Can Dark Roofs Stay as Cool as White?

It’s common knowledge that dark colors get hotter than light ones in the sun. But new research from Lawrence Berkeley National Laboratory reveals an exception. Some dark pigments stay just as cool as white thanks to fluorescence, which occurs when fluorescent materials immediately re-emit absorbed light.

Led by scientist Paul Berdahl, the research team focused on ruby red coatings created from a mix of aluminum oxide and chromium. They first discovered that white paint covered with a layer of synthetic ruby crystals remained cooler than an off-white surface.

The scientists then created several shades of ruby pigment to mix into coatings, which were applied over white substrates. The ruby paint samples stayed just as cool as white materials, though Berdahl notes that creating ruby powder that was as deeply colored as the crystals was significantly more labor-intensive. In both applications, fluorescent cooling boosts the pigments’ performance by re-emitting some of the visible light that the surface absorbs to appear dark rather than passively reflecting it.

Berdahl also identifies blue materials that fluoresce and can be combined with other colors to yield green and black materials that remain cool. Prototype fluorescent coatings are currently undergoing weathering tests, and the commercialization of the pigments is not expected to be costly, Berdahl says.

“Rubies have a reputation for being expensive, but they’re mostly aluminum oxide, which sells for about 30 cents per pound,” Berdahl adds.

The full results were published in the journal Solar Energy Materials & Solar Cells.