Optical Properties
Mousumi Rahaman Hashi Senior Lecturer
Dept. of Textile Engineering
Faculty of Engineering
Optical Properties
The Optical properties of fibres are:
1. Birefringence : Basis on refraction/
Transmission
2. Dichroism : Basis on absorption.
3. Lusture : Basis on reflection.
Birefrengence
When a beam of light falls on textile materials/fibres, it splits up into two refracted beam. One polarized parallel to the fibre axis and other polarized perpendicular to the fibre axis.
The difference between the refractive index for light, polarized parallel to the fibre axis and the refractive index for light, polarized perpendicular to the fibre axis is called Birefringence value of the fibre.
Let, nII and nL are the refractive index for light, polarized parallel to the fibre axis and perpendicular to the fibre axis respectively.
So, the Birefringence value= nII - nL
Greatest values of Birefringence indicate the most of molecules are lined up parallel to the fibre axis. So, orientation is high.
Dichorism
The variation in the absorption of radiation by a colored fabric (Pre dyeing) with direction of polarization of light is called Dichroism, which may result in the difference of depth of shade or even in the actual colour.
For dyed fibre exhibiting Dichroism, its magnitude is used as a measure of orientation of the molecules in the fibre. We get, Φ=KII/KL.
KII= Absorption of co-efficient for light polarized parallel to the fibre axis.
KL= Absorption of co-efficient for light polarized perpendicular to the fibre axis.
Φ= Dichric/ Dichroitic constant
Requirement for Dichorism
1. The dye molecule must be symmetrical so that its absorption of radiation varies with direction of the electric field.
2. The dye molecule must be absorbed into the fibre molecule in a particular direction so that all the dye molecules make the same angle with the axis of the chain molecules.
3. The chain molecule must be preferentially oriented.
LUSTURE
Lusture is an important property of textile fibres. If a beam of light fall onto a fibre surface. The reflection may vary with the angle of incidence and with the colour and polarized of light. Lusture depends on the following factors:
1. Whether the light falls across the fibre or along the fibre:
If a fibre behaved as a perfect reflecting circular cylinder,
it would reflect light. It is clear that if the light falls across
the fibre, then it will be i≠r and it is reflected at various
angles, where as if it falls along the fibre, it is reflected at
a constant angle.
LUSTURE
2. Whether fibre is finer or coarser:
For the same types of smoothness and regularities, the coarser fibre will have more lusture than the finer one.
3. Irregularities of fibre surface:
Irregularities of surface and cross sectional shape will cause light to be reflected in various direction and will reduce the lusture. It is essential that the fibre should be uniform along its length. For this reason, lusture id greater in regular filament such as silk and manmade fibre. Lusture must be more in smooth surface than in rough surface.
LUSTURE
4. Cross sectional shape:
Fibre shape is an important factor for lusture. The particular types of lusture associated with Nylon, Rayon, and Silk. It must be partly due to pattern of light reflection by their respective circular, serrated and triangular shapes.
Circular- More lusture, Serrated- Dull Lusture, Triangular- Less lusture.
5. Whether the fibre contains small particles like TiO2:
When a beam of light falls on a fibre it is not only reflected but also is transmitted. Some of thin transmitted light will be reflected from the internal surface. If the fibre contains small particles like TiO2, this will scattered, transmitted light at varying angles and cause it to immerse as apparently diffuse reflection.
LUSTURE
6. Maturity of Fibres:
If fibres maturity is high, then the fibres cross section will be near about circular. So, the reflection of the fibre will be better and lusture will be higher.
