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                                Photoluminescence  (Glow-in-the-Dark)                    

How it works
Luminescence is the emission of light, without significant amounts of associated heat.
There are many types of luminescence, each defined by where the exciting energy comes from:

Radioluminescence: - where the energy comes from radioactive decay. 
These materials are sometimes called self-emitters. 
Cathodoluminescence: - where the energy comes from fast moving electrons, as in television
 and monitor screens (CRTs - cathode ray tubes).  Powered.
Chemiluminescence: - where light is emitted as a result of a chemical reaction.
 Bioluminescence, is chemiluminescence:- in animals, such as that seen in 
some insects and underwater creatures.
Thermoluminescence: - where the exciting energy comes from absorption of heat.
Triboluminescence: - where the energy comes from the action of frictional forces.
Electroluminescence (EL): - where materials emit light when placed in a moving electric field.  Powered.

But the most common form of luminescence is Photoluminescence (PL)
 where the exciting energy comes from the absorption of light. Note this is not reflected light.
Photons are actually absorbed, their energy is dissipated within the material, 
then new photons are later re-emitted.

Glow4u products are not radioactive.

Types of Photoluminescence Photoluminescent materials fall into one of two general classes;

Fluorescence - ( Fluorescent): where the absorption/emission cycle happens very quickly.

Phosphorescence - ( Glow-in-the-Dark): where there is usually (but not always) an appreciable delay between 
absorption and emission of light, creating an afterglow. However this is a very approximate distinction.
 A more accurate description depends upon a fuller understanding of what happens at a molecular level.

The Physics of Photoluminescence: All physical materials exist in an energy state, the lowest 
and most comfortable of which is the ground state. Higher, excited states exist for any given material, 
and these can be achieved by absorbing energy, in this case that carried by photons, from the surrounding environment. 
These higher energy states exist at discrete levels, with no 'half energised' states in between.

Simple Fluorescence:

When a photon of exactly the right energy comes along it may be absorbed by the system, causing the material to assume 
a higher energy level. When this happens the photon ceases to exist, its energy having been dissipated internally. 
This absorption process happens very quickly, usually over approximately 10 -17th of a second.

With many simple materials, especially gases, this excited state is very unstable and the system quickly resumes
 the ground state, emitting a photon which carries away the discarded energy. This is resonance radiation, the simplest
 form of fluorescence, where the energy and wavelength of the absorbed photon is the same as that of the photon emitted. With most solids however,
 including all common photoluminescent pigments, the process is slightly more complicated. 

Typical Photoluminescence:

Once the energy of an absorbed photon has raised the energy level of a system to an excited state, some energy may be
 lost internally before return to the ground state. In this case the photon going in will have more energy, or higher wavelength, 
than the photon going out. For example, an ultraviolet photon may be absorbed, then a visible photon emitted.
 The emission process happens very much more slowly, around 10-5th of a second. Absorption/Emission Spectra:

Energy levels are distributed in a known sequence, and different jumps from all available levels to all other
 available levels is usually happening furiously. So, rather than absorbing at one clearly defined wavelength, and 
emitting at another, a typical photoluminescent material or pigment will usually absorb over a broad band of wavelengths, 
and will emit over a slightly dissimilar but similarly 
broad band of wavelengths. These absorption and emission bands, or spectra, of a typical fluor or phosphor are 
conventionally expressed on a diagram as follows. The difference between the absorption peak and the emission 
peak is referred to as the Stokes Shift. Typical fluorescent pigments may, for example, absorb 
in the ultraviolet and near visible, and emit lower energy visible light, perhaps orange and red,
with the difference in energy being lost internally.

