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Fluorescence

Fluorescence? and Phosphorescence? are forms of photoLuminescence?/" style="text-align: justify; ">Luminescence?.

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, Reflectionhttp://missionscience.nasa.gov/ems/03_behaviors.html 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. 

Fig 1

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.