recorded by Sky Quality Meter (SQM) detectors belonging to several monitoring networks, permanently installed outdoors for periods ranging from several months to several years. The sensitivity drifts were estimated by means of parallel measurements made at the beginning and at the end of the evaluation periods using reference detectors of the same kind that were exposed to little or no weathering in the intervening time. Our preliminary results suggest that SQM detectors installed outdoors steadily increase (darken) their readings at an average rate of +0.036 magSQM/arcsec2 per MWh/m2 of exposure to solar horizontal global irradiation, that for our locations at mid latitudes (39°-43° N) in temperate climate zones translates into approximately +0.06 magSQM/arcsec2 per year.

astro-ph.IM↗
arXiv2026

Skyglow inside your eyes: intraocular scattering and artificial brightness of the night sky

The visual perception of the natural night sky in many places of the world is strongly disturbed by anthropogenic light. Part of this artificial light is scattered in the atmosphere and propagates towards the observer, adding to the natural brightness and producing a light polluted sky. However, atmospheric scattering is not the only mechanism contributing to increase the visual skyglow. The rich and diverse biological media forming the human eye also scatter light very efficiently and contribute, in some cases to a big extent, to the total sky brightness detected by the retinal photoreceptors. In this paper we quantify this effect and assess its relevance when the eye pupil is illuminated by light sources within the visual field. Our results show that intraocular scattering constitutes a significant part of the perceived sky brightness at short distances from streetlights. These results provide quantitative support to the everyday experience that substantial gains in naked-eye star limiting magnitudes can be achieved by blocking the direct light from the lamps that reaches the eye pupil. Urban lighting designs that reduce the illuminance over the eye pupil and locate the sources at large angles with respect to the visual axis are expected to help decreasing the artificial intraocular skyglow.

astro-ph.IM↗
arXiv2026

Can we illuminate our cities and (still) see the stars?

Could we enjoy starry skies in our cities again? Arguably yes. The actual number of visible stars will depend, among other factors, on the spatial density of the overall city light emissions. In this paper it is shown that reasonably dark skies could be achieved in urban settings, even at the center of large metropolitan areas, if the light emissions are kept within admissible levels and direct glare from the light sources is avoided. These results may support the adoption of science-informed, democratic public decisions on the use of light in our municipalities, with the goal of recovering the possibility of contemplating the night sky everywhere in our planet.

astro-ph.IM↗
arXiv2025

Trust, but verify: Reducing artificial light emissions and monitoring compliance

In order to reduce light pollution we have to reduce the overall amount of artificial light emissions. This is a consequence of the basic rules governing the propagation of light in the terrestrial environment. In this work I revisit the physical laws causing that (i) "all" artificial light emitted outdoors is lost or pollutant, (ii) the negative effects of light pollution depend monotonically on the local concentration of photons of anthropogenic origin, and (iii) this concentration depends linearly on the total artificial light emissions, weighted by the light pollution propagation functions. Setting total emission limits becomes necessary in order to ensure that the negative effects of light pollution do not surpass red-lines of unacceptable degradation of the natural night. Once these red-lines are socially agreed, monitoring compliance becomes a relevant task. Several complementary methods for assessing total light emissions are being used nowadays, including public inventories of installed lights, direct radiance measurements from ground or low Earth orbit satellites, and scattered radiance measurements using ground based detectors (night sky brightness monitoring). While updated administrative inventories could in principle be trusted, independent verification is a must. The required measurements pose in turn significant challenges: we discuss here how the variability of the terrestrial atmosphere sets a lower limit on the minimum emission changes that can be reliably detected by measurements, and propose some ways to improve this performance.

astro-ph.IM↗
arXiv2025

Diverging evolution of light pollution indicators: can the Globe at Night and VIIRS-DNB measurements be reconciled?

The radiance of nighttime artificial lights measured by the VIIRS-DNB instrument on board the satellite Suomi-NPP increases at an average rate ~2.2 %/yr worldwide, whereas the artificial radiance of the night sky deduced from the Globe at Night (GAN) unaided-eye observations of the number of visible stars is reported to increase at an average rate ~9.6 %/yr. The difference between these two estimates is remarkable. This raises the question of whether the diverging temporal evolution of these indicators could be due to changes in the spectral composition of outdoor artificial light, consequence of the current process of replacement of lighting technologies. This paper presents a model for evaluating the temporal rate of change of different light pollution indicators and applies it to the VIIRS-DNB vs GAN issue, based on available data. The results show that the reported difference could be explained by spectral changes alone, if the visual GAN observations are made with scotopic or mesopic adaptation at definite times under some particular transition conditions. In case of photopic adapted observers, however, reconciling these two measurement sets requires the existence of GAN-specific light sources that affect the Globe at Night observations but do not show up in the VIIRS-DNB data. The lumen emissions of these GAN-specific sources for photopic observers should increase at a rate larger than 9%/yr worldwide.