Typical Phosphorescence

Phosphorescent materials undergo a further process during luminescence. It is the electrons that jump between energy levels
 and electrons have spin, usually spin paired with partner electrons so that there is no nett spin to the local system.
 However an excited electron may have its spin reversed, or inverted, perhaps by interaction with an otherwise
 unassociated low energy photon. After this has happened it is impossible for the electron to assume its
 former condition as it would no longer be spin paired with its partner, so this would constitute a forbidden transfer,
 and the local system has entered a metastable phase, described as a transfer from the singlet state where there is no
 nett angular momentum, to a triplet state which has angular momentum. In this condition, the excited electron drifts 
off over the crystal leaving a positively charged hole, or exciton, behind it, until it finds a suitable new hole to fall into. 
The exciton may also drift off in a different direction. Over time, electrons and excitons find suitable
 discharge locations and with every discharge a completely new photon is emitted carrying the surplus energy away. 
But this matching up takes time and the delay is usually (but not always) seen as an afterglow once the irradiating 
source has been removed. The characteristic rate of emission of these photons in a phosphorescing crystal
 drops off exponentially, and is usually sufficiently regular for a brightness half life to be calculated,
 i.e. the time it takes for the afterglow brightness to reduce by half.

A phosphor afterglow may be very shortlived, dying away after a fraction of a second, a so called flash afterglow,
 or may continue for hours or days, depending on the material phosphorescing. With certain inorganic phosphors
 the afterglow decay is so fast as only to be recognisable using specialist scanning equipment.

Anti-Stokes Photoluminescence with careful engineering of equidistant energy levels, it is possible to prompt
 a photoluminescing system to absorb two or more photons of the same wavelength sequentially, and thereby
 be excited to a higher state, before returning to the ground state in a single discharge.

By using this technique it is possible for some special phosphors to absorb low energy light, and emit higher energy light, 
for example absorbing infrared and emitting visible. Although initially this may appear to be a contravention of the 
Conservation of Energy Law, the total energy of the two or more photons absorbed is always more than that 
of the single photon emitted in a single cycle.

In practice internal energy losses are high and increase significantly with each photon absorption. With up-conversion 
the Stokes diagram flows backwards, from longer wavelengths to higher wavelengths, so the Stokes Shift is negative, 
hence the term anti-Stokes applied to these pigments. With anti-Stokes photoluminescence the phosphor wants to 
discharge half way through the process so must be prevented from doing so. This is done by forcing lower energy light
 into the phosphor by using a laser, or some other high intensity device such as a light emitting diode at shorter range.

Other Photoluminescent Processes:
The reverse photoluminescent process has been proposed, namely absorption producing a single very large energy transition, 
followed by return to the ground state by several equal low energy discharges. By using such a technique it would be 
possible to produce very large positive Stokes Shifts, say 
directly from ultraviolet to infrared. This process has been reported using very exotic dopants, such as a promethium,
 however a viable pigment still lies some way off. With anti-Stokes photoluminescence the phosphor wants to discharge 
half way through the process so must be prevented from doing so. This is done by forcing lower energy light into 
the phosphor by using a laser, or some other high intensity device such as a light emitting diode.
The usual method of producing large positive Stokes Shifts is by cascading, where the luminescent output of 
one material is matched to the input of another. However this matching is never entirely accurate 
so there is always significant energy loss with every cascade cycle.

Measurement of Afterglow: Afterglow brightness or, more accurately, luminance, is a measure of luminous intensity 
of a surface over a given area. The luminance of phosphorescent pigments and surfaces is measured using light 
sensitive equipment under carefully controlled laboratory conditions. Several standard test specifications are in use,
 the most commonly quoted being DIN 67510. Whichever test is used, luminance measurements 
are usually quoted in millicandelas per square metre (mcd/m2) after a quoted time.

Rate of luminance decay can be plotted graphically, in which case it is usually in a log/log format giving something
 approximating a straight line, which can be extrapolated. DIN67510 The brightness and duration of luminescent
 afterglow depends on the amount of light energy absorbed, or the charge received, by the test piece and DIN67510 
was an attempt to create a level playing field by saturating test pieces with light. However, now that newer much
 more efficient phosphors have been developed, the light level specified by DIN67510 is no longer sufficient to 
saturate test pieces, but at 1000 lux neither is it a practical representation of typical light conditions. Consequently 
the test is now viewed with some scepticism and not considered a complete indication of performance.
 It remains, nevertheless, the best simply expressed measure of afterglow.

