Viennese street lighting the night effect of exchange (1701876-2025)

The aim of the study is to update the direct measurement of the proportion of urban public lighting in night brightness, in line with the ‘Roadmap’1 on light over Vienna by 2028, which was prepared and made available by the Kuffner-Sternwarte association. The determination of the ‘Stadt-Wien share’ of night brightness is based on known patterns, so-called ‘steps’ in the time series of horizontal irradiance2. These patterns correlate with the circuit and regulation of public street lighting, also known as partial night circuits. The measured values are recorded by the monitoring network of the Kuffner-Sternwarte association. Here we present, for three measuring stations at different distances from the city center and with different proportions of the usage classes of the surrounding light sources, as well as the years 2015 to 20243, the first analysis of the temporal development of the resulting night light proportions, which can be assigned to the public street lighting of the city of Vienna. The main result is a sharp decline in these shares to around a quarter of the 2015 figures, i.e. at the beginning of the changeover. The decline is in line with the progress of the installation, as measured by the cumulative installation numbers of the new types of luminaires. For 2025 to 2026, with further progress of the exchange with the current installation rates and assuming that the majority of the luminaires are operated overnight with reduced light output, a disappearance of the contributions of the public street lighting of the city of Vienna to brighten the night, below the current detection limit of around 1% of the night brightness is to be expected.