**1. Introduction**

UV VIS Absorption spectrophotometer (applicable for coloured liquid) and UV VIS reflectance spectrophotometer (applicable for flat samples of coloured solids) are the two major equipment now being used in colorimetric evaluation related labs for textiles and other industries. Colour measurement of liquid dye solution or colorimetric titrations is known with the advent of UV Vis Absorption spectrophotometer. Colour measurement of solid substances was quantified by CIE internationally in 1923, which was further revised in 1976 and is continuing [1–4]. Colour

matching theory was made commercially applicable in 1950s. Till then, so many varied applications of colorimetric evaluations have made precision process control and product control possible for coloured textiles.

## **2. Principles of colorimetric/spectrophotometric evaluation of coloured substance using UV: VIS absorbance spectrophotometer**

**Beer's Law** states that the amount of light absorbed is directly proportional to the concentration of the colored solute in the solution.

$$\text{Log}\_{10}\frac{\text{I}\_0}{\text{I}\_t} = \text{εc}\tag{i}$$

where, **Ɛ** = proportionality constant and c = concentration of the solute in solution.

**Lambert's Law** states that the amount of light absorbed is directly proportional to the length and thickness of the solution (thorough which light is passed through) under analysis.

$$\mathbf{L} \mathbf{Q}\_{10} \frac{\mathbf{I}\_0}{\mathbf{I}\_t} = \varepsilon \mathbf{b} \tag{ii}$$

where **Ɛ** = proportionality constant and b = length/thickness of the quartz cell in which solution is tested.

So, combining these two laws, called Beer-Lambert Law [3, 4]:

$$\text{A(Absortivity or Absolute)} = \text{Log}\_{10} \frac{\text{I}\_0}{\text{I}\_t} = \text{ebc} \tag{iii}$$

where *I*<sup>0</sup> = intensity of the incident light, *It* = intensity of the transmitted light, *c* = the concentration of absorbing substance/solute in the solution, *b* = the path length/the distance the light is passing through the absorbing solution, and *A* = absorptivity/Absorbance and **Ɛ** = proportionality constant dependent upon the absorbing substance, the wavelength of light used, and the units used to specify *c* and *b.*

In simple colorimetry, the entire visible spectrum (white light) is used to pass through the solution and consequently the complementary colour of the one absorbed, is observed as transmitted light. In UV VIS absorbance Spectrophotometer, a monochromatic light or a narrow band of light radiation is used, replaced the colorimeter and then this instrument is called Absorbance spectrophotometer or reflectance spectrophotometer, differentiating by measurement parameter i.e. measuring as absorbency or optical density of transmitted light intensity for colored solution.

Limitations and Cares for measuring absorbance/optical density parameter of liquids:


#### *Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

Example: Benzoic acid in Benzene solvent form dimer, i.e., aggregates as dimer, and Potassium dichromate on higher dilution, dichromate ions are ionizing or dissociating into chromate ions, which are the causes of deviation of correct reading in both these two cases.


While for Solid coloured samples, surface reflectance values are measured for any solid-coloured substance, the measurement parameter is reflectance (R values at different wavelengths user-chosen wavelength or preferably at maximum absorbance wavelength, i.e., λmax) and the instrument used for R values of solid coloured substance, is called UV-VIS Reflectance Spectro-photometer.

Thus, when it is required to measure colour from a solid dyed/printed surface, the measurement parameter is not absorbancevalues, but is Reflectance values (R), i.e., reflected light intensity from a solid surface of dyed textiles/coloured/coated polymeric film/plastics etc. RL Rs UV-VIS reflectance spectrophotometer is used having different viewing angle with setting facility of measuring specular reflectance or diffused reflectance, with UV-in (On) and out (Off), with large viewing angle or small viewing angle, with D65 or other standard illuminant light ambience etc. having options of changed testing parameters. Both these spectrophotometers are not limited to the visible spectrum only and are often employed to make measurements in the ultraviolet and infrared regions too. So presence of any colored chemical agents/dyes/pigments can be thus calorimetrically or spectrophotometrically identified by absorbance spectrophotometer and can be quantitatively estimated frequently using a dilute solution at concentrations smaller than one part of the constituent in several hundred million parts of a selected coloured solution of specific solute. While by UV-VIS Reflectance Spectrophotometer, K/S values (i.e., surface colour strength) of any opaque coloured substance can be determined easily by Kubelka Munk Equation [2, 3] from the measured reflectance values at different wavelength by measuring intensity of reflected diffused light beam or intensity of specular reflected light beam.

Besides surface colour strength, the colour difference and other colour interaction parameters [2, 3, 5, 6] like Total colour differences (DE), Lightness/Darkness (DL\*), Red-ness/Green-ness (Da\*), Yellowness/Blueness (Db\*), Changes in Chroma (DC) and Changes in hue (DH) can be calculated by CIE formulae [1–4]. Also, non-coloured surface appearance properties of any flat sample including

textile fabrics can be determined easily in terms of whiteness index, yellowness index, and brightness index values using appropriate and respective formulae of CIE/ASTM or other standards [1–3, 5, 6] to compare any changes in its surface texture for any chemical treatment or physical intervention on the sample, which is very useful for industry.

