1. The document discusses the optical properties of fibers including refractive index, birefringence, absorption, dichroism, reflection, and luster. 2. These properties are determined by how light interacts with the fiber structure when it is transmitted, absorbed, or reflected. They provide information about the fiber's molecular orientation and structure. 3. The refractive index, birefringence, dichroic ratio, and luster of different fibers vary based on factors like density, moisture content, surface smoothness, and molecular orientation. Optical properties are useful for characterizing fibers.

1. first year Masters program in Fiber science and
technology
on optical
properties of fiber
By:belete Baye
ID:1108713
December,2018
EiTEX
Bahir dar University
Bahir Dar University
Ethiopian Institute of Textile and Fashion Technology

2. OUTLINES
Introduction
Refractive index
BIREFRINGENCE
Absorption and dichroism
Reflection and luster
Conclusion
References

3. When light falls on a fiber, it will be
partly transmitted, absorbed or reflected depending on the
structure of the fiber and the wave length of the light.
These determine the visual appearance of an individual fiber.
are source of information about structure of fiber:
In particular, the orientation of the polymer molecules
can be estimated from differences in the refractive indices and
in the absorption of light polarized in different directions
relative to the fiber axis.

4. POLARIZATION AND LIGHT
Polarization refers to the separation of charge, either momentarily or
permanently.
Polarization Nature of light
The process of transforming unpolarized light into polarized light is known as
polarization. Polarized light has its E field in one direction

5. EMW causes eclectic polarization and Change of
velocity of light
eclectic polarization is Large when E is parallel to
the joining atoms
But in simple materials (fibers); molecules are in
random fashion, it is Summation of the
polarizabilities of bonds present in the fiber
molecules.

7. Double refraction
nature
Incident ray
When light passes through isotrops, it splits in two rays
called ordinary(O) and extraordinary rays(E)
This lead to the variation in refractive index with the
direction of vibration

8. Absorb
ed ray
REFRACTION
REFLECTION
ABSORPTION

9. •Refractive index
•Absorbance
•Reflectivity
•Optical density
•Transmittance
•Color
•Photosensitivity
•UV visibility
•IR & NIR visibility
•Radar visibility
The main Optical Properties of Fiber include

10.  ratio of the velocity of light in a to the velocity of
light in the .
various interference phenomena occur
Varies with temperature and wave length of light and with
direction of electric field
Refractive index
Refractive index [n] =sine of angle of incidence
sine of angle of refraction
n= sin /sin

11. Refractive index is affected by the fiber
density; and moisture regain
For many fibers, as the density of fibers
increase(number of molecules present
increase),
n will also increase.
Principal refractive
indices of a fiber
(n-1)/ρ =constant=k
n-1= k
n=k+1

12. It decrease with increase in moisture
regain
n of cellulose with regain
Where
V0 = volume of 1 gm of dry fiber
VMR = volume of sample fiber at a frictional
moisture reign of MR
n0 = refractive index of dry fiber
nMR= refractive index of moisture fiber
vMR(nMR-1) = v0(n0-1) + 0.333MR
nMR-1=[v0(n0-1) + 0.333MR]/vMR
nMR=[v0(n0-1) + 0.333MR]/vMR +1

13. Fiber Birefringence
 the difference of
refractive index parallel to
the fiber axis and index at
right angles to the fiber
axis
 phenomenon of double
reflection
 It depends on degree of
orientation and degree of
asymmetry of the
molecules
Increase with degree of orientation
of molecules
optical orientation factor f: the
ratio of the birefringence of the
fiber to an ideal fiber in which
the molecules are perfectly
oriented parallel to the fiber
axis.
Δn=0; for random arrangement,
Δn=1; for perfectly oriented
(Δn) = nιι-n⊥

14. Expressed in terms of average angle
of inclination of the molecules
f = 1 – 3/2 * Sin2 φ
Isotropic material
f = 0, Sin2 φ = 2/3, φ ≈ 55°,
Chain conformation in a fiber
will be
Zigzag, Extended, Helical
Side groups form atomic bonds
perpendicular to main chain axes
Variationofnandϕwithf

15. When fibers absorb a large amount of
moisture, the birefringence become
greater
• For dry fibers, the difference
between the refractive indices of
the crystalline and amorphous
phases is relatively small
• in wet fibers, the refractive index
difference between the crystalline
and amorphous phases become
larger
Birefringence is used to study molecular
orientation
refractive indices, in a fiber
with spiral orientation of
chain molecules.
1/4/2019 15optical property

16. Refractive indices and birefringence of some fibers

17. refractive indices and birefringence of polypropylene
fibers prepared with different draw ratios.

18. Refractive index of the different species of cotton

19. ABSORPTION AND DICHROISM
• the absorption of light is the result of the interaction between
the electro-magnetic waves and the electron clouds in the fiber.
The color of a fiber is determined by the selective absorption of
light wave lengths (or frequencies).
• Color is a result of selective absorption of light
=dichroic ratio
Where
I =intensity of light after passing for a
distance d through a fiber,
Io= the intensity of the incident light,
k= the absorption coefficient.
Lambert’s law of absorption

20. Values of dichroitic
constant vary from unity in
an isotropic material to
infinity in a perfectly
oriented fiber.
Example:Dichroic constant
for ramie is 9 & for viscose
rayon is 1.4 – 3.33.
The dichroic ratio,θ,
is used to assess the
orientation in fibers.
In general, the
dichroic ratio increases
with increasing
orientation in fibers.
cross-sectional of
fiber absorbs radiation
than parallel cross-
sectional of a fiber

21. Reflection and Luster
important aesthetic property
• When light falls on the fiber, part of light is reflected.
• The reflaction may be

22. • When the light is incident along the longitudinal direction,
reflected at a constant angle as long as the fiber surface is
sufficiently smooth.
• However, when the light falls across the fiber on the
transverse direction, it is reflected at various angles. but,
many fibers don't have smooth surface or circular cross-
section.
• Fibers with Rough surface and irregular cross sectional
shapes increase the diffuse reflection.
Reflection and Luster

23. incident
Transverse
direction.
incident longitudinal
direction,
The reflection of flight is affected by how the incident ray act
upon the fiber.
The luster of a single fiber is determined by total visual
appearance of these reflections from the fiber surface.

24. Lustrous fibers are those with smooth surface and regular cross-
sectional shape, such as silk and some synthetic polymer fibers.
Primary and secondary reflections

25. Secondary reflection on fibers with
i. hollow tunnel,
ii. cavities,
iii. par-ticulatefillers.
Hollow tunnels, cavities and particulate fillers scatter the
transmitted light and cause apparent diffuse reflection.

26. Factor affecting on luster:
❶ Incident angle of light
❷ fiber Finess
❸ Irregularity of the fiber surface
❹ Cross-section of fiber
❺ Presence of TiO2 or Cavities
Presence of TiO2 or
cavities will scatter the
transmitted light and
cause apparent diffuse
reflection

27. Lustre of a range of cottons

28. Conclusion
Optical property of fibers always affect to the visual
apperance of fibers and psychological impacts on human
beings.
it is always expressed when light on a fiber is absorbed,
reflected, or refracted and expressed in terms of Dichroism,
lustur and refractive index.
Refractive index of fiber affected by density, moisture, and
orientation of fibere molecules.
Optical properties of fibers provide a convenient measure of
many structural characteristics, especially the molecular
orientation of fiber physics and study, it is the very important
parameter.