Measuring Light Pollution with a Calibrated High Dynamic Range All-Sky Image Acquisition System

Combining series of exposures with different exposure times made with a digital SLR camera and a fish-eye lens allow to create highresolution images that contain the full dynamic range of daylight, including the Sun, usable as scene background and for skylight illumination computation purposes. These High Dynamic Range (HDR) images can also be calibrated with a luminance meter, so the image data contain meaningful values, and the system becomes a measuring device. At night, long-time exposures can be combined, and the setup, once fine-calibrated with a Sky Quality Meter or another low-level light measuring device, becomes a valuable tool to provide absolute values of sky brightness. Using data from the astronomical literature, false-colour plots of sky luminance can be created that closely match visual estimation of visible stellar magnitudes.