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Photoluminescence vs. fluorescence

Reflective Traffic Signs A photoluminescent sign with the lights off…

Reflective Traffic Signs
…and lights on.

Many people confuse photoluminescent materials with fluorescent materials, but the difference between the two is quite significant. Fluorescent materials take light at a low wavelength and emit it at a higher one — so fluorescent sign hit by a pair of weak old headlights will reflect light back at a brighter luminosity. In contrast, photoluminescent materials absorb — and store — visible light. While photoluminescent signs are always emitting a bit of light, the glow can only be seen in the dark. Unlike fluorescent signs, they continue to emit light without an immediate light source, but they are not as bright. Fluorescent signs are especially useful at dawn and dusk, and photoluminescent signs are valuable after the lights have been turned off in a building.

Photoluminescent signs are extremely important in case of a power outage because they glow in the dark, and after illumination they can withhold light for a period of time. This duration of illumination time, however, is indirectly proportionate with the brightness of the photoluminescent sign. This means that the longer the photoluminescence period, the duller the light intensity during that time. Often there are two types of signs: a bright short-lasting kind, and a dim kind that can retain its luminosity for several hours. Emergency signs often use the brighter material, as it is imperative that the sign is seen quickly.

Photoluminescent material contains pigment molecules that, when exposed to light, absorb photons with wavelengths in the visible light range. In a process called photo-excitation, pigment molecules become excited, and emit photons back out at a lower energy level. This happens when the electrons of the pigment molecule absorb light energy, move into their excited state, and then return to their lower-energy state.

The extra energy released from the difference in electron excitation levels can be dissipated through the emission of light. Because the pigment molecules are rechargeable, this can happen over and over again, day after day. The absorption and emission process usually occurs in a matter of nanoseconds, but under special circumstances, it may take a few minutes or even hours. Photoluminescent signs are able to delay this process, allowing them to remain bright after minutes or hours, depending on the quality of the photoluminescent material.

Photoluminescent signs are strongly favored in emergency situations and to illuminate dark hallways and staircases. However, photoluminescent signs have almost no place in the traffic and road signage where fluorescent signs shine brightest.

 

 

 
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