NIGHT SKY BRIGHTNESS MEASUREMENTS
Monitoring the brightness of the night sky is pivotal in addressing the recent increase in light pollution. Measurements are essential to assess the impact of new lighting systems, particularly in urban and suburban areas, but also to track the light levels in rural and remote regions that remained largely unaffected until a few decades ago. In these previously dark areas, the modern expansion of human infrastructure can have a significant impact on once undisturbed ecosystems.
Measurements of light pollution can be either continuous or sporadic. Continuous measurements are taken by instruments installed in fixed locations, operating under consistent experimental setups. This approach allows for long-term monitoring but is influenced by varying weather and environmental conditions. A clear example is the Moon’s phase and elevation, which significantly affect the light intensity measured. Cloud cover also plays a crucial role. Therefore, the analysis of such data requires careful consideration of these factors if aiming to obtain meaningful insights about light pollution. On the other hand, sporadic on-the-spot measurements can be conducted under optimal conditions, such as clear skies during moonless nights, but they lack the consistency of continuous monitoring and are often more difficult to compare with other datasets.
Light pollution can be monitored using both ground-based and space-based instruments, each of which present their own advantages and limitations. In all cases, it is important to emphasise that what is being measured is the total light level, which includes both natural and anthropogenic components. Distinguishing between these two sources is challenging. However, natural components tend to remain constant or exhibit periodic variations over short time scales, whereas the anthropogenic contribution primarily drives long-term trends.
Another important factor to consider is the spectrum of the emitting source in relation to the spectral response of the measurement device. While the human eye detects visible light between roughly 400 and 650 nm, many light sources emit outside this range, depending on their design or technology. Similarly, measurement devices have specific spectral sensitivities. Therefore, the result of a measurement depends on the overlap between the source’s emission spectrum and the device’s sensitivity. A key example is the growing use of LED lighting, which shifts emissions towards shorter (bluer) wavelengths, while many sensors are more responsive to longer (redder) wavelengths, thus potentially leading to the underestimation of light pollution in recent times.
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