The effect of the spectral response of measurement instruments in the assessment of night sky brightness
Introduction
Artificial light at night (ALAN) is constantly increasing around the world. A recent research [1] revealed that the lit surface of Earth is increasing in brightness and extend despite the renovations of lighting installations that were widely implemented. This is mainly caused by the rebound effect of price competitiveness of LED light sources compared to traditional gas discharge lamps [2]. The excess of light in outdoor lighting leads to the so-called light pollution and to the waste of energy [3]. Light pollution can be observed either from ground level or from space via remote sensing [4]. A major effect of light pollution is the increase of the skyglow during the night. The intensity and the variation of skyglow depends on the number, the concentration and the type of artificial light sources [5], [6], [7]. Weather conditions and cloud coverage also affect the brightness of a light polluted sky [8]. Researchers concluded that light pollution has an environmental impact [9] and is affecting human circadian cycle [10], [11]. A recent research links also the ALAN with breast and prostate cancer risk [12]. Thus, the monitoring and the evaluation of light pollution is of high scientific interest and a challenging topic for environmentalists, biologists, anthropologists, astronomers, engineers, etc.
The assessment of light pollution includes the measurement of dark sky brightness. The current trend is to monitor the skyglow using various types of instruments. The most common instruments are presented in the next chapter. The target is to investigate how much “bright” is the sky during the night and how this brightness varies across different areas, time, and period of the year. Measurement of brightness includes the radiation in the region of visible lighting (approx. 380–780 nm). An accurate assessment of light pollution can be useful in case of light remodelling and infrastructure renovation [13], [14].
This task could have good accuracy and repeatability if sky's spectrum was constant during twilight and night period. However, the spectrum of the sky is not stable due to natural and artificial light variation. In addition, sky spectrum close to horizon may be significantly different compared to the spectrum at zenith. This is due to physical reasons but mainly depends on the artificial lighting installations where upward lighting scatters in the atmosphere. The scheduling of city lights switching on and off, grouped in various source types, affects the spectrum synthesis of the skyglow. Light pollution is also reflected in the clouds during night-time. The recent trend of replacing street luminaires is also a parameter that changes the type of emitted light in certain areas of a city.
Thus, using instruments with different spectral behaviour introduces potential uncertainties and non-repeatability of measurements. Considering that the light pollution deals with the effects of ALAN in humans, animals, insects and plants, shows that assessment of skyglow is a complicated procedure which can easily lead to confusing results. Especially in cases where instruments are used to monitor the dark sky for long periods, the comparison of these measurements cannot be considered valid.
This paper investigates the theoretical correlation between the measurements from instruments that are used for skyglow assessment. The study considers a variable spectrum of the sky. Instruments under consideration are equipped with different filters like photopic, astronomical or custom ones. It illustrates the variation between factors that can be used for spectral correction of different instruments. This research works towards the development of a scientifically acceptable skyglow assessment methodology.
Section snippets
Status quo in dark sky measurements
Dark sky brightness assessment can be realized by measuring the radiation in various spectral bands. In terms of light pollution research, several different procedures and instruments are used. A recent work [15] describes the current situation in this field and gives an analysis of the available instruments. From this work it is clear that there is no standardized instrument or measuring procedure for dark sky assessment. This is mainly due to the involvement of different scientists in the
Calculation of spectral correction factors
As explained above, a spectral correction factor is needed for each measurement, of a target with certain spectrum, taken using an instrument with a specific spectral response. The main scope of this research is to calculate the theoretical variation between the spectral correction factors as a function of the instrument, the spectrum of the measured target and of the reference source used for calibration. These calculations reveal the potential “risk” when someone is trying to compare
Calculation results
The calculation results of the SMCFs are formed in groups of Figures and are presented in Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11. Each Figure refers to one of the eight typical spectral responses as the reference response/instrument. This means that the measurements of the remaining instruments must be corrected to the readings of the reference instrument according to Eq. (3). Each Figure includes eight sub-figures. These sub-figures refer to one of the alternative
Conclusions – discussion
A review of the calculation results of this study shows that the results of sky's brightness measurement with a varying spectrum using diverse instruments cannot be compared without using spectral correction factors. A direct comparison of instruments readings can be safely done only if these systems are calibrated under same reference source and the spectrum of the sky patch is constant and known. As the spectrum varies, errors are introduced and readings are not comparable anymore. These
References (33)
- et al.
“Modelling of light pollution in suburban areas using remotely sensed imagery and GIS
J Environ Manage
(2006) - et al.
Contributions of artificial lighting sources on light pollution in Hong Kong measured through a night sky brightness monitoring network
J Quant Spectrosc Radiat Transfer
(2014) - et al.
"Limiting the impact of light pollution on human health, environment and stellar visibility
J Environ Manage
(2011) - et al.
Qualifying lighting remodelling in a Hungarian city based on light pollution effects
J Quant Spectrosc Radiat Transfer
(2016) - et al.
Artificially lit surface of Earth at night increasing in radiance and extent
Sci Adv
(2017) - et al.
Redefining efficiency for outdoor lighting
Energy Environ Sci
(2014) - et al.
On the environmental pollution and energy waste due to urban lighting
WIT Trans Ecol Environ
(2003) - et al.
Worldwide variations in artificial skyglow
Sci Rep
(2015) - et al.
An investigation of LED street lighting's impact on sky glow
(2017) - et al.
How clouds are amplifying (or not) the effects of ALAN
Int J Sustainable Lighting
(2016)
Evaluating potential spectral impacts of various artificial lights on melatonin suppression, photosynthesis, and star visibility
PLoS ONE
“Measuring and modelling light pollution at the Zselic Starry Sky Park
J Phys Conf Ser
Measuring night sky brightness: methods and challenges
J Quant Spectrosc Radiat Transfer
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