astro-ph.IM↗
arXiv2024

Detecting changes in anthropogenic light emissions: limits due to atmospheric variability

Monitoring the evolution of the anthropogenic light emissions is a priority task in light pollution research. Among the complementary approaches that can be adopted to achieve this goal stand out those based on measuring the direct radiance of the sources at ground level or from low Earth orbit satellites, and on measuring the scattered radiance (known as artificial night sky brightness or skyglow) using networks of ground-based sensors. The terrestrial atmosphere is a variable medium interposed between the sources and the measuring instruments, and the fluctuation of its optical parameters sets a lower limit for the actual source emission changes that can be confidently detected. In this paper we analyze the effect of the fluctuations of the molecular and aerosol optical depths. It is shown that for reliably detecting changes in the anthropogenic light emissions of order ~1% per year, the inter-annual variability of the annual means of these atmospheric parameters in the measurement datasets must be carefully controlled or efficiently corrected for.

astro-ph.IM↗
arXiv2024

Quantifying the visual impact of wind farm lights on the nocturnal landscape

Wind farm lights are a conspicuous feature in the nocturnal landscape. Their presence is a source of light pollution for residents and the environment, severely disrupting in some places the aesthetic, cultural, and scientific values of the pristine starry skies. In this work we present a simple model for quantifying the visual impact of individual wind turbine lights, based on the comparison of their brightnesses with the brightness of well-known night sky objects. The model includes atmospheric and visual variables, and for typical parameters it shows that medium-intensity turbine lights can be brighter than Venus up to ~4 km from the turbine, brighter than alpha CMa (the brightest star on the nighttime sky) until about ~10 km, and reach the standard stellar visibility limit for the unaided eye (m_v=+6.00) at ~38 km. These results suggest that the visual range of wind farms at nighttime may be significantly larger than at daytime, a factor that should be taken into account in environmental impact assessments.

physics.soc-ph↗
arXiv2024

An enhanced version of the Gaia map of the brightness of the natural sky

The GAia Map of the Brightness Of the Natural Sky (GAMBONS) is a model to map the natural night brightness of the sky in cloudless and moonless nights. It computes the star radiance from the photometric data in Gaia and Hipparcos catalogues, adding the contributions of the diffuse galactic and extragalactic light, zodiacal light and airglow, and taking into account the effects of atmospheric attenuation and scattering. The model allows computing the natural sky brightness in any given photometric band for a ground-based observer, if appropriate transformations from the Gaia bands are available. In this work we present the most recent improvements of the model, including the use of Gaia EDR3 data, the inclusion of a wide set of photometric bands and the derivation of additional sky brightness indicators, as the horizontal irradiance and the average hemispheric radiance

astro-ph.IM↗
arXiv2023

Artificial light at night: a global disruptor of the night-time environment

Light pollution is the alteration of the natural levels of darkness by an increased concentration of light particles in the nighttime environment, resulting from human activity. Light pollution is changing in a deep way the environmental conditions of the night in wide areas of the planet, and is a relevant stressor whose effects on life are being unveiled by a compelling body of research. In this paper we briefly review the basic aspects of artificial light at night as a pollutant, describing its character, magnitude and extent, its worldwide distribution, its temporal and spectral change trends, as well as its dependence on current light production technologies and prevailing social uses of light. It is shown that the overall effects of light pollution are not restricted to local disturbances, but give rise to a global, multiscale disruption of the nighttime environment.

physics.soc-ph↗
arXiv2023

On the Posch ratio for irradiance in coastal waters and the high seas

The horizontal irradiance at the sea surface is an informative light pollution indicator to study Artificial Light at Night (ALAN) effects on marine biodiversity (e.g.: zooplankton diel vertical migration). The Posch ratio (PR) for the horizontal irradiance (that is, the ratio of the horizontal irradiance to the zenith radiance) is a useful tool for estimating the irradiance from easily available measurements of the zenith night sky brightness. The PR definition has already been generalized for any pair of linear radiance indicators in any pair of arbitrarily chosen photometric bands, and can also be applied to estimate e.g. the average sky radiance or the radiance at some elevation above the horizon as a function of the radiance in any other direction of the sky. The PR for a single light source depends on the distance from the source, its angular and spectral emission pattern, and the state of the atmosphere. The PR for any set of sources is a linear combination of the individual PRs that each one would produce separately, with weights that can be easily derived from the relative contribution of each source to the zenith radiance. Whereas in populated lands the ALAN PR varies relatively fast from one location to another, due to the particular spatial distribution of lights, in coastal waters and the high seas the light pollution PR is a smooth function of the distance to the shoreline, due to the progressive lack of neighboring sources and the absence of obstacles. In this work we present the fundamental equations of the model and an example of application for the waters surrounding the Iberian Peninsula, North Africa and the West Mediterranean islands.

