What is traditional Zinc Sulphide pigment?
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.
What is Strontium Aluminate pigment?
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:
|Chymistry component||MO·aAl2O3·bSiO2·cL: Eu2+Dy3+||ZnS: Cu|
|Average grain (um)||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)||＞2000||200|
|Exciting time (min)||20||4|
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.