Paris by night, source.
Pollution projector is a projector that uses the intensity of available sunlight during the day to visualize different levels of light pollution caused by big cities at night. Light pollution is excessive or obtrusive artificial light.
When the projector is placed in bright sunlight, this light and the city-name of the most light-polluted city get projected onto a surface. If the intensity of the sunlight changes, by clouds or the setting sun, the projection gets less intense and a city-name of a corresponding less light polluted city will be shown.
The projector works by reflecting the sunlight into a lens with a mirror. In front of the lens is a light sensor that measures the intensity of the sunlight. Also in front of the lens is a transparent disk that holds the different city-names. A servo engine changes the city-name placed in front of the lens. This servo is controlled by an Arduino that uses the input from the light sensor.