Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and reemits it for a much longer time after the radiation source is removed.
How do some paints and stickers glow in the dark
In a general sense, there is no distinct boundary between the emission times of fluorescence and phosphorescence (i.e.: if a substance glows under a black light it is generally considered fluorescent, and if it glows in the dark it is often simply called phosphorescent).[1] In a modern, scientific sense, the phenomena can usually be classified by the three different mechanisms that produce the light, and the typical timescales during which those mechanisms emit light. Whereas fluorescent materials stop emitting light within nanoseconds (billionths of a second) after the excitation radiation is removed, phosphorescent materials may continue to emit an afterglow ranging from a few microseconds to many hours after the excitation is removed.[2]
Everyday examples of phosphorescent materials are the glow-in-the-dark toys, stickers, paint and clock dials that glow after being charged with a bright light such as in any normal reading or room light. Typically, the glow slowly fades out, sometimes within a few minutes or up to a few hours in a dark room.[5]
The term phosphor had been used since the Middle Ages to describe minerals that glowed in the dark. One of the most famous, but not the first, was Bolognian phosphor. Around 1604, Vincenzo Casciarolo discovered a "lapis solaris" near Bologna, Italy. Once heated in an oxygen-rich furnace, it thereafter absorbed sunlight and glowed in the dark. In 1677, Hennig Brand isolated a new element that glowed due to a chemiluminescent reaction when exposed to air, and named it "phosphorus".[8]
There was much confusion between the meanings of these terms throughout the late nineteenth to mid-twentieth centuries. Whereas the term "fluorescence" tended to refer to luminescence that ceased immediately (by human-eye standards) when removed from excitation, "phosphorescence" referred to virtually any substance that glowed for appreciable periods in darkness, sometimes to include even chemiluminescence (which occasionally produced substantial amounts of heat). Only after the 1950s and 1960s did advances in quantum electronics, spectroscopy, and lasers provide a measure to distinguish between the various processes that emit the light, although in common speech the distinctions are still often rather vague.[10]
In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for glow-in-the-dark materials which are "charged" by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in laser mediums like the common ruby, phosphorescent materials "store" absorbed energy for a longer time, as the processes required to reemit energy occur less often. However, timescale is still only a general distinction, as there are slow-emitting fluorescent materials, for example uranyl salts, and, likewise, some phosphorescent materials like zinc sulfide (in violet) are very fast. Scientifically, the phenomena are classified by the different mechanisms that produce the light, as materials that phosphoresce may be suitable for some purposes such as lighting, but may be completely unsuitable for others that require fluorescence, like lasers. Further blurring the lines, a substance may emit light by one, two, or all three mechanisms depending on the material and excitation conditions.[11]
When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude. Because the atoms usually begin in a singlet state of spin, favoring fluorescence, these types of phosphors typically produce both types of emission during illumination, and then a dimmer afterglow of strictly phosphorescent light typically lasting less than a second after the illumination is switched off.
The release of energy in this way is a completely random process, governed mostly by the average temperature of the material versus the "depth" of the trap, or how many electron-volts it exerts. A trap that has a depth of 2.0 electron-volts would require a great amount of thermal energy (very high temperatures) to overcome the attraction, while at a depth of 0.1 electron-volts very little heat (very cold temperatures) are needed for the trap to even hold an electron. Higher temperatures may cause the faster release of energy, resulting in a brighter yet short-lived emission, while lower temperatures may produce dimmer but longer-lasting glows. Temperatures that are too hot or cold, depending on the substance, may not allow the accumulation or release of energy at all. The ideal depth of trap for persistent phosphorescence at room temperature is typically between 0.6 and 0.7 electron-volts.[17] If the phosphorescent quantum yield is high, that is, if the substance has a large number of traps of the correct depth, these substances will release significant amounts of light over long time scales, creating so-called "glow in the dark" materials.
Persistent phosphorescence is the mechanism of most anything commonly referred to as glow in the dark. Typical uses include toys, frisbees and balls, safety signs, paints and markings, make-ups, art and décor, and a variety of other uses.
