Darkness card, sky brightness with greenhouse lighting on

The map concerns the sky brightness in North Holland in 2011-2012 with greenhouse lighting on. If there is no direct illumination nearby, the night sky determines the degree of darkness. The sky is illuminated by radiant light from a city or greenhouse from an area with a radius of about 30 kilometers. In 2011 and 2012, the brightness of the sky in the point straight up (zenit) was measured at 225 locations in the province of Noord-Holland, as a measure of the ‘darkness’. Previous measurements in Amsterdam and on the island of Texel have also been included in this darkness map.