1.1 through spectrophotometer. Color difference formula CMC will

1.1    
 BACKGROUND

Towel
industry is one of the most emerging industries of Pakistan. Pakistan is
exporting huge number of product worldwide and has number of foreign customers.
A towel is an absorbent fabric which draws moisture when it comes in contact
with a wet surface. It has piles on its surface which increases the absorbency
of it as compared to other fabrics. Terry product is used globally. A terry
towel is manufactured with loop pile on one side or both sides generally
covering the whole surface. Terry towels are manufactured with different pile
heights according to their end use.   

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In
Pakistan, color difference of dyed towel is visually evaluated only. Most industries
rely on visual assessment and the consignments are approved on this basis only.
The technology is not used properly, hence making the instrument useless. Towel
is a textured fabric as it has piles on its surface, so when color difference
is evaluated on instrument i.e. spectrophotometer, results extracted are not in
the tolerance range. The color difference evaluation varies from observer to
observer, as one evaluates with piles at centre and some evaluates with piles
at extreme left or extreme right on instrument. Color difference evaluated in
each case will be different.

It
is needed to make the use of technology prevail in our industry. So that the
color difference can easily be evaluated on the instrument and gives
satisfactory results. As perception of color differ from human to human and we
cannot rely on visual assessment only.

1.2    
 AIMS AND
OBJECTIVES

The aim of the project is to improve the correlation between
the visual to instrumental color difference evaluation so as if a colorist is
passing a batch on the basis of visual assessment, the instrument also gives
the result within the tolerance limit irrelevant of the pile height.

In order to achieve this, batches are prepared of different
pile height (i.e. 4mm, 5mm and 6mm) against the standard pile height of 3mm.
The batch standard pair will be dyed in three different hues with shade depth
of 1% and 2.5%. The standard batch pair will be evaluated through
spectrophotometer. Color difference formula CMC will be used to calculate ?E values. Visual
assessment will be performed using grey scale technique. The polynomial
equation will be used to convert the grey scale readings to ?V scale.

1.3    
 REPORT
OUTLINE

Chapter 1 defines the aims and objectives for the project. Background
includes the problem faced by industry in evaluation of color difference of
terry towel.

Chapter 2 formulates basic understanding to achieve the aims
and objective of the project. It includes fundamental definitions of color
measurement, difference between the CMC and CIEDE 2000 color evaluation formula
along with detail discussion about spectrophotometer.

Chapter 3 formulates basic understanding about the
manufacturing of towel and other fundamental definitions.

 

 

 

 

 

 

 

 

 

 

 

 

Chapter 2        
COLOR EVALUATION SYSTEM

2.1    
COLOR ATTRIBUTES

Color is a sensation produced by the eye in the mind of a
human. The perceived color is a result of interaction of a light source, an
object and an observer. The perception of color differs from person to person.
The following perceptual attributes help us to understand a color.

2.1.1    hue

It is one of the main properties of color perception. It is
the degree by which stimuli can be described. It is described as a pure color
like red, yellow, green, blue etc. it can be quantitatively represented by a
single number. It is also one the three dimensional property in some color
space.

2.1.2    chroma

Chroma is defined as the quality of a color’s purity,
intensity or saturation. It is the perceived strength of a surface color. It is
also termed as “colorfulness” of an object relative to the brightness of white
object similarly illuminated.

2.1.3    lightness

Lightness is technically defined as the perceived brightness
of an object. It is normally measured on a scale of 0-100.

2.1.4    brightness

Brightness is an attribute of visual sensation according to
which a surface appear to exhibit more or less light.

2.2    
color difference formula

To measure a color perceived in a fabric there are number of
formula developed for this purpose. Color difference formula is important tool
for measuring the color difference of a standard batch pair. It predicts color
difference based on the stimuli difference therefore they are the tool used for
quality control.

Many color difference equations have been proposed since the
development of the CIE system in 1931. For the accuracy of the formula many
samples are tested. Their color difference is measured by recording the
required tri stimulus values and calculating the ?E for that particular equation. Then these ?E values are
compared with the visual assessment result involving number of observers and
calculation their ?V. An ideal color difference formula allows the pass/fail
decision on a single numbered result.

However,
no formula has given a uniform result but the amendments have been made to
these formulas to minimize the inaccuracy. The CMC (l:c) formula was developed
which gave best results except for the shades in saturated blue region. This
problem was overcome in the CIEDE 2000 formula.

2.2.1   
CMC (l:C)

CMC color difference formula was developed in 1984 by the
members of the Color Measurement Committee of the SDC. It is the modified form
of JPC formula. It currently is the ISO standard for the textile industry. It
was noticed that the JPC formula was not sufficient enough to give the best
result regarding the chroma and lightness of the color. It was noticed that
different weighing factors are needed for different industrial sectors. It was
proposed that for acceptability of color difference for textile l and c should
be 2 and 1, respectively.

The l (lightness weight) and c (chroma weight) were included
to allow different weights to be used according to the circumstances. The two
anomalies were corrected in this formula

Ø 
Discontinuity of chromatic differences close to
the achromatic axis.

Ø 
Over prediction of lightness differences close
to black.

The color difference (?E) formula proposed in the CMC meeting is (1)

?ECMC
=                              
Equation 2.1

Where,

?L = L2-L1                                                                                                                 Equation
2.2

?C = C2-C1                                                                        
                                       Equation 2.3

?H = H2-H1                                                                                      Equation 2.4

SL =                                                        
 Equation
2.5

Unless
L1