Around the year 1603, Italian shoemaker and amateur alchemist Vincenzo Casciarlo attempted to smelt some particularly dense boulders found on the slopes of Mount Paderno near Bologna. No gold, silver, or other precious metals were found as he had hoped. But after the stone cooled, Casciarolo discovered something interesting: if he exposed the material to sunlight and then moved it into a dark room, the stone would glow.
That was the “Bologna Stone” The first artificially produced, continuously luminescent substance, Many more were to follow—and today, the luminescent material is consistently used for decoration, emergency lighting, sidewalk markings, and medical imaging.
Someday they may give us glowing cities that are cooler and use less electricity.
a New generation of luminescent materials It has the potential to cool cities by re-emitting light that would otherwise have been converted to heat. They can also cut down on energy use, as luminescent sidewalks, glowing street markers, or even glowing buildings can replace some street lighting. Already, some cities in Europe have installed shiny bicycle lanes, and some researchers have studied using Glowing Paint for Road Marking,
“It’s better for the environment,” says Paul Burdahl, an environmental physicist now retired from Lawrence Berkeley National Laboratory in Berkeley, California. “If the technology can be improved, we can use less energy … it’s a worthwhile job.”
The Bologna Stone, a form of the mineral barite, attracted natural philosophers of the time, but was never particularly useful. But in the 1990s, chemists developed new types of persistent photoluminescent materials, such as strontium aluminate, that maintained a strong glow for hours after exposure to light. Most of these newer materials glow blue or green, although some glow yellow, red or orange.
Such photoluminescent materials serve to “trapping” the energy of a photon and then re-emitting that energy as shorter-wavelength light. Sometimes light is emitted immediately, as in a fluorescent light bulb. Other materials, which are said to be continuously luminescent, store energy for longer and emit it more slowly.
These materials that glow strongly for hours open possibilities, such as “glow-in-the-dark” cities illuminated by luminescent sidewalks and buildings. Since then Lighting accounts for 19 percent of total global energy use, and in Europe About 1.6 percent exclusively for street lightingThe potential energy savings are huge, write building engineer Anna Laura Pisello and colleagues In the 2021 Annual Review of Materials Research,
One problem with the approach is that most luminescent materials will not glow overnight. Better materials could help solve that problem, says Pisello of the University of Perugia, who studies energy-efficient building materials. In the meantime, existing materials can be combined with electric lighting that will come on long enough for road signs to recharge before shutting down again.
Luminescent paint can also provide lighting in an outdoor area. Pisello’s lab developed such a bright color and, in 2019 report, simulated what would happen if they painted a public path near a railway station with it. The scientists found that by shining overnight, the paint would reduce the energy required to light in the immediate area by about 27 percent.
If this keeps the whole city’s worries lit up all night and connects harmful light pollution, Pisello says it’s unlikely. Luminescent materials will probably only replace existing lighting, not contain it. The color of the flashing material can be chosen to avoid blue frequencies that have been found to be particularly harmful to wildlife.
Luminescent materials can also help fight what’s known as the urban heat island effect. Roofs and sidewalks absorb energy from the sun and emit it as heat, giving the city summer temperatures an average of 7.7 °C higher than the surrounding countryside. Higher temperatures are a potential health hazard and result in more energy being used to cool buildings.
An increasingly common solution is to use “cooler” materials that reflect light, such as white paint and light-colored asphalt. It turns out that adding luminescent material can help even more.
In the Lawrence Berkeley Lab, Bardahl and his The team experimented with synthetic ruby, a material that is luminescent when exposed to sunlight, for making colored coatings that remain cool. In an early experiment, they reported that a ruby-pigmented surface remained cooler in the Sun than a similarly colored material without special pigments.
Pisello’s lab went a step further and Multiple consecutive luminescent materials added—those who store and release light energy gradually—to solidify, Compared to non-luminescent surfaces of the same color, the best of them reduced the surrounding air temperature by 3.3 ° C on sunny days.
“You can make [a surface] As reflective as possible. But can you go beyond that? The idea is that maybe you can go a little further than that by using continuous luminescence as another way to transfer energy… It’s interesting,” says Patrick E. Phelan, a mechanical engineer at Arizona State University, that co-author of a paper on the urban heat island effect In the annual review of the environment and resources.
There are 250 known luminescent materials, many of them not yet studied for practical applications. Pisello says there’s the potential for glossy paint and sidewalls that last longer and shine in more colors.
“In the short term, the best and easiest solution is to improve what we already have,” she says. This includes tweaking materials so that they light up longer, more strongly, or in different colors, and ensuring that they continue to work in real-world environments.
In the long term, she adds, new classes of engineered materials may work even better. For example, one can turn to “quantum dots” – tiny semiconductor particles that can be made to glow and which are already used in biological imaging—or perovskites, materials used in solar cells that are also being studied for their luminescent properties.
Kurt Kleiner is a freelance science journalist based in Toronto.
This story originally appeared on know-how magazine,