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Luminescence

Luminescence? is the emission of light without the emission of heat. Luminescent materials have a variety of stimulus to provoke this reaction, but the term Luminescence? is the general term which refers to the emission of light within the visible, infra red or Ultraviolet regions without the production of heat. The stimulus for light emission is wide ranging for example;

  • Chemiluminescence, result of a chemical reaction
  • Crystalloluminescence, result of crystallisation
  • Electroluminescent, result of electrical charge
  • Trioboluminescence, result of material bonds being broken through mechanical action such as rubbing or scratching
  • Photoluminescence, result of absorption of photons
  • Thermo Luminescence?, result of heat

Fluorescence? and Phosphorescence? are forms of photoluminescence.

The ability to emit light without heat is useful in a range of textile applications, the ability to do so without an external power is a further attraction.

Luminescence? is used as a safety alert in a range of applications, in textiles it is used in clothing to alert to the wearers presence, such as in apparel, sportswear and PPE?, most commonly as the fluorescent strips in the High Visibility vest which are a health and safety necessity in many industrial working environments. Safety is also the paramount reason for their use in textile products across engineering, agricultural and building, in such products as tarpaulins, nettings and covering in order to draw attention to their presence.

The advancement of luminescent materials in textiles has led to an increased interest from the fashion industry, where the technology is no longer viewed as a purely functional attribute. The exploration of novel ways to incorporate luminescent materials has led to some innovative and striking creations. Luminant use electroluminescent fibres to produce a range of products including garments and interiors, requiring a low power input.

Luminous textiles are being explored for interior or architectural textiles implemented as both a design feature and a promoter of well being reacting to sound or temperature to instruct a luminous colour display.

Lumigram pic

Photo courtesy of Lumigram.

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
  • Use of Optical fibres
  • Use of Light emitting Wires

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.

Rare-earth minerals with luminescent properties have been added through a melt spinning process. 

Optical fibres have been modified from those used in data transmission to emit the light being carried along them, instead of just at the ends as displayed in traditional optical fibres, displaying luminous along the length of the fibre. Although optical fibres have been developed as sensors to monitor and transmit data from the wearer, ie, biological readings from military personnel, or from other textile applications such as load stresses in engineering textiles. 

Optical Fibres 

Optical fibres transmit data in the form of light waves and are effective as the wave can travel down the length of the fibre bouncing off the edges similar to how light bounces off a mirror, being carried throughout the length of the fibre. As they are made from glass, silica, or polymeric materials, unlike metal wires they do not come under risk from electromagnetic interference or voltage surges. The term Polymeric Optical Fibres or POF’s is often applied but this relates specifically to polymeric materials and not silica or glass. Their ability to transmit data and their fibrous nature making suitable in textile applications has seen optical fibres used in textile based sensors, such as biosensors able to transmit data on the wearers condition, or in industrial type applications able to monitor load stresses.

The European Union began a research project, Optical Fibre Sensors Embedded into technical Textile for Healthcare, (OFSETH) 

Optical Fibre content resized

Figure 1. Photo from; http://en.wikipedia.org/wiki/Optical_fibre

Figure 1 displays the way the light travels down the optical fibre, with it being emitted at the end. In applications where light is required through the length of the fibre, the surface is modified in order to allow the light ways to be emitted down the length of the fibre, see figure 2. These particular applications are more for aesthetics to create illuminated and sensorial textiles. 

Lumigram optical fibre

Figure 2. Image from; Polymeric optical fibres and future prospects in textile integration.

Displays a lumitex fibre. 

Photonic Crystal Fibres 

Photonic Crystal fibres are an advancement of optical fibres with a Cross section? containing many micron sized air voids, as shown in Figure 3. 

Photonic Crystal Fibre

Figure 3. Image from http://spie.org

It is argued that in textile applications photonic crystal fibres are superior to those optical fibres used to provide light emission. This is because the outer surface can remain intact, unlike optical fibres, which require incisions into the fibre length to allow light emission, shown in Figure 2, optical fibre.  Further claimed advantages are improved resistance to corrosion and fatigue, lightweight and ease of integration into textiles. 

The unique Cross section? allows for a range of colours to be produced from a singular light source and fibre, without the need for added pigments which can deteriorate over time. The displayed colour change is due to slight variation in how the light travels through the fibres microstructure.  This concept of colour creation through microstructure rather than by pigments is a concept based on bio mimicry, inspired by how colour is displayed on a butterfly’s wings. 

It has even been claimed that the remittance of infra red light signals within these fibres have been applied for security purposes, which could pose an interesting future for the anti-counterfeiting technology in textiles. 

Useful links for Photonic Crystal Fibres; 

Electroluminescent wire

Electroluminescent wires require an electrical current to illuminate, they can display different colours dependant upon the applied pigment in the protective sleeves and because of this each wire can only display the colour applied in the sleeve.

Their make up consists of a conducting, often copper core then a phosphor coating intertwined with a thin wire, followed by a singular or multiple layers of insulating sleeves, as shown in figure 4. 

Electro luminescent wire

Figure 4. From Wikipedia 

They have been applied in fabric and clothing, but have their disadvantages due to their requirement of electrical voltage to power and due to their construction can have limited ductility and durability. However they have been used to create some quire striking effects, see figure 5. 

Electroluminescent clothing

Figure 5. Electroluminescent wire used in clothing. Image from Urban Space. 

Useful links for electroluminescent wire;

Standards & Regulation

As luminescent materials in textiles are a relatively new area there is as yet no specific standards or regulation of their use.

A working group on smart textiles is currently assessing the need for standards and regulations on smart textiles looking at;

  • Functionality
  • Safety
  • Durability

Including defining smart textiles and the integration with other Directives.

They must conform under the general product safety directive.

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2002:011:0004:0017:en:PDF

If chemical compounds are being used these must conform under the REACH regulation.

Where visibility is a requirement for PPE? equipment as such it will fall under PPE? regulations.

As Photoluminescnce is the most widely used mechanism in safety wear and PPE?, regulations and standards for this can be found in the Fluorescence? section. 

Basic Priciples - Fundimental Science

Light is emitted from an atom when an external stimuli, such as heat, chemical reaction, electronic current, or electromagnetic radiation causes the atom to vibrate. This 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;

  1. Electron in normal state, on the lower orbital
  2. Energy imposed on atom makes electron move to outer orbital
  3. As it drops back to lower orbital, energy is released

Fig 2

 

Figure 2. Image show how an atom emits light. From:  www.explainthatstuff.com &

http://www.explainthatstuff.com/quantum-dots.html

Some atoms emit electromagnetic energy within the light range when excited; this excitement can be caused by a variety of mechanisms, such as heat, chemical reaction, electronic current, or electromagnetic radiation.

The below video explains the basic principles of how light is emitted from an atom. 

The measurement unit for luminance is candela per square metre (cd/m2)

Innovation

Below you will find information/links to lastest research or product development; 

Swicofil supply a range of yarns and fibres for Luminescence? or glow yarns.

Phillips partnered with Kvadrat Soft Cells to product textile panels that absorb sound and produce luminous displays. 

Nanotechnology;  Toward Optoelectronic textiles, reviews current methods of producing optoelectronic textiles, including the electro spinning of luminescent fibres.

Most recently electroluminescence yarns have been explored for creating localised Luminescence? in automotives. Development and analysis of novel electroluminescent yarns and fabrics for localised automotive interior illumination