Estimation of Garstang emission function parameters from skyglow monitoring by all-sky camera

A knowledge of an emission function of gound-based light sources is crucial for skyglow modelling. In the case of complex light sources as cities, it is in practice impossible to specify accurately their emission characteristics. Therefore various semi-empirical functions determined by some suitably defined parameters are usually used for description of city light emissions. Assessment of these parameters is obviously extremely difficult. Whether for the complexity of inverse problems of radiative transfer, or for the lack of specialized measurement devices. The aim of the article is to present a simple and effective method of retrieval of city emission function parameters from easily available experimental data. The method is applied on the Garstang emission function because of its generally wide spread utilisation in the light pollution research area. However, it can be in principle applied also on another suitably parametrized emission functions of ground-besed sources. We start from a theoretical model of diffuse radiation intensity distribution over the clear sky, which is based on the successive order of scattering method and which allows relatively simple inverse retrieval of the Garstang parameters. Ratios of zenith radiance to horizontal irradiance at various distances from a source serve as the experimental input data. Such data can be obtained by obvious inexpensive devices what forms the main advantage of the introduced method. In the paper we present utilisation of all-sky cameras which allow simultaneous measurement of the both demanded quantities.