Lightlead photoluminescent materials (commonly called "glow-in-the-dark") has the ability to absorb light (sunlight, fluorescent, incandescent, etc) and then to emit light. When ambient darkness occurs, it becomes highly visible, lasting at least 8 hours. Our best products could last more than 20 hours.
Upon removal of the light source, the stored light is gradually released, the strongest glow is produced during the first 30 minutes of darkness, the most critical period following a power failure or other emergency. Then it fades over a period of time, when there is light again, it stores energy again. So the photoluminescence process is circulatory.
The bases of most photoluminescent products are photoluminescent pigments that can be incorporated into coat,paint, ink , fabric, ceramic glaze or porcelain enamel, glass,flexible and rigid molded plastics. Typical products include self adhesive flexible vinyl tapes, rigid PVC marker strips, and silk- screened signage. Photoluminescent fabric,enamel-coated sheet metal and ceramic tiles are also available.
　

Photoluminescence Explained Many inorganic and organic materials exhibit luminescence, which means that they emit visible or invisible light during and after exposure to a source of excitation energy. For photoluminescent materials, the excitation source is electromagnetic radiation - visible and invisible (typically ultraviolet) light.
　

The basic principle of photoluminescence is simple: electrons orbiting atoms or molecules absorb energy through collision with photons during excitation. They then emit this excess energy as photons of (usually visible) light at a later time.
　

The afterglow decreases (or decays) over time after the excitation source has been extinguished. Some photoluminescent pigment manufacturers state that the decrease is exponential, but this is usually incorrect. Most long-persistence photoluminescent pigment (including those commonly used in commercial products) exhibit what is called hyperbolic decay.
　

Zinc Sulphide

Some of the first phosphors to be developed were inorganic zinc sulphide compounds. (German submarine commanders during World War I reportedly rubbed these compounds on their hands to read documents during blackout conditions.) These phosphors typically absorb energy from deep blue and ultraviolet light and emit it as yellow-green light. This is useful in that the peak spectral distribution of the emitted light roughly coincides with the peak spectral sensitivity of the human visual system under isotopic (low-level) lighting conditions (which is around 510 nanometers). Zinc sulphide occurs in crystalline form, but is not photoluminescent by itself. This requires the addition of activator ions to the crystals, such as copper atoms. These ions absorb the excitation energy of the ultraviolet or visible light and later release it as visible light. The copper-activated zinc sulphide crystals (identified with the chemical symbol ZnS:Cu) are typically ground into a fine powder with a grain size of 3 to 15 micrometers to avoid light trapping and light piping effects. This must be done carefully however, because the crystals can darken (or gray) if subjected to heavy pressure or excessive mechanical stress, due to disruption of the crystalline structure.

Strontium Aluminate

Recent research efforts have produced several new inorganic compounds whose photometric characteristics greatly exceed those of zinc sulphide compounds. These materials include oxides of strontium aluminate (identified as SrAl) and other proprietary inorganic compounds. These compounds offer much brighter and longer-lasting photoluminescence, and they can formulate (unlike zinc sulphide compounds) to produce a range of colors. While there are many different strontium aluminate compounds, the following is an example of comparative performance characteristics: Properties SrAl ZnS:Cu

Chymistry component SrAlO4：Eu2+，Dy3+ ZnS：Cu Average grain（μm） 10-60 35 Exciting wavelengh（nm） 300-450 200-450 Luminescent peak value（nm） 520 530 Afterglow brightness（mcd/m2） �@ 350 30 Afterglow time（min）             �A ＞2000 200 Exciting time（min）              �B 20 4 lightfastness （hr）               �C ＞1000 10-24 Specific gravity 3.6 4.1

Strontium aluminate products are currently available in flexible vinyl and rigid PVC sheets, and can be substituted wherever commercial zinc sulphide products are used for life safety applications. For the example shown above, it is some fifteen times brighter than commercial zinc sulphide products, and is clearly visible after many hours of total darkness. Strontium aluminate and other proprietary compounds represent a revolution in the photoluminescent industry, which has relied on zinc sulphide pigments for the better part of a century. There will undoubtedly be new and better photoluminescent pigments and products introduced in the near future, including those with different emission colors.

You might ask, "How bright are these photoluminescent products?" Many manufacturers refer to the extinction time of their products, which is defined as the time required for the afterglow to diminish to one masb (0.032 mcd/m2, or about 100 times the limit of human perception). In practice, this is very difficult to see unless your eyes are fully dark-adapted and you are in a completely dark environment.
For practical purposes, luminances of one to two millicandela per square meter are more appropriate limits for life safety applications, and even this assumes a smoke-free environment. Thus, zinc sulphide products are useful for perhaps 30 to 45 minutes after their excitation source has been extinguished. The afterglow of strontium aluminate products, on the other hand, can be visible for several days or more.
At the other end of the time scale, strontium aluminate products can provide surprising amounts of initial afterglow. For example, a four-inch square of material held a few inches away from a magazine page can provide enough light to read by, at least for the first minute or so. In addition, microprismatic retroreflectors and other brightness enhancing techniques can increase the materials luminance by several times.
　

Lightlead photoluminescent materials are non-toxic, non-radioactive and contain no phosphorus or lead, or any other hazardous element or chemical. The term phosphorence is sometimes used to describe luminescence, but Lightlead photoluminescent materials contain no phosphorus.
　

Lightlead photoluminescent materials differ from reflective materials, which amplify and increase relative brightness when light is applied. Fluorescent materials are quite different from photoluminescent materials. Fluorescent materials (such as the 3M fluorescent work zone film or a "hot pink" tag) actually serve to borrow nearby light energy in nearby wavelengths and concentrate the amount of light returning to the viewer in a certain (for example, day-glo orange) wavelength. Fluorescent films are especially useful in dawn and dusk viewing times, when they appear particularly bright.Lightlead photoluminescent materials require prior exposure to a light source in order to glow in the dark.
　

Lightlead photoluminescent materials were originally designed to complement existing emergency lighting systems but continuous improvements in luminance performance mean that in certain circumstances high quality photoluminescent products can replace emergency lighting. They can be sited at waist or skirting level so they can still function if the ceiling lights become obscured by smoke. Similarly, life saving appliances and fire fighting equipment become more visible and are easily located in an emergency. Some emergency lighting can take up to 15 seconds to activate. In these instances, photoluminescence can be a lifesaver.
　

Photoluminescent Application Questions