Abstract
The measurement of night sky quality has become an important task in night sky conservation. Modern measurement techniques involve mainly a calibrated digital camera or a spectroradiometer. However, panchromatic devices are still prevalent to this day, even in the absence of determining the spectral information of the night sky. In the case of multispectral measurements, colour information is currently presented in multiple ways. One of the most frequently used metrics is correlated colour temperature (CCT), which is not without its limitation for the purpose of describing especially the colour of natural night sky. Moreover, visually displaying the colour of the night sky in a quantitatively meaningful way has not attracted sufficient attention in the community of astronomy and light pollution research—most photographs of the night sky are post-processed in a way for aesthetic attractiveness rather than accurate representation of the night sky. The spectrum of the natural night sky varies in a wide range depending on solar activity and atmospheric properties. The most noticeable variation in the visible range is the variation of the atomic emission lines, primarily the green oxygen and orange sodium emission. Based on the accepted models of night sky emission, we created a random spectral database which represents the possible range of night sky radiance distribution. We used this spectral database as a learning set, to create a colour transformation between different colour spaces. The spectral sensitivity of some digital cameras is also used to determine an optimal transformation matrix from camera defined coordinates to real colours. The theoretical predictions were extended with actual spectral measurements in order to test the models and check the local constituents of night sky radiance. Here, we present an extended modelling of night sky colour and recommendations of its consistent measurement, as well as methods of visualising the colour of night sky in a consistent way, namely using the false colour enhancement.
Highlights
In the last few decades, urbanisation and decreasing energy cost caused a dramatic increase in the extent of artificial lights at night (ALAN) [1]
We demonstrated a method with which we could visualize the real night sky colour based on digital camera measurements and with the same colour conversion technique, we defined false colour enhancement (FCE) in order to highlight the sources of light pollution and any enhanced natural sources
Based on realistic models of night sky spectra and spectral observations, we determined the possible range of colours of natural sky emission during moonless astronomical night
Summary
In the last few decades, urbanisation and decreasing energy cost caused a dramatic increase in the extent of artificial lights at night (ALAN) [1]. Light pollution hinders astronomical observations, and influences the natural behaviour of nocturnal animals, affecting foraging, reproduction, communication, and other critical behavioral patterns [2,3,4,5,6]. The mechanism of the effect of light pollution on a cellular level is widely researched, revealing that increased illumination at night change the circadian rhythm and inhibits melatonin production causing adverse health issues in non-nocturnal species [7,8,9,10]. Overillumination implies a waste of energy, which, in turn, means unnecessary carbon emission
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