All-Sky brightness monitoring of light pollution with astronomical methods
Introduction
The indiscriminate emission of light into the atmosphere is another type of environmental pollution along with as the emission of fumes, the discharge of hydrocarbons into the ocean by vessels, and the dumping of pollutants into rivers by factories, In fact, light pollution has a negative impact on biological rhythms, has important psychological and physiological effects on human health, and is a source of environmental degradation (Borg, 1996, Shaflik, 1997).
Examples of how light pollution affects biological rhythms are the following:
- i)
Migratory birds: Artificial light projected onto the sky dazzles and disorients birds (Longcore and Rich, 2004).
- ii)
Zooplankton: High levels of light can reduce the population of Daphnia, a crustacean at the bottom of the marine food chain (Moore et al., 2000).
- iii)
Fish: The light of coastal cities confuses fish in their migrations, and may induce a reduction of the plankton population (Moore et al., 2000).
- iv)
Insects: Light pollution affects the reproductive cycles of insects because they must cover long distances to mate, and are unable to cross the “light barrier” of the illuminated city center (Longcore and Rich, 2004).
- v)
Other species: The population equilibrium of other species is severely affected because some species are blind to certain wavelength intervals, while others are able to perceive them. As a consequence, predators can prosper, whereas the species blind to these wavelengths cannot.
- vi)
Flora: Certain plants are affected by the decrease in the populations of insects required to pollinate them. Although more research is needed, this may affect the productivity of certain crops.
- vii)
Human health: Various research studies demonstrate the impact of light pollution on human health, e.g. the suppression of melatonin by exposure to light at night is closely related to higher rates of breast and colorectal cancers (Schernhammer et al., 2003, Blask et al., 2005) or the negative effects on the human circadian clock (Pauley, 2004) due to the relationship between the blue-light-sensitive retinal ganglion cell light receptors and the modern high-intensity discharge and white LED lamps.
The astronomical scientific community was the first to be concerned by light pollution, and regulations were proposed to reduce the contribution of artificial light sources to the night sky background. In recent years, scientists have assessed light pollution in various ways. For example, they have studied the relationship between the light emitted by a city and its population (Walker, 1977), or have analyzed the atmospheric properties used to calculate luminance due to scattered light (Treanor, 1973, Berry, 1976). Their studies are based on measurements of sky brightness with professional telescopes at astronomical observatories (Garstang, 1986) or satellite measurements of the light scattered upward by cities at night (Cinzano et al., 2000, Falchi and Cinzano, 2003, Chalkias et al., 2006).
This paper describes the a portable instrument, similar to others, such as the WASBAM instrument (Cinzano and Falchi, 2003), or similar (Duriscoe et al., 2007), which are composed of a wide-angle photographic lens and a CCD to take images of small areas of the sky in order to make a complete map of the sky. Another example is the TASCA instrument (Smith et al., 2004), an all-sky camera of commercial parts, which is used to monitor atmospheric effects, air glow variations, and light pollution.
The technique described in this paper is innovative, and is based on the data analysis method used by astronomers in classical CCD stellar photometry to derive accurate measurements of light pollution in the photometric bands B, V and R of the Johnson-Cousins photometric system. By means of software, which we specifically developed for this purpose, it was possible to automatically calculate artificial sky brightness from the images obtained with the prototype.
Our study demonstrated that light pollution can be measured by combining classical astrophotometry with the use of wide-angle cameras. This is essentially the novelty of our work.
Section snippets
Prototype description
The hardware of the instrument used consists of three separate elements. The first element and core of the apparatus is a research-grade, thermoelectrically cooled, astronomical CCD camera, equipped with an internal filter wheel in which the standard B, V and R filters are installed. The CCD camera in our study was an SBIG STL-11000 M camera with an array of 4008 × 2672 active pixels of 9 microns square per pixel. The second element is a fish-eye photographic lens, capable of taking a full sky
Physical background
The core idea of the method described in this paper was to compare the radiant flux received from the sky background with the radiant flux received from stars of known brightness. Conceptually, if the stellar surface is admitted to have uniform brightness, the radiant exitance leaving the star per unit area can be defined as follows:where φe is the total radiant flux emitted by the star, and R is the stellar radius.
As a consequence, the average irradiance at the Earth is the following:
Analysis and results
Fig. 2, Fig. 3, Fig. 4 show the images taken with our prototype with exposure time of 60 s for the B, V and R filters of the Johnson-Cousins Photometric System. The geographical coordinates where the images were taken were 37°09′02″N Lat., 03°36′22″W Long. and the location was 670 m above the sea level.
With a free open source software application called Stellarium, we identified the stars (see labels in Fig. 2, Fig. 3, Fig. 4). To correctly plot the sky, Stellarium only needs as input the
Conclusions
This paper has described a study involving a prototype and an innovative method for evaluating light pollution. Standard techniques of astronomical photometry were applied to assess this type of pollution, and to obtain measurements of the night-sky brightness for each photometric band. Our study is innovative because this is the first time that conventional astronomical photometry methods have been used to analyze all-sky images. Our method has the advantage of ringing all the power of
Acknowledgments
This research study was funded by the Research Result Transfer Office (Oficina de Transferencia de Resultados de Investigación) of the University of Granada. The authors wish to thank José Luis Ortíz Moreno for his research contribution to this project, and Evaristo Molero for the technical support provided.
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