Some examples of glow-in-the-dark materials do not glow by phosphorescence. For example, glow sticks glow due to a chemiluminescent process which is commonly mistaken for phosphorescence. In chemiluminescence, an excited state is created via a chemical reaction. The light emission tracks the kinetic progress of the underlying chemical reaction. The excited state will then transfer to a dye molecule, also known as a sensitizer or fluorophor, and subsequently fluoresce back to the ground state.
The development of strontium aluminate pigments in 1993 was spurred on by the need to find a substitute for glow-in-the-dark materials with high luminance and long phosphorescence, especially those that used promethium.[18][19] This led to the discovery by Yasumitsu Aoki (Nemoto & Co.) of materials with luminance approximately 10 times greater than zinc sulfide and phosphorescence approximately 10 times longer.[20][21] This has relegated most zinc sulfide based products to the novelty category. Strontium aluminate based pigments are now used in exit signs, pathway marking, and other safety related signage.[22]
A common use of Phosphorescence is decoration. Stars made of glow-in-the-dark plastic are placed on walls, ceilings, or hanging from strings make a room look like the night sky.[29] Other objects like figurines, cups, posters,[30] lamp fixtures, toys[31] & bracelet beads may also glow.[32] Using blacklights makes these things glow brightly, common at raves, bedrooms, theme parks & festivals.
A shadow wall is created when a light flashes upon a person or object in front of a phosphorescent screen which temporarily captures the shadow. The screen or wall is painted with a glow-in-the-dark product that contains phosphorescent compounds.[33] Publicly, these shadow walls can be found at certain science museums.[34][35]
Modern luminous powders based on rare earth doped strontium aluminate have transformed glow in the dark applications. They are available in a number of colours but the green one appears to be the brightest and will outshine and outlast zinc sulphide/copper formulations by a factor of ten. Many products that use luminous formulations such as clocks often disappoint the purchaser as they simply do not last the night. The use of this recent technology really does get over this problem.
One factor does make their application slightly difficult in that they are a little coarse in size making it difficult to hold the material in suspension in a liquid medium for easy painting and it might be more informative to describe the material as 'glow in the dark crystals'. You cannot take the obvious step of grinding the material because the performance is heavily dependant on the crystal structure and grinding has a negative effect.This may be a factor in the performance of luminous paints and sprays that have been used in many projects.
This Instructable shows a way around the problem, at least for the small areas involved in the production of stickers. We paint/drip a lake of nail varnish on the sticker and then apply an excess of 'glow in the dark' powder on the top. The nail varnish then mops up the powder until it forms a cake. When the nail varnish is dry we are left with a solid 'cushion' of 'glow in the dark' powder and dry nail varnish. A final coat of nail varnish then stabilises the structure.
Working quickly, because the nail varnish will dry rapidly, spoon an excess of 'glow in the dark' powder on to the lake of nail varnish. The result is shown in the picture above. Now allow to dry for several hours or overnight.
There are an enormous number of potential applications--anything that a sticker will adhere to and that you want to find in the dark, light switches being an obvious one. Travelling alarm clocks often have a press switch that activates a light for a few seconds but one of these stickers will help you to find it first. If you are using this in a dark place which does not receive much daylight then you can 'charge up' the 'glow in the dark' material by holding it near a light bulb or applying a torch for a very few seconds. The resulting glow will now last for the night.
On the left of the composite picture shown above you can see what must be one of the earliest commercial LED torches--just a couple of white LED's driven by a pair of lithium cells. This still does good service as our travel torch and has illuminated our nocturnal perambulations in many dark hotel rooms. The problem comes when trying to locate the torch in darkness and in doing so it is easy to make a noise whilst groping for it or even knock it on to the floor resulting in an awoken partner and commensurate loss of household credit. Problem solved! A small 'glow in the dark' pad has been affixed to the torch and held against a light for a few seconds resulting in a glow that will last the night allowing for immediate location of the torch. The right hand side of the picture shows a picture taken in darkness after the pad received a very few seconds activation with a separate torch. 2ff7e9595c
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