Part 1
Following exposure at 1000 lux for 5 minutes the light source is extinguished and luminance measurements are 
taken after ten minutes and sixty minutes. DIN67510 Part 1 results are expressed in the format a/b  c d 
a = luminance after 10 minutes b = luminance after 60 minutes
c = time in minutes to 0.3mcd/m2
d = excitation colour code e = afterglow colour code
c is referred to as the time to extinction and is usually extrapolated from a plotted decay curve. However, a recent
 directive issued by the PSPA states that this should not be quoted unless actually measured, extrapolation
 being considered too inaccurate

Part 2
This part of the standard DIN test concerns the measurement of phosphorescent items at the place of use and 
is therefore more relevant to end applications rather than pigment performance. Luminance readings are
 expressed in tabular form.Application guidelines:   Do not grind, mill or blend with non luminescent pigments, 
or use opacifiers or solid functional fillers. Do disperse by high speed stirring, use lubricants with plastics 
where possible apply over white surface as this will improve the performance.

Inorganic UV Responsive (reactive) Pigments:

                    Physical Properties:                    
Pigments phosphoresce brilliantly, with little or no afterglow. They are insoluble and have good stability under prolonged 
exposure so may be used in applications unsuitable for organic pigments.  
These may be blended together, but not with opaque colourants.

Application guidelines: Pigments:    
Do not mill or grind as this will reduce luminescent yield, use Red/Orange/Yellow/White in low pH applications, 
blend with non-luminescent colourants or other opacifiers. disperse by high speed stirring, 
test before using in water based formulations.

                                                     Organic UV Responsive (reactive) Pigments:                                                        
UV Series are high intensity pigments for use in inks, paints, plastics and other applications, where their presence is 
not meant to be apparent under normal lighting conditions. Under long or short UV, these materials fluoresce intensely,
with no afterglow once the irradiating source is removed. Ideal for security and identification,

Physical Properties:
They are many times stronger than inorganic equivalents, but are not intended for  
prolonged outdoor exposure. They are soluble in some polar solvents, but dispersing agents may be necessary
 in some water based formulations. Lightfastness is typical of organic fluorescent compounds, but effective life may 
be extended by the use of UV absorbing overlacquers to block damaging shorter incident wavelengths.

Application guidelines:
Do mill for colour development, blend to achieve intermediate shades,
Do not use for outdoor applications.

                Up-Converters Physical Properties:                         
Anti-Stokes pigments, or 'up - converters', absorbing lower energy light and re-emitting the energy as
 higher energy light. This is achieved by emitting one unit of high energy for every two or more units of low energy absorbed.
 Because this process is the reverse of what usually happens in phosphors and fluors, the process is said to be
 anti-Stokes luminescence. The UC range of up-converters absorbs near infrared around 980nm, and emits in the visible spectrum. 
The activating energy should be by infrared laser, The UC range does not luminesce under ultra violet. 

Application guidelines:
Do not mill or grind excessively, Do treat as a inorganic phosphor, 
Specially manufactured, battery powered, infrared laser pens are available for activation. 
A low powered IR diode may be use to activate UC-G at short range.

Formulator's Guide: 
Do not mill or grind inorganic phosphors This degrades the crystalline structure of phosphors and reduces their
 luminescent efficiency, so that emissions will be less bright and shorter lived. Where essential, very gentle 
milling can be tolerated, but formulators should test for luminescent degradation. As a general guide, the bigger 
the particle size, the better the luminescence of any given inorganic phosphor. 
High speed stirring is always to be prefered to milling. Do not blend with conventional colourants 
This quenches luminescent emissions very effectively. It is not possible
 to tint phosphors and fluors effectively with non-luminescent colourants do not use opacifying fillers
 or other additives This will also quench luminescent emissions use UV absorbers where appropriate Apart
 from the very stable inorganic aluminates phosphors and fluors benefit from short UV protection, while allowing 
the longer UV wavelengths to get through. Tests have shown that a carefully formulated UV absorbing overlacquer will double
 effective lightfastness, whereas incorporating he same UV absorbers into a photoluminescent coating only increases 
lightfastness by about 25%. the key to the best luminescent performance is good dispersion.

