Vincenzo Casserole, an amateur shoemaker, tried to make shoes out of some dense stone he found on the slopes of Mount Paderno. He had hoped for gold, silver, or other precious metals to result. The stone would glow if he exposed it to sunlight and then took it into a dark room.
"Bologna Stone" was the first artificially prepared substance. Today, persistent luminescent materials are used for decorations, emergency lighting, pavement markings, and medical scans.
Someday, they might give us glowing cities that use less electricity.
A new generation of luminescent materials has the potential to cool cities by re-emitting light that would otherwise be converted into heat. They could also cut down on energy use by replacing street lighting with glowing sidewalks, road markers, or buildings. Some cities in Europe have installed glowing bicycle lanes, and some researchers have studied the use of glowing paint for road markings.
"It's better for the environment," said Paul Berdahl, an environmental physicist who retired from Lawrence Berkeley National Laboratory. If technology can be improved, we can use less energy.
Natural philosophers were fascinated by the Bologna Stone, but it was never useful. New types of photoluminescent materials, such as strontium aluminate, were developed in the 1990s. Most of the new materials give off a blue or green glow, although a few glow yellow, red, or orange.
The photoluminescent materials work by trapping the energy of a photon and then emitting it as lower-wavelength light. Sometimes the light is emitted immediately, such as in a fluorescent light bulb. Other materials emit energy more slowly and store it longer.
There are more than 250 kinds of luminescent materials. The trace materials that act as the luminescent center are grouped with the host compound and the color of the material.
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The materials that glow strongly for hours can be used to illuminate "glow-in-the-dark" cities. In the "Annual Review of Materials Research", Anna Laura Pisello and colleagues write that the potential energy savings are large since 19% of all global energy use is for lighting.
Most luminescent material won't light up all the way through the night. Pisello is a researcher at the University of Perugia who studies energy-efficient building materials. In the meantime, existing materials could be combined with electric lighting that would come on long enough to replenish the road markings.
The paint could provide outdoor lighting. The lab of Pisello developed glow-in-the-dark paint that could be used to paint a public path near a railway station. The scientists found that by glowing throughout the night, the paint would reduce the amount of energy needed for lighting.
Pisello says that it's unlikely that this will cause a lot of worry in the cities. Existing lighting would likely be replaced by luminescent materials. The blue frequencies that have been found to be harmful to wildlife could be avoided with the color of the glowing materials.
The urban heat island effect is caused by the use of luminescent materials. The city's summer temperatures are higher than the countryside because rooftop and pavement absorb energy from the sun and emit it as heat. The high temperatures can cause a health hazard and result in more energy being used to cool buildings.
White paint and light-colored asphalt are examples of materials that reflect light. Adding luminescent materials can help even more.
Anna Laura Pisello and her colleagues at the University of Perugia are trying to create practical pavement that glows in the dark. They are experimenting with different luminescent substances, and testing how to add them to pavement material to get the best performance. There are samples of luminescent materials and a paving stone embedded in them.
Anna Laura Pisello is a person.
Berdahl and his team at the Lawrence Berkeley Lab were experimenting with synthetic Ruby, a material that is luminescent in the sun, to make colored coating that stayed cool. They reported that a Ruby-pigmented surface was cooler in the sun than a similar colored surface.
Pisello's lab added several persistently luminescent materials to concrete, which gave off light energy slowly. The best of them lowered the surrounding air temperature on sunny days by up to 3.3C.
You can make a surface reflective. Can you go beyond that? The idea is that you can use persistent luminescence as another way to transfer energy. Patrick E. Phelan is a mechanical engineer at Arizona State University and co-authored a paper on the urban heat island effect.
Many of the luminescent materials are not yet studied for practical applications. Pisello says there is a chance for glowing paints and pavement that will last longer.
She said that the best and easiest solution was to improve what we already have. Making sure that the materials give out light longer, more strongly, or in different colors is part of that.
New classes of engineered materials could work better in the future. quantum dots are tiny semiconducting particles that can be made to glow and are already used in biological.
