Glow in the dark materials
Photoluminescence (PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. It consists in an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. The period between absorption and emission is typically extremely short, in the order of 10 nanoseconds.. The most familiar such effect is fluorescence. An even more specialized form of photoluminescence is phosphorescence. The energy trapped transit very slowly than other transitions and the result is a slow process of radiative transition back to the singlet state, sometimes lasting minutes or hours: this is the basis for “glow in the dark” substances.
So trying to imagine that photoluminecence can be integrated in the cities and can also last longer than one hour, we can imagine, walls, streets, signs, details, of our cities in a luminescent atmosphere as the one imagined for avatar in the Planet Pandora where plants and living creaures from the forest become luminescent at night. The followng examples are all related to inanimated surfaces and materials that are able to be charges by UV and light and re-emitt it during the night. This is no bioluminescence but a property of pigments included into urban materials.
Starting with the possibility of the Glow-in-the-dark materials that shine with the whole range of visible colors, and can even produce white light: a team of Japanese researchers in 2007, from Ryukoku University, Kyoto, say they can be used to provide lighting and clearly readable signs in emergencies without the need for electricity. Using glow-in-the-dark – or phosphorescent – materials in this way is rare. But the International Commission on Illumination (CIE), the international authority on lighting, has suggested buildings be fitted with emergency lighting and signs that work without power as standard. New York and Tokyo are considering the idea, says one of the researchers, Mitsunori Saito. He thinks the new material’s ability to emit light across the whole visible spectrum makes glow-in-the-dark emergency lighting more attractive.
The most important question is: how long they last? “Conventional blue or green phosphors create an eerily uncomfortable illumination environment in which people feel anxiety,” Saito explains. They also give poor contrast when used for signs which is a problem when people need to find exits through heavy smoke or dust, he says. Warmer colours like orange and red will produce more legible signs. “Combining red, green, and blue colours even enables us to create white light, which may provide more natural illumination,” Saito says. Phosphorescent materials absorb energy when exposed to light and emit that energy as light again over long time periods. Good blue and green phosphorescent compounds have been around for some time but until now red ones lasted minutes instead of hours. Saito and colleagues Naoki Adachi and Hiroyasu Kondo got around this by adding red dye to green and blue phosphors. So the problem is that warmer red colours shuold last more. The dye molecules absorb light emitted by the base phosphor and emitt red light of their own. Mixing the red dye in different proportions with green or blue phosphors makes it possible to produce the full range of visible colours. Adding the dye does not change the efficiency of the phosphors noticeably, the researchers claim. A 5-minute charge from a fluorescent lamp is enough to make them glow for about three hours. “Using glow-in-the-dark is not a current strategy for emergency lighting,” says Roy Webb at Heriot-Watt University, UK, “but I can see the appeal,” he says. Emergency lighting does not need to be very bright because the human eye adapts well to dim conditions, he points out. But Webb added that the specific benefits of having more colours would need proper testing.
The Japanese team are also exploring other uses for their technology: as a very interesting example, one company is developing a night-light lamp shade that shines all night.
On the other end our knowledge today is that photoluminescent pigments such as grade 0, strontium aluminate pigments can glow throughout the night on a single charge but they only glow brightly enough for the first 2 hours to have any use on the road. In addition, they start this discharge when they lose direct sunlight. An example is the Signal Delightcoating material based on an alkaline strontium aluminates pigment, a novel and environmental friendly material. The material is charged when there is enough ambient light around. The material has a long and bright afterglow. It is durable, heat, UV and Weather resistant, and is easy applicable. The chemical stability is better than that of ZnS products. Applicable for any surface, from tarmac to Photo luminescent ink, from clothing to toys or for ready applied material on tiles or aluminum strips.(www.signaldelight.com)
Luminous Glass is basically a luminous float glass through the usage of luminous pigments. These pigments are silk-screened on the back of the float glass, although spraying is also possible. The photo luminous pigments are loaded by (UV) light. The loaded energy will be emitted in a dark environment through a optically perceptible luminous surface. The maximum size of the float glass is 2250x4200mm, thicknesses between 5 and 19mm. This standard glass is extra clear, hardened and mass coloured green or blue. The pigments come in two basic variations: yellow (turning green) and light blue (turning gaudy blue), other colours are on request. The pigments can be painted in a pattern or custom design, also multicoloured. Besides float glass, other glasses are also possible, for example isolation glass, stratified glass and curved glass. Even other materials like plastics can be painted with the luminous pigments. Applications are: facades, roofs, floors, tiles (floor and wall), ceilings, inner walls, furniture, counter-tops etc. This material is UV resistant, scratch resistant and weather condition resistant; it is also hard, heavy and glossy with a high translucency.