Paints: (Solvent)
Aluminate phosphors can be stirred directly into virtually any solvent
based base provided it is at least translucent, and at best transparent. Do not stir
phosphors into a titanium dioxide white base or paint as this will immediately quench any luminescent emission.
 Generally use the biggest particle size grade and do not mill or grind the phosphor into the base.
The key to good luminescent performance is good dispersion.

Recommended concentration is 10% to 40% by weight, which gives a matt or semi matt finish, so an over lacquer
is recommended. Always apply over an opaque white undercoat. With large particle size, aluminate phosphors 
will tend to settle in most paint formulations. Anti- settling rheological control additives can be used,
 or a translucent filler/extender such as a fumed silica can be used effectively.

Paints: (Waterbourne)
Aluminate phosphors can be stirred directly into waterbourne binders such as an acrylic
base provided it is at least translucent, and at best transparent but better water white.
. Do not stir phosphors into a titanium dioxide white base or paint as this will immediately quench any luminescent emission.
 Generally use the bigger particle size grade (30 micron recommended) and do not mill or grind the phosphor
 into the base.The key to good luminescent performance is good dispersion.

Recommended concentration is 10% to 40% by weight, which gives a matt or semi-matt finish, so an 
overlacquer is recommended. Always apply over an opaque white undercoat. With large particle size, aluminate
 phosphors will tend to settle in most paint formulations. Anti- settling rheological control additives 
can be used, or a translucent filler/extender such as a fumed silica can be used effectively.

Paints: Waterbourne Horological (watches)
Only use Treated Aluminate phosphors can be stirred directly into waterbourne binders such as acrylic base provided 
it is at least translucent, and at best transparent or better water white. Do not stir phosphors into a
 titanium dioxide white base or paint as this will immediately quench any luminescent emission. 
Generally use the smallest particle size grade (3µ micron recommended) 
and do not mill or grind the phosphor into the base.

The key to good luminescent performance is good dispersion.
Recommended concentration is 30% by weight, which gives a matt or semi matt finish, so an over lacquer
 is recommended. Always apply over an opaque white undercoat. With large particle size, aluminate phosphors
 will tend to settle in most paint formulations. Anti-settling rheological control additives
 can be used, or a translucent filler/extender such as a fumed silica can be used effectively.

Paints: Solvent Horological (watches)
Any Aluminate phosphors can be stirred directly into solvent binders for the best results use optically
 waterwhite clear binders these will always give the best results  translucent, and at best transparent.
 Do not stir phosphors into a titanium dioxide white base or paint as this will immediately 
quench any luminescent emission. Generally use the smallest particle size grade (3µ micron recommended)
 or 10µ - 15µ and do not mill or grind the phosphor into the base.The key to good luminescent performance
 is good dispersion. Recommended concentration is 30% by weight, which gives a matt or semi-matt finish,
 so an overlacquer is recommended. Always apply over an opaque white undercoat. With large particle size, 
aluminate phosphors will tend to settle in most paint formulations. Anti- settling rheological
 control additives can be used, or a translucent filler/extender such as a fumed silica can be used effectively.

Screen Inks:
By far the most common use of phosphors and fluors is in screen inks, which produces the thickest films of 
any printing process. Always use the coarsest mesh available and the coarsest phosphor grade at maximum concentration.
A single pass at high concentration is more effective (and cheaper) than two or more at lower concentration. 
Pigment concentrations of 50% or more are common with long afterglow aluminate phosphors. 
A single pass with at 50% through a coarse mesh will achieve PSPA Class B comfortably. Always apply over a white base.
 A clear, non-yellowing overlacquer will improve mar resistance and provide a more pleasing finish.

Gravure Inks:
Phosphorescent gravure inks require finer aluminate grades, usually 15 micron or less which has been used successfully
 in gravure inks for beverage labels. Additionally, special fine grade aluminate phosphors are available to special 
order for gravure and other applications, although these have reduced photoluminescent properties.
 Phosphor concentration should be as high as possible, 30% to 40% is normal. It should be noted that
 aluminate phosphors are extremely hard materials, approximately 6 on the Moh Scale, and this can cause 
abrasion to gravure cylinders and associated equipment after prolonged printing runs. Certain modifiers, and 
indeed some cell release agents, can reduce this damaging effect.

