Retroreflective Spray Paint

This bright silver aerosol can is full of a sprayable mix of skin-safe glue and microscopic glass spheres that are transparent in daylight but retroreflective in the dark. It allows its users to liberally spray themselves, their clothes, their bikes and even their reindeer with a retroreflective mist of miniscule beads that makes them illuminate in the bright light of a car’s headlights (or a camera flash - if you want to test this for yourself). 

Shiny mirrors, metallic films, glossy ceramics and even white T-shirts readily reflect light, but these standard reflective surfaces scatter the light that hits them in many directions, making them visible from many angles, but relatively dim. Retroreflective materials, on the other hand, are designed to reflect most of the light that hits them directly back to the original source, no matter what angle it hits the surface at. Retroreflective materials use a few different technologies, but the main ones incorporate either glass beads, tiny prisms or little lenses embedded in a fabric or plastic substrate. This transparent, aerosolized ‘paint’ is made up of millions of tiny, perfectly round glass balls that are fired out of the can by a butane and propane propellant. 

The extent to which a glass bead retroreflects is determined by the refractive index of the glass used, its transparency and colour, and the roundness of the bead. A glass with a high refractive index is normally used for this kind of retroreflective sphere: usually something like barium titanate glass. This has a large percentage of barium oxide and titanium dioxide in it, as opposed traditional glass formulas which are mostly silica, and these additives lend the glass a higher refractive index than ordinary glass. When light strikes a glass surface at an angle, one side of the light wave enters the denser medium and slows down first. The other side, still traveling in the air, continues at a higher speed, which forces the overall path of the light to bend. As light enters a barium titanate bead it slows down more quickly than it would when entering ordinary glass, and is bent at a much sharper angle, reflecting more light back to the viewer. A more transparent and round glass bead with fewer inclusions or air bubbles will also retroreflect more of the light hitting it. Retro-reflectivity can be further improved by coating the back of the sphere in a metal coating to make it mirror-like. Light passes through the clear half of the glass bead and bounces off the reflective aluminium-coated back, directing the light back to the source. Perhaps the most familiar example of an aluminized retroreflective glass bead can be seen in the cat's eyes embedded in our roads. 

When they were first used in the 1920s, retroreflective glass spheres were ten to twenty millimetres in diameter, but manufacturers have been working on miniaturising these glass beads. In the 1930’s the American company Potters made tiny accurate glass spheres only a fraction of a millimetre in diameter which were first used on the cinema’s ‘silver screens’, giving a much brighter image. 

This ability to concentrate light into a bright and directional beam makes retroreflective materials particularly useful for creating high visibility in low light conditions. These high-shine materials have become crucial for nighttime road safety and are liberally used in thermoplastic road markings, traffic signs and car license plates. They also contribute to safety on construction sites and for emergency responders, embedded in helmets and high-vis tactical and safety vests. In architecture, retroreflectors have been explored for use in pavements, roofs and building facades to reflect solar radiation and reduce heat absorption, with the potential to mitigate the ‘urban heat island effect’ observed in cities (whereby the materials and geometry of the built environment, the lack of vegetation and waste heat from air conditioning and cars can create localised thermal bubbles).

This particular can of paint was produced as part of a road safety campaign by advertising agency Grey London for Volvo, working with specialist Swedish paint manufacturer Albedo100. One of the earliest uses for Albedo100s retroreflective paint was in 2014 when Finnish reindeer herders tried spraying the antlers of their sled-working reindeer to prevent some of the 3500-5000 reindeer road kills that happen every year in Finland. Sadly, this proved ineffective because the reindeer rubbed it off within a few days, but images of the magical glowing creatures went viral on social media, drawing attention to the issue.