astro-ph.IM↗
arXiv2023

Light pollution is skyrocketing

Artificial light at night is a pollutant that is rising fast, as demonstrated by Kyba et al. (1) work by analyzing ten of thousands observations by citizen scientists in the last 12 years. The study found that the dimmest stars are vanishing, progressively hidden by a 10 percent yearly increase of the sky background due to artificial lights. This increase is difficult to be detected by the global coverage satellites now in operation, due to detector's blindness to the blue peak of white LEDs that are progressively replacing older technology lamps. This shows the need for a satellite with nighttime multi band capability in the visible light to study and control future evolution. More importantly, a call for a strong reverse in the light pollution rising trend is extremely urgent to avoid all the cultural, scientific, energetic, ecological and health negative effects of artificial nightlights.

astro-ph.IM↗
arXiv2023

Aggregate Effects of Proliferating LEO Objects and Implications for Astronomical Data Lost in the Noise

The rising population of artificial satellites and associated debris in low-altitude orbits is increasing the overall brightness of the night sky, threatening ground-based astronomy as well as a diversity of stakeholders and ecosystems reliant on dark skies. We present calculations of the potentially large rise in global sky brightness from space objects, including qualitative and quantitative assessments of how professional astronomy may be affected. Debris proliferation is of special concern: since all log-decades in debris size contribute approximately the same amount of night sky radiance, debris-generating events are expected to lead to a rapid rise in night sky brightness along with serious collision risks for satellites from centimetre-sized objects. This will lead to loss of astronomical data and diminish opportunities for ground-based discoveries as faint astrophysical signals become increasingly lost in the noise. Lastly, we discuss the broader consequences of brighter skies for a range of sky constituencies, equity/inclusion and accessibility for Earth- and space-based science, and cultural sky traditions. Space and dark skies represent an intangible heritage that deserves intentional preservation and safeguarding for future generations.

astro-ph.IM↗
arXiv2023

Modelling the artificial night sky brightness at short distances from streetlights

Contrary to some widespread intuitive belief, the night sky brightness perceived by the human eye or any other physical detector does not come (exclusively) from high in the sky. The detected brightness is built up from the scattered radiance contributed by all elementary atmospheric volumes along the line of sight, starting from the very first millimeter from the eye cornea or the entrance aperture of the measuring instrument. In artificially lit environments, nearby light sources may be responsible for a large share of the total perceived sky radiance. We present in this paper a quantitative analytical model for the sky radiance in the vicinity of outdoor light sources, free from singularities at the origin, which provides useful insights for the correct design or urban dark sky places. It is found that the artificial zenith sky brightness produced by a small ground-level source detected by a ground-level observer at short distances (from the typical dimension of the source up to several hundred meters) decays with the inverse of the distance to the source. This amounts to a reduction of 2.5 mag/arcsec2 in sky brightness for every log10 unit increase of the distance. The effects of screening by obstacles are also discussed.

astro-ph.IM↗
arXiv2023

Quantitative evaluation of outdoor artificial light emissions using low Earth orbit radiometers

Low Earth orbit radiometers allow monitoring nighttime anthropogenic light emissions in wide areas of the planet. In this work we describe a simple model for assessing significant outdoor lighting changes at the municipality level using on-orbit measurements complemented with ground-truth information. We apply it to evaluate the transformation effected in the municipality of Ribeira (42° 33 23 N, 8° 59 32 W) in Galicia, which in 2015 reduced the amount of installed lumen in its publicly-owned outdoor lighting system from 93.2 to 28.7 Mlm. This significant cutback, with the help of additional controls, allowed to reduce from 0.768 to 0.208 Mlm/km2 the lumen emission density averaged across the territory. In combination with the VIIRS-DNB annual composite readings these data allow to estimate that the relative weight of the emissions of the public streetlight system with respect to the total emissions of light in the municipality changed from an initial value of 74.86% to 44.68% after the transformation. The effects of the sources spectral shift and the photon calibration factor on the radiance reported by the VIIRS-DNB are also evaluated.

astro-ph.EP↗
arXiv2022

Light pollution and the concentration of anthropogenic photons in the terrestrial atmosphere

Light pollution can be rigorously described in terms of the volume concentration of anthropogenic photons (light quanta) in the terrestrial atmosphere. This formulation, consistent with the basic physics of the emission, scattering and absorption of light, allows one to express light pollution levels in terms of particle volume concentrations, in a completely analogous way as it is currently done with other classical pollutants, like particulate matter or molecular contaminants. In this work we provide the explicit conversion equations between the photon volume concentration and the traditional light photometry quantities. This equivalent description of the light pollution levels provides some relevant insights that help to identify artificial light at night as a standard pollutant. It also enables a complementary way of expressing artificial light exposures for environmental and public health research and regulatory purposes.

astro-ph.IM↗