![]() ![]() More than 250 kinds of luminescent materials have been identified. Other materials, which are called persistently luminescent, store the energy longer and emit it more slowly. Sometimes the light is emitted immediately, such as in a fluorescent light bulb. Such photoluminescent materials work by “trapping” the energy of a photon and then re-emitting that energy as lower-wavelength light. Most of these new materials give off a blue or green glow, although a few glow yellow, red or orange. 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. The Bologna Stone, a form of the mineral baryte, fascinated natural philosophers at the time, but was never especially useful. “If the technology can be improved, we can use less energy.… It’s a worthwhile thing to do.” “It’s better for the environment,” says Paul Berdahl, an environmental physicist now retired from Lawrence Berkeley National Laboratory in Berkeley, California. ![]() Already, some cities in Europe have installed glowing bicycle lanes, and some researchers have studied using glowing paint for road markings. They might also cut down on energy use, since luminescent sidewalks, glowing road markers or even glowing buildings could replace some street lighting. Someday they might give us glowing cities that stay cooler and use less electricity.Ī new generation of luminescent materials has the potential to cool cities by re-emitting light that would otherwise be converted into heat. That “Bologna Stone” was the first artificially prepared, persistently luminescent substance. Many more were to follow - and today, persistent luminescent materials are used for decorations, emergency lighting, pavement markings and medical imaging. But after the stone had cooled, Casciarolo discovered something interesting: If he exposed the material to sunlight and then took it into a dark room, the stone would glow. No gold, silver or other precious metals resulted as he had hoped. Around the year 1603, Italian shoemaker and amateur alchemist Vincenzo Casciarolo tried smelting some especially dense stone he had found on the slopes of Mount Paderno, near Bologna.
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