Limitations and Cares for measuring Reflectance/Surface Colour parameters of solids:


*Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

> practical purposes, if non-linear curve appears in the plots between K/S values vs. concentration of dyes, it is needed to re-dye to recheck linearity or to modify the curve by simply eliminating data of one or two erroneous dyed samples to make it linear for precision colour measuring and match prediction. Also, some mordanted textile samples dyed with a variety of natural dyes, do not show linearity for plots between K/S values vs. concentration of dyes.


### **3. Colourimetric evaluations for process/product control of dyed textiles**

Different types of selective colorimetric evaluation methods are described one by one in brief with examples/case studies with experimental data below mentioning the importance of each method.

#### **3.1 Colorimetric identification and estimation of purity/concentrations of any colorants/dyes/pigments using UV: VIS absorbance spectrophotometer**

An optical UV-VIS absorbance spectrophotometer records the absorbance values at different wavelength range at which absorption occurs, together with the degree of absorption at each wavelength and thus a pictorial curve of wavelength (X-axis) vs. Absorbance (Y-axis) called UV-VIS spectrum of that solute from its very dilute solution (preferably 1/100th dilution). The resulting UV VIS spectrum is presented as a graph of absorbance (A) versus wavelength showing maxima (λmax) and minima (λmin) of absorbance at different wavelength in both UV and visible region.

Solute molecules absorb ultraviolet or visible light from a monochromatic beam of incident light beam and the rest are transmitted through the solutions of fixed path length (b or d) in a cuvvete/quartz cell holding the sample solution. As optical density or absorbance is directly proportional to the Path length, *b*, and the concentration, *c*, of the solute/absorbing species of the coloured solution, *Beer's -Lambert Law stands here as*, *A =* **Ɛ***bc*, (where **Ɛ** is a constant of Proportionality, called the *absorbtivity Constant) and is Optical density/Absorbance*.

Different solute molecules absorb UV-VIS light/radiation of different wavelengths depending on its chemical nature and structure with or without interference, if any, as depicted by corresponding absorption spectrum showing absorption peaks and troughs/bands according to the chemical structural groups present in the respective solute molecules present in the coloured dilute solution. Thus, the UV-VIS spectral scan (For absorption or optical density) of a particular-coloured compound/dyes/pigment at a particular wavelength (at λmax) is deterministic and identifiable instrumentally, which is the basis of the identification and estimation of purity/concentrations of any colourants/dyes/pigments by UV VIS spectrophotometric (Absorbance) evaluation.

CASE STUDY 1: As a case study, UV–VIS Absorbance spectrophotometric method of determination of purity and concentrations of rubia/madder as a natural colorant is discussed:

Calibration curve (**Figure 1**) is prepared by using 1,2,3,4,5,6 to maximum of 10 mg of natural *Rubia*/madder (Madder or Manjistha) containing manjishthin and purpurin as natural colourant powder per ml of methanol, after extracted in aqueous solution and purified by Soxhlet extraction under methanol. The solutions were filtered through Whatman filter paper-40 and then used for UV Calibration method and the absorbance was measured at 426 or 430 nm as λmax.

Once the calibration curve is ready in UV VIS absorbance spectrophotometer screen or manual graph paper, the unknown solution of the same compound having unknown quantity of solution is placed in UV VIS scanning taking 10 ml of sample solution of unknown concentration, diluted to known times i.e. say 50 to 100 times until a very fent colour appears in the solution and from that dilute solution 2–3 cc is poured in quartz cell of sample solution and mounted in UV-VIS absorbance Spectro, to measure its Optical density/absorptivity values. Once the absorptivity/ Optical density values of sample of unknown concentration are obtained, the

*Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

**Figure 1.** *Calibration curve for the* Rubia *dye natural colorant for determining concentration of a solution of unknown concentration of colour component extracted from Rubia/Madder.*

concentration can now be easily obtained by putting measured absorbance or OD (optical density) values in calibration curve of **Figure 1**, to find the Concentration/ purity of the content of rubia/madder coloured component in it in specific unit, after correcting the value with dilution factor and converted into proper unit like % or g/lit etc. as per requirement. .

#### **3.2 Identification of colorants/dyes/pigments powder or from its dyed textiles by using UV: VIS absorbance spectrophotometer**

CASE STUDY 2: Identification of Natural dye Madder/*Rubia* as a natural colorant is discussed below and is also compared with the synthetic Alizarin coloured compound to distinguish them by UV-VIS Spectrophotometric evaluation method.

