Photometric indicators of visual night sky quality derived from all-sky brightness maps

https://doi.org/10.1016/j.jqsrt.2016.02.022Get rights and content

Highlights

  • Photometric indicators of severity of light pollution are derived from all-sky images.

  • A natural reference condition is derived for comparison to artificial sky brightness.

  • Example observations encompassing a range of artificial sky brightness conditions are presented.

Abstract

Wide angle or fisheye cameras provide a high resolution record of artificial sky glow, which results from the scattering of escaped anthropogenic light by the atmosphere, over the sky vault in the moonless nocturnal environment. Analysis of this record yields important indicators of the extent and severity of light pollution. The following indicators were derived through numerical analysis of all-sky brightness maps: zenithal, average all-sky, median, brightest, and darkest sky brightness. In addition, horizontal and vertical illuminance, resulting from sky brightness were computed. A natural reference condition to which the anthropogenic component may be compared is proposed for each indicator, based upon an iterative analysis of a high resolution natural sky model. All-sky brightness data, calibrated in the V band by photometry of standard stars and converted to luminance, from 406 separate data sets were included in an exploratory analysis. Of these, six locations representing a wide range of severity of impact from artificial sky brightness were selected as examples and examined in detail. All-sky average brightness is the most unbiased indicator of impact to the environment, and is more sensitive and accurate in areas of slight to moderate light pollution impact than zenith brightness. Maximum vertical illuminance provides an excellent indicator of impacts to wilderness character, as does measures of the brightest portions of the sky. Zenith brightness, the workhorse of field campaigns, is compared to the other indicators and found to correlate well with horizontal illuminance, especially at relatively bright sites. The median sky brightness describes a brightness threshold for the upper half of the sky, of importance to telescopic optical astronomy. Numeric indicators, in concert with all-sky brightness maps, provide a complete assessment of visual sky quality at a site.

Introduction

Escaped artificial light at night degrades natural environments in several ways. Sky glow refers to light that scatters in the atmosphere and returns to earth. Glare refers to the degradation of vision due to high contrast in luminance within the visual field. Glare, and exposure to excessively lit environments, has an extended effect due to degraded dark adaptation. Excess illuminance, due to scattered and direct ray paths, degrades the integrity of nocturnal environments and wildlife behavior [1], [2]. The multiplicity of effects suggests that no single metric of excessive luminance or illuminance will adequately describe light pollution, or even night sky quality.

Light pollution was first described as “astronomical” [3], [4], [5] referring to the masking or veiling of astronomical objects by anthropogenic sky glow and hindering scientific investigation in optical wavelengths. In this paper, the term “visual night sky quality” is used to describe a human esthetic or recreational opportunity, which may be degraded by light pollution. Ecological and human health effects may also result from human-caused light which escapes to the environment [6], [7], [8]. In protected areas such as nature preserves or parks, a description of the intensity and distribution of anthropogenic sky glow may be used to quantify these impacts. In particular, in U.S. Wilderness areas, any departure from a landscape that is essentially unaffected by human modifications represents a degradation of the characteristics for which such areas were set aside [9]. Therefore, it is also important to define a natural reference condition for the night sky photometrically. This may be accomplished using selected numeric indicators representing a summary of the all-sky brightness distribution [10].

Human visual indicators of night sky quality, such as Bortle Class [11] and naked eye limiting magnitude [12] may be subject to observer bias [13], [14]. Photometric measurements of sky brightness at the zenith or the darkest part of the sky have been utilized in long term monitoring and sky quality assessment studies, particularly for astronomical observatories [15], [16]. Larger areas of the sky may be included with wide-field single-channel devices such as the Unihedron Sky Quality Meter [17], [18], [19]. These methods provide an inexpensive means for detecting trends over time in night sky brightness near the zenith. Predictions of zenith sky luminance from sky glow models assess impacts on a landscape scale [20], [21], and have been reported as a ratio of anthropogenic brightness to a constant natural background value [22].

Digital array sensors in wide angle cameras may be used to provide high resolution photometrically calibrated images of sky brightness [23], [24], [25], [26]. In locations experiencing relatively small amounts of artificial sky glow, subtraction of the natural background is an important step in tracking the artificial (anthropogenic) component, requiring a high resolution natural sky model and specialized software [27]. Once an all-sky image of artificial sky glow is obtained in an equal-area projection, the extraction of a suite of measures defining sky quality with high precision and accuracy is possible with appropriate image processing algorithms.

This paper describes photometric indicators which may be synthesized from all-sky images, including either total observed brightness or derived artificial sky glow only, and compares them to calculated natural background conditions. The derivation of the natural reference condition from a high resolution model of the natural night sky is also described.

Section snippets

All sky brightness maps

The background brightness of the night sky over the entire hemisphere was extracted from 406 observations taken between 2003 and 2015, using methods of the U.S. National Park Service Night Skies program [23]. These methods call for clear “photometric” skies and sites with minimal horizon obstructions, usually un-vegetated areas such as mountain tops, away from human development. The observations produce all-sky mosaics at a scale of 0.05°/pixel, calibrated with standard stars in the original

Sky Brightness indicators

Indicator values were calculated for the six example data sets presented in Fig. 1. The results are shown in Table 4 for the observed sky brightness maps and Table 5 for the artificial sky brightness maps. Artificial sky glow at the zenith and darkest part of the sky was lower than the measurement threshold of 17 μcd m−2 at site (a). The sites are ordered by increasing observed zenith brightness.

Examination of Table 4 reveals that, while all of the indicators show a general increasing progression

Conclusions

Numeric all-sky summaries of anthropogenic sky glow provide a robust method of assessing impacts to visual night sky quality. The average all-sky brightness is a simple and accurate assessment when derived from both observed sky brightness maps and maps of estimated artificial sky brightness. When expressed as a ratio to the natural baseline, comparison from location to location and night to night is possible. If a suitable site with clear horizons is available, fisheye or mosaicking camera

Acknowledgments

The author acknowledges the guidance and creative suggestions of Chad Moore, Christian Luginbuhl, Kurt Fristrup, and two anonymous reviewers which greatly improved the manuscript. Field data collection and processing over more than ten years, primarily by Chad Moore, Teresa Jiles, Bob Meadows, and Jeremy White resulted in the large data base which made the data exploration described herein possible. Jeremy White assisted in scholarly research and edited portions of the manuscript. This research

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