Invisible or UV responsive phosphors: 
Are regularly used in gravure inks for covert product identification and coding, and also to achieve novelty 
effects under ultraviolet display lamps. Inorganic phosphors should be the first choice for long term display under
 ultraviolet and in sunlight. Much brighter organic invisibles can be used but only for shorter term displays. 
Many formulators blend inorganic phosphors and organic fluors in the same formulation to reach a compromise 
between initial brightness and long term stability.

Litho Inks
Inorganic phosphors are only used in litho inks in very exceptional circumstances, 
as it is largely impractical to mill them down to sub-micron size. 
However UV responsive organic fluors (UV Series) are commonly used, as milling these usually
 develops the luminescent effect rather than reducing it.

All pigments may be used in plastics, either directly or in masterbatch form.
Aluminate phosphors are very hard materials, approximately 6 on the Moh Scale, and this can cause abrasion in 
extrusion, calendaring and moulding equipment. The first signs of this will be a discoloration of the extrudate, 
usually to a light grey, and a reduction in the luminescent effect. Lubricants should always be used, 
such as a suitable wax or similar additive. Pigment concentrations depend on wall thickness, 
and can be anything from 40% in films and thin walled applications, to 5% in more massive mouldings.
 Generally, the higher the pigmentation, the better the effect, provided the pigments are well dispersed.

Inorganic phosphors are very stable in all plastics processes, however it should be noted that
 organic fluors (UV Series) have limited temperature stability 
(decomposition points around 200 C and low tolerance to extended dwell times.)

Safety Signs:
A very significant use of aluminate phosphors is in safety signs 
The current PSPA guideline is that Class A is the minimum requirement for photoluminescent safety signs
 (refer to the appendix for details of the PSPA Classification structure). 

Most sign producers make their own phosphorescent inks as required by dispersing aluminate phosphors 
directly into a preferred screen ink base. In this way waste is minimised and compatibility is maintained.

Meshes should be as coarse as possible to ensure the thickest coatings. Always apply the phosphor on a white substrate 
or over a white undercoat. A clear, non-yellowing overlacquer based on the same ink system as the
 phosphorescent coating, will improve mar resistance, and produce a more pleasing finish.

Security Marking and Tracing:
The use of UV responsive phosphors and fluors in covert security marking is well established as a 
simple but effective method of product identification and coding. Most security markings are printed.

Although often used singly, carefully formulated combinations of fluors and phosphors can be created to produce 
unique and complex responses under different light sources, known only to the formulator, for example combining 
a short UV responsive pigment with a long UV responsive pigment, and an afterglow phosphor, all in the same ink. 
Or, prints can be superimposed to produce unique effects.Furthermore, phosphors and fluors can be combined with
 other non opacifying materials such as flops and pearls. Combination with metallics is not recommended 
as this can lead to quenching (Reduction) of luminescent emissions.

Phosphors and fluors are regularly used as tracers to mark otherwise difficult to follow materials, 
for example on continuous production lines, or in leak detection. 
They are also used to guide automatic machinery such as drilling equipment and recycling machinery.

Glow4u has considerable experience in these applications and will be pleased to offer recommendations and suggestions. 
All advice and cooperation in discrete applications is given in complete confidence and secrecy.   

Although phosphors and fluors are used in cosmetics they are used only as effect additives and not as colours, 
so they do not appear on the positive colours lists. They do not have formal INCI names, and do not have FDA approval.

Phosphors and fluors: 
Are generally considered to be non- toxic if used as recommended, however it must always remain the users
 responsibility to ensure that the use of phosphors and fluors is appropriate for their own application. 
Users must test the products in their own system before adopting them on a commercial scale.

All the above information is supplied in good faith to help formulators use Phosphors and Fluors
and Glow4u can not be held responsible for any poor performances that may be due to other factors.