Qualifying lighting remodelling in a Hungarian city based on light pollution effects

https://doi.org/10.1016/j.jqsrt.2016.02.025Get rights and content

Highlights

  • Changes of the skydome of a Hungarian city were measured after lighting remodelling.

  • The observations were compared with Monte Carlo radiation transfer calculations.

  • Photopic measurements demonstrate improvement in light pollution after remodelling.

  • However, blue rich lighting increases the risk of negative ecological effects.

Abstract

The public lighting system has been remodelled in several Hungarian cities. In some cases the majority of the old luminaries were fitted with high pressure sodium lamps and they were replaced with white LED lighting with a typical correlated colour temperature of about 4500 K. Therefore, these remodelling works provide a testbed for methods in measurements and modelling. We measured the luminance of the light domes of selected cities by DSLR photometry before and after the remodelling.

Thanks to the full cut off design of the new lighting fixtures we obtained a slight decrease even in the blue part of the sky dome spectra of a tested city. However, we have to note that this positive change is the result of the bad geometry (large ULR) of the previous lighting system. Based on Monte Carlo radiative transfer calculations we provide a comparison of different indicators that can be used to qualify the remodelling, and to predict the possible changes in light pollution.

Introduction

Because of the increasing level of light pollution, night sky quality monitoring becomes an important part of nature conservation. We performed imaging sky luminance measurements in Hungary as part of the designation procedure of natural park areas for International Dark Sky Parks (IDSP) recognized by the International Dark Sky Association (IDA). A well-known method of qualifying the loss of energy in the direction of the sky is the measurements based on satellite images (see e.g. [1]). This provides an efficient tool also to survey the changes of light pollution after modifications in the light sources. However, we have to take into considerations in some of the satellite imageries the spectral response of the cameras. The lack of sensitivity in blue light may indicate a decrease in the escaped light in contrast to the real variation.

A possible method to qualify the light pollution over a large area is to make maps of night sky brightness in the selected area. There are different procedures to generate such maps, the simplest one is to measure the mean luminance of a portion of the night sky by a luminance meter (e.g. Sky Quality Meter). To provide indicators on the ecological effects on light pollution it is important to collect as complete data as possible – like the luminance distribution of the whole sky together with spectral information. Single zenith radiance measurements by SQM devices may provide only limited information of the changes during lighting remodelling.

If such measurements are made on a dense enough geographic grid, the skyglow of the territory can be mapped. However, significantly more information can be gathered by imaging photometry of the whole sky. In addition, recent techniques to survey light pollution (e.g. [2], [3], [4]) provide the spatial distribution of artificial sky luminance as a function of different parameters (wavelength, sky position, etc.). The improvements of Digital Single Lens Reflex (DSLR) cameras provide a new opportunity to monitor the artificial skyglow and light pollution. DSLR cameras that are able to save images in an unaltered raw format, can be calibrated to get measurements of the luminance of the sky on a physical scale. Then the photos of the night sky can be converted to false colour images, which represent the distribution of sky brightness [4]. Such calibrated images of the light domes over cities provide enough information to interpolate or even extrapolate them to a larger area if the measurements are combined with numerical radiation transfer modelling.

In order to interpret the sky brightness measurements and to provide models of the effects of city-lights, numerical calculations should be performed. The general procedure of light pollution modelling uses the spatial distribution and the characteristic properties of light sources together with the physical parameters of the atmosphere as an input for the calculations. Then the solution of the radiation transfer equations constrained by the input parameters provides observable quantities, like the distribution of the luminance of skyglow or illuminance values at different locations. Different methods exist to calculate the solutions of this problem, e.g. by direct integration of the full radiation transfer equations, or some approximate solutions based on simplification of the problem. The basic elements of these methods can be found e.g. in [5], [6] and references therein. Another possible way is to perform Monte Carlo simulation of photon packets (see e.g. [7]). This method is widely used in general atmospheric research, but does not yet spread widely in light pollution related works. To simulate the effect of different lighting systems, light sources, weather and atmospheric conditions we have developed a Monte Carlo radiation transfer code [8].

Section snippets

Measurements before and after a lighting remodelling

SLR cameras provide a new opportunity for scientific measurements of radiance or luminance distributions [9]. The great advantage of DSLR measurements is that it results in imaging photometry or radiometry. Together with a fisheye lens the system provides information on the whole upper hemisphere with a single exposure. Compared to other imaging photometric systems (like CCD cameras) it is highly transportable, suitable for field work in remote locations. The method has been proved to be an

Models

In clear air (no clouds), the propagation of light is determined by Rayleigh scattering for molecules and by Mie scattering for aerosols. The mean free path of photons in terrestrial atmosphere is several tens of kilometres, depending on the aerosol content and elevation. It gives a natural choice to use Monte Carlo simulation of the light propagation since the observed photometric quantities are statistical averages of photon packets. For detailed description of Monte Carlo radiation transfer

Model results

To predict the effect of different spectral distributions and ULR values on the observable quantities of light pollution, we simulated different indicators based on the Monte Carlo radiation transfer calculations. Here we present results for a luminance meter with the standard photopic V(λ) visibility function. The most widely used measuring device is the Unihedron Sky Quality Meter (SQM). We simulated measurements for the ‘L’ version with narrow angle (10° HWHM, as given by the manufacturer)

Summary

We presented on site measurements and numerical simulations to predict the possible effects of lighting remodelling, when Sodium lights are replaced with white LEDs. Our measurements and radiative transfer calculations clearly demonstrate the importance of fully-shielded design (ULR=0) if the new lighting sources have high CCT value. The photopic measurements are in agreement with the model results within the errors of the measurements. However, the scotopic, or blue light measurements have a

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