Lumistone instead is an acrylic solid surface that contains the unique characteristic of long lasting photoluminescence. This surfacing material is an elegant warm white color that emits a cool calming blue after glow in darkness. It was originally conceived for emergency exodus during a power failure. However, it is now also considered as an aesthetic feature. Its classification as solid surface ensures long durability when exposed to high traffic areas, cleaning agents and a high flame resistance. Lumistone products consist of strips for use as moldings, trims, inlays and inserts. It is also available as a liquid inlay, packaged in a ready to use dispensing cartridge. Lumistone works wherever it is exposed to natural daylight or artificial lighting such as fluorescent lights. Optimum lighting provides 12 hours or more of a cool blue afterglow in darkness. It is UV resistant. In the picture an example of application of Lumistone in a project of a “Senior Spot Bench” designed by Studio ABK Architects, produced by Sterling Surfaces and located at Choate Rosemary Hall, Wallingford, CT USA. The bench is 131 feet long in Corian Solid Surface and has also 7,000 inlayed letters of glow in the dark Lumistone Solid Surface. (www.sterlingsurfaces.com) (vistas.ws/MSDSLumistoneSheetsStrips.pdf)
Lumi-tiles are tiles with a luminescent layer. It is possible to provide these tiles with an anti-slip layer. They are suitable for heavy use and are frost- and scratch resistant. The tiles save energy in comparison with other emergency lights and signs, and it meets all ISO-standards. Lumi-tiles can be made into different luminescent colours, of which the green has the best light yields. Lumi-tiles are durable and deliverable in different formats up to 40×10 cm. Lumi-tiles are usable for wall and floor application and it is ready used as emergency lighting and emergency. Other applications possible are in stair steps of public places or in terraces and (outdoor) walking paths in vacation and bungalow parks.
Ambient Glow Technology is a photo-luminescent stone and sand aggregate. High-performance and non-toxic, it illuminates for over 12 hours after being exposed to a light source for only 10 minutes. This self-emitting light technology requires no electricity and is a safety-conscious alternative light source with a lifespan of 15 years indoors or outdoors. It decreases the use of ambient electrical lighting by 80% and can be integrated into any type of design or structure which utilizes cement, concrete, terrazzo, stucco, epoxy or resins. This photoluminescent aggregate AGT is produced in the following formats: 3 different meshes of sand, 1/4 inch stone and 1/2 inch stone. It can be applied by various methods ranging from hand-broadcasting & GFRC to integral finish-coat batch mixing. It can be utilized for exterior flooring, landscaping, walkways, driveways. It is produced in Canada and it assures LEED points for its sustainable approach. (www.ambientglowtechnology.com)
Similarly photoluminescent aggregate is the ones of Terrazzo Luminoso developed together with S. Lövenstein BV (www.eternoterrazzo.nl). Another similar aggregate is an artificial concrete made with synthetic polymers instead of lime cement. It can be reinforced with fibreglass. Polymer concrete can be used for facade cladding, tiles and floor covering. Because of its non capillary properties it will not attract water or dirt. Luminous concrete has glass beads in the surface that has been treated with photoluminescent and fluorescent materials. These photos show the variation with glassbeads which will glow up when exposed to blacklight. Without this light, the bright colours are hidden. In the dark, only the photoluminescent materials will glow. (www.verborgenveiligheid.nl) (www.ppgevelbouw.nl)
Gravel Floor is a photoluminescent materials made of particles that are integrated creating a seamless surface. As such the material adds ever lasting after glow effects to a wide variety of colorful gravel flooring combinations. The particles can be evenly distributed over the surface or grouped in patterns like circles, lines or arrows, thus providing additional safety features in the dark. Unobtrusive at light, photolum upgraded gravel floors blend well with interior design, unlike traditional floor markings. Typical applications are offices, lobbies, libraries.
Another example of integration of pigment in particols is Glowcrete, a luminescent concrete by Vergelabs that emits light by means of phosphorescence. Ultraviolet light is absorbed and re-emitted at higher wave-lengths, creating a visible glow. When the phosphorescent pigment is added to expansion cement, an uneven distribution provides record of mixing strokes. When the phosphorescent pigment is used as an aggregate in concrete, the even distribution promotes average uniformity in light emission. As the concrete surface erodes, virgin aggregate is exposed, extending the lifespan of luminescence. The researchers used phosphorescent pigment in two ways to produce glowing concrete: they added the pigment to expansion cement, the pigment, when distributed unevenly, left a glowing trail that served as a record of the mixing process. More than this they also added the phosphorescent pigment to the concrete as aggregate. The even distribution of pigment in the second case creates a uniform distribution of light emission. In each case, as the surface of the concrete weathers and erodes, the researcher says that new phosphorescent aggregate is exposed, which extends the lifespan of the luminescence. (vergelabs.com/?p=74)
It is important to note in conclusion, that these solutions should be applied intelligently in conditions and situations where they are required or needed or where they can be responsibly useful to assure security and safety, orientation and a better perception of the space and of the image of the city. It is also evident that a deep critical reflection about these material need to take into account evaluation of the effective economical costs compared with their real efficacy and durability. We should also open up the vision from an olistic pint of view and try to evaluate their use from a sustainable point of view, estimating the environmental weight of their production and dismantling phase in order to understand the effective importance of their LCA. Despite of all these considerations, it is undeniable that they promise a new creative, genuine, more articulated and layered approach to the lighting design of the streets and cities that today are completely overwhelmed by the superimposed power of the standardized poles lacking details, a deeper meaning, the juxtaposition of darkness and lightness and an animated result in a more human-sized project.
From
Daria Casciani 