A fluorescent material is defined in European standards as a material that emits optical radiation at wavelengths longer than those absorbed.
It therefore has the ability to not simply reflect light- it return it at a longer wavelength, so it appears brighter than the original light source. In Fluorescence?, this ability to emit light stops immediately upon removal of the original light source (within around 10 nanoseconds).
Phosphorescent materials continue to emit light following the removal of a light source, all though how long they can do so varies greatly from minutes to hours, the light source acts as a charger and once removed phosphorescent materials begin to re-emit this energy.
Whereas Luminescence?, Phosphorescence? and Fluorescence? all describe the ability to emit light, Reflection is the ability to reflect it, with Retroreflection being the ability to reflect light, but only in the direct opposite direction to which the light waves entered, meaning that the wave is not scattered so remains stronger. Retroreflective materials can be made Fluorescent, but not all fluorescent materials are retroreflective. Retroreflective materials are of relevance because they are currently widely used in textile applications, and can be used alongside luminescent materials in order increase the brightness.
The ability to give out light without the requirement of a power source that does not generate potentially hazardous heat, is useful in a range of textile applications.
Fluorescence? and retroreflection are seen in apparel and sportswear to alert of the wearers presence, for the same reason this is seen in PPE?, most commonly as the fluorescent strips in the High Visibility vest which is a necessity in many industrial working environments as a safety feature.
As retroreflective materials reflect light rays, this includes rays within the UV range and can therefore pose a sunburn risk, so the use of them on areas which could reflect onto the skin is avoided, such as the shoulders.
What mechanism is used will depend on the required outcome, Photoluminance? requires light in order for it to re-emit it, therefore is only useful in situations where it will be exposed to light, such as reflective strips illuminated by car head lights. If this light exposure is not possible, such as in underground environments experienced in mining, Photoluminance? is not appropriate, therefore other forms must be used such as electroluminance or chemoluminance.
The luminous materials have to be in small solid dust like particles in order to allow the textile structure to maintain its tactile properties such as bending, shearing and drape ability. Luminous properties can be given to textile materials in a variety of ways, generally this can include;
- Coating fabrics with luminous particles in a resin mix
- Introduction into synthetic fibres at spinning stage
- Fibre coating
- Textile finishing or domestic laundering
The coating of textiles with light emitting materials in a resin is perhaps the most widely used in safety garments and this is seen in such items as the strips in high visibility vests, where retroreflective or fluorescent materials are affixed to the textile in a resin. This affects the durability of the textile and can limit its production method, making sewing difficult and limiting end use.
Fluorescent materials in textiles are most widely used as optical brighteners in laundry additives to make whites appear whiter by reflecting the light.
Optical brighteners or fluorescent whitening materials are introduced to the textile, usually in finishing or home laundering. They absorb UV and visible light and emit it back at a different wave length in order to give a brighter appearance and hence disguise impurities. The wave length emitted is usually in the blue-violet region of visible light in order to enhance whiteness and disguise any yellow/brown tones. It is more widely used in natural fibres where impurities are more prevalent than in synthetic. The widespread use of optical brighteners and the fact that they are often washed out of the fabric during laundering has led to environmental concerns.
Not only used to enhance the appearance of white textiles, they are also used on colours, however, due to their ability to wash out, in certain colours a noticeable hue change can be visible between washes.
Fluorescent pigments are widely used in inks to enhance appearance. They have applications in safety wear and apparel such as enhancers in dyes and prints.
Fluorescent pigments can suffer poor fade resistance due to their molecular excited states caused by prolonged exposure to UV, which is why they are often used alongside UV absorbers.
Like optical brighteners the widespread use of fluorescent pigments have been investigated for their environmental impact. However a further concern of optical dyes is that they can penetrate human skin.
Light is emitted from an atom when an external stimuli, in this case electromagnetic energy within the light region, causes the atom to vibrate, which in turn vibrates the electrons of the atom, causing them to move to different energy states, or orbitals, as they move back their grown orbital they emit energy. In the case of fluorescent materials the energy emitted is that in the visible light range. As figure 2 demonstrates, depicting just 1 electron to give a clear illustration;
- Electron in normal state, on the lower orbital
- Energy imposed on atom makes electron move to outer orbital
- As it drops back to lower orbital, energy is released
Figure 2. Image show how an atom emits light. From www.explainthatstuff.com &
Video shows how atoms emit light
More on the scientific principles of Fluorescence? and Phosphorescence? available from; http://www.physik.unibas.ch/Praktikum/VPII/Fluoreszenz/Fluorescence_and_Phosphorescence.pdf
Regulations and Standards
As fluorescents is often implemented as a safety function in personal protective equipment it is regulated as such. Personal Protective equipment is subject to mandatory CE marking.
Currently in draft for public consultation is 10/30200375 DC, ISO 20471 High visibility clothing for professional use.
There are a range of test standards for high visibility clothing;
BS 6629 Specification for optical performance of high visibility garments and accessories for use on the highway
Specification for colours for high visibility clothing
BS EN 1150 Protective clothing. Visibility clothing for non-professional use.
BS EN 13356 Visibility accessories for non-professional use.
REMA- The Retroreflective Equipment Manufacturers Association have published an article on the selection and use of high visibility clothing for professional use, within which information on standards and regulation is outlined.