A thin film that reflects light could be used to make road signs that shine brightly and change color at night. The film consists of polymer microspheres laid down on the sticky side of a transparent tape. The material’s physical structure leads to an interesting phenomenon: When white light shines on the film at night, some observers will see a single, stable color reflected back while others will see changing colors. The phenomenon depends on the angle of observation and whether the light source is moving.
The material can be used to make smart traffic signs; for example, if a person is listening to loud music or isn’t paying attention while they’re walking or driving, a color-changing sign can help to better alert them to the traffic situation.
In one set of experiments, a speed limit sign was created with letters and numbers made from the film. A white light was placed nearby to illuminate the sign and when a fast-moving car drove past, the color of the characters on the sign appeared to flicker from the perspective of the driver as the driver’s viewing angle changed.
In other tests, the new material was applied to a series of markers lining the side of a road, denoting the boundary of the driving lane. As a car approached, the markers lit up in bright colors, reflecting light from the vehicle’s headlights. From the driver’s perspective, the markers’ color remained stable. But to a pedestrian standing at the side of the road, the color of the markers appeared to flicker as the car and its headlights sped past. If the car goes faster, the pedestrian will see the color change more quickly.