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 < 16 when SL = 0.511 SC =                                                        Equation 2.6 SH = SC (Tf + 1- f)                                                   Equation 2.7 f =                                                                 Equation 2.8 T =                                                 Equation 2.9 Unless h1 is between 164° and 345° When, T = 0.56 +                                     Equation 2.10       L1, C1 and h1 refer to the 'standard' of a pair of samples. ?L, ?C and ?H are the differences of lightness, chroma and hue of the standard batch pair respectively. SL, SC and SH are the weighing factors for lightness, chroma and hue respectively. It indicates the length of the semi axes of the ellipsoid defining unit ?E.     The CMC tolerance system describes hue difference moving around the circumference and chroma as the vector that moves from the centre to the edges. The value of lightness, chroma and hue creates an ellipsoid of the standard in a color space. The color that falls inside this ellipsoid is acceptable, while the color that falls outside the ellipsoid is rejected. This formula more closely aligns with how the human eye sees color. 2.2.2    ciede 2000 CIEDE 2000 is the current standard by CIE. It was introduced in 2001 by the members of the CIE technical committee. The problem that rose in previous formula of CIE and CMC was the poor result in the saturated blue region. All the ellipses were pointing away from the neutral, ellipses for all other colors were generally pointing towards the neutral. Human eye cannot differentiate some colors if they are widely different. The area of such colors on chromaticity diagram is called color discrimination threshold of human eye. In other words, human eye cannot differentiate color difference with in the same ellipse. The sensitivity to the color difference is low with the high saturation. The shape of the ellipses apparently becomes close to the circle with low saturation, and narrower in the direction of hue and longer in the direction of chroma with high saturation. Therefore human eye cannot differentiate color difference for high saturation whether their color difference is high. (2) figure 2.2.2 Visually acceptable ellipses The CIEDE2000 formula includes five corrections to the CIELAB. Ø  A lightness weighting function (SL) was included Ø  A chroma weighting function (SC) was included Ø  A hue weighting function (SH) was included Ø  An interactive term (RT) between chroma and hue differences for improving the performance for blue colors Ø  A factor (1+G) for rescaling the CIELAB a* scale for improving the performance for grey colors. The CMC formula included the first three terms. The CIEDE2000 color difference formula is not an attempt to build a color space in which the width of the color discrimination thresholds of the human eye is uniform. Instead, it defines a calculation so that the color difference calculated by the color meter become close to the color discrimination threshold of the human eye to the solid color on the L*a*b color space. The color difference ?E proposed by CIE is (1) ?E00 =              Equation 2.11 Where, SL =                                                                       Equation 2.12 SC =                                                                                              Equation 2.13 SH =                                                                                           Equation 2.14 T =                                                                                                                                   Equation 2.15 RT =                                                                                          Equation 2.16 L' = L*                                                                                                                       Equation 2.17 a' = (1+G) a*                                                                                                    Equation 2.18 b' = b*                                                                                                              Equation 2.19 C' =                                                                                                 Equation 2.20 G =                                                                                  Equation 2.21 SL, SC and SH means the same as in CMC, they are the weighting factors for the lightness, chroma and hue respectively. A new factor RT was introduced to improve the performance of the color difference equation in the saturated blue region, as discussed earlier. This formula has proved to give the best result in the saturated blue region and closely matches the perception of the human eye so far.  2.3         visual assessment Traditionally, assessments were carried out by visual method under the natural daylight. Daylight is highly variable depending upon the geographical location, time of the day, time of the year and the climatic condition. To overcome this issue light cabinet was developed which consist of multiple standard illuminant so that the sample can be viewed in various conditions. (3) It provides a standard procedure for evaluating a color difference by placing batch side by side with a standard. Standard batch pair is evaluated in a light box at 0/45 illuminating/viewing geometry. To examine the color perceived visual assessment is carried out by a panel of number of observers. This panel consists of both male and female in order to achieve best results because the color observing natures of both genders are different. The visual assessment is performed using common grayscale method. The specifications are according to the ISO A02 grayscale for assessing the color change. The color difference is evaluated by comparison of gray standards with the batch and the amount of contrast in color is rated from 1-5, 1 being the most color change to 5 being the least color change. Observers are asked to examine the standard batch pair and then rate their color difference according to the grayscale. They can also rate in decimals, as if they feel that the difference is between two whole numbers they can rate in the value of (0.5).   figure 2.2.3 gray scale for color change 2.4     instrumental assessment Instrument was developed to eliminate the human error during assessment of color difference of the sample. Instrument allowed the reproducibility of the color difference of the same standard batch pair. Spectrophotometer is used widely to evaluate color difference. 2.4.1    spectrophotometer Spectrophotometer is commonly used for the measurement of transmittance or reflectance of solution and opaque solids. It measures the ratio of reflected to incident light from a sample at many points across the visible spectrum. The instrument operates by passing a beam of light through a sample. It sends electromagnetic radiation into a target and measure the resulting interaction of the energy and the target. The reflected or transmitted light is then passed onto a spectral analyzer where the light is split into its spectral components. This allows the light detector and control electronic to make measurement at any point across the visible spectrum. The spectrophotometer used for color measurement is Data color SF650 spectrophotometer. 2.4.1.1   working The basic function of a spectrometer is to take in light, break it into its spectral components, digitize the signal as a function of wavelength, and read it out and display it through a computer. The first step in this process is to direct light through a fiber optic cable into the spectrometer through a narrow aperture known as an entrance slit. The slit vignettes the light as it enters the spectrometer. In most spectrometers, the divergent light is then collimated by a concave mirror and directed onto a grating. The grating then disperses the spectral components of the light at slightly varying angles, which is then focused by a second concave mirror and imaged onto the detector. Alternatively, a concave holographic grating can be used to perform all three of these functions simultaneously. This alternative has various advantages and disadvantages, which will be discussed in more detail later on. Once the light is imaged onto the detector the photons are then converted into electrons which are digitized and read out through a USB (or serial port) to a computer. The software then interpolates the signal based on the number of pixels in the detector and the linear dispersion of the diffraction grating to create a calibration that enables the data to be plotted as a function of wavelength over the given spectral range. This data can then be used and manipulated for countless spectroscopic applications.     Block Diagram figure 2.5 block diagram of spectrophotometer The Datacolor 650 includes an option to measure the transmission properties of transparent and translucent samples. It can measure the "regular" transmittance of transparent solid and liquid samples, as well as the "total" and "diffuse" transmittance of translucent samples. 2.4.1.2   Accessories All models come with the following standard accessories: ·         Six foot power cable ·         Serial cable with connectors on either end ·         Black Trap ·         White Tile ·         Green Tile ·         USB Cable Calibration Tiles A black trap, white tile and green tile are provided with all instruments: ·         The black trap and white tile are used each time the instrument is calibrated. ·         The green tile is used to perform an optional diagnostic test. ·         A CD and diskette containing the calibration values for the white tile are also provided. Aperture Plates Multiple aperture plates with openings of different sizes are included as standard accessories with all models. The identification is engraved on each plate. figure 2.6 calibration tiles Standard Aperture Plates The apertures listed below are provided with every model: ·         Large Area View (LAV) ·         Ultra-Small Area View (USAV) Optional Aperture Plates ·         Medium Area View (MAV) ·         Small Area View (SAV) ·         Extra Ultra Small Area View (XUSAV) ·         Medium Area View (MAV), barium coated (standard with the Datacolor 650)