Heliograph

Heliograph: A heliograph (literally sun writing) is a communicating device to send a code with reflected sunlight. This was used by the British in southern Asia, and during the Civil War in the United Stated. It shares etymology with heliotrope, plants that turn towards the sun, and all the many graph words: telegraph, phonograph, computer graphics, xerographic, and pornographic.

Far down the road, a tiny cloud of orange dust was rising and a windshield heliographed as it passed in and out of shade.