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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+
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ZnS: Cu
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Average
grain (um)
|
10-60
|
35
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Exciting
wavelengh (nm)
|
300-450
|
200-450
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Luminescent
peak value (nm)
|
520
|
530
|
|
Afterglow
brightness (mcd/m2)
|
350
|
30
|
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Afterglow
time (min)
|
＞2000
|
200
|
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Exciting
time (min)
|
20
|
4
|
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Lightfastness
(hr)
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＞1000
|
10-24
|
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Specific
gravity
|
3.6
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4.1
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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.
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