The two red dyes– [(i) Natural Rubia (Manjishtha/Madder) which contains manjisthin (similar to alizarin as coloured compounds) in its natural extract and (ii) synthetic alizarin coloured compounds as synthetic same coloured dye] were weighed separately (0.1 gm) and dissolved in 1000 ml dichloromethane/methanol and then wavelength scan under UV-Visible absorbance spectrophotometer was taken for both. For visible spectral analysis, this solution may be used, but for UV spectral analysis this solution needs to be further diluted by 510 times for better results. Comparative Identification of Synthetic alizarin and madder (Rubia) as natural colourant/dye, by this UV VIS spectral analysis, involves a comparison of the minute details of UV–VIS peaks/bands of UV VIS spectrum (at λmax) of *Rubia*/ Madder as natural colourant and synthetic alizarin red colour. Such a Comparative spectral analysis of both with corresponding UV-VIS peaks is compared in **Figure 2**. The details method of identification of rubia/madder as natural colourant is available in new IS standard**s 17,085:2019** [9] in Annexure-E as one of the confirmatory tests.

Thus from **Figure 2** Comparative analysis of UV-Vis Spectrum of Natural Rubia/ Madder extract and Synthetic alizarin (as shown in **Figure 2** indicate that Natural *Rubia/Madder colourant shows uv–vis peaks at* 250 nm (with 0.954 OD) and at 491 nm (with 0.171 OD), while Synthetic Red alizarin shows *uv–vis peaks at* 250 nm (with 1.38 OD) and 426 nm (with 0.309 OD). This different OD at 250 nm in UV zone and Peaks in Visible Zone at two different wavelengths (at 491 nm with 0.171

**Figure 2.** *UV spectrum of* Rubia *and Alizarin dyes: [Source -IS standard- 17,085: 2019 [9]].*


#### **Table 1.**

*UV VIS spectral peaks analysis of natural* Rubia *colorant and synthetic alizarin.*

OD for Rubia/madder natural colourant and at 426 nm with 0.309 OD) are identifying factors confirming presence or absence of them, as shown **Figure 2** and **Table 1**.

Individual UV VIS absorbance Spectrum at visible region only at 390–700 nm, when is partly enlarged for 390–450 nm, it is also observed that the UV–Vis absorbance spectrum of aqueous solution of natural Rubia/Madder extract (extract of Indian Madder i.e., natural manjishthin) also shows small hump like peaks at 398 nm (with 0.801 OD) and also indicating large hump like peak at also 426 nm (with 0.838 OD), which are vivid from **Figure 3**. Therefore, the appearance of the above said two respective peaks in the said wavelength region lead to indicate the presence of natural Rubia/Madder with manjisthin (not synthetic alizarin), which is more clearly understandable in the enlarged peak of highlighted part of UV–VIS Spectrum (**Figure 3**) of extracted solution of *Rubia-cordifolia (used as Natural dye) dyed cotton textiles. Thus, even if Rubia/Madder shows peak at 426430 nm showing* λmax at around 426 nm (with 0.838 OD) i.e., at the same wavelength where synthetic alizarin has also shown its peak at 426 nm (with 0.309 OD), but OD values are different and thus these two red dyes are easily distinguishable by this method.

*Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

**Figure 3.** *Part of enlarged UV-Vis Spectrum of Natural Rubia/Madder colorant (containing manjisthin).*

#### *3.2.1 Confirmation of identification of natural* RUBIA*/madder as compared to synthetic alizarin from UV: VIS spectral scan analysis*

Optical density/absorbance at λmax for extract of natural *Rubia/Madder* colourant and synthetic alizarin dyes are thus found to be quite different from UV VIS Spectral analysis. In the UV–VIS spectrum, although the peaks are at 250 nm, they have shown different optical densities/absorbance values (**Figure 2** and **Table 1**). In the visible region, peaks at 426 nm for alizarin and peaks at 398 nm (0.801) and 426 nm (0.838) for Natural *Rubia*/madder dye-containing manjishthin (a natural coloured compound similar to alizarin, but bio synthesised as natural colorant in Madder/Rubia plant in association with other natural ingredients) are the characteristic peaks of differentiating alizarin and manjisthin (from natural rubia/madder). There are other different methods available for identification of natural Madder/Rubia colourant by either HPLC-DAD analysis or LC–Ms/UPLC-MS analysis or also by FTIR analysis and NMR analysis, some of which are detailed in IS standard**s-17,085:2019** [9] and International ISO Standard: **ISO/Standards-22,195-1-2019** [10] published in recent past in 2019.

#### **3.3 Colour quantification and measurement of surface colour strength (K/S values) of any dyed textiles using UV–VIS**-**reflectance spectrophotometer**

Colour quantification in mathematical term is necessary to develop a systematic understanding of the principles of colour perception and measurement for understanding the differences between colours of two samples i.e., match and mismatch for any method of colour encoding/imaging and communications, to give a more realistic picture for colour reproduction. Hence, TRISTIMULUS VALUES (X, Y, Z) are defined as three coordinates to define any colour for communications, where X, Y and Z values are as follows:

Thus, Tristimulus values X, Y, Z can be calculated from measured total reflectance of a textile or similar flat surface with its derivative formula function as shown below [2–3, 6–8]:

$$\mathbf{X} = \sum \mathbf{P}\_{\lambda} \mathbf{x}\_{\lambda} \mathbf{R}\_{\lambda} \tag{1}$$

$$\mathbf{Y} = \sum \mathbf{P}\_{\lambda} \mathbf{y}\_{\lambda} \mathbf{R}\_{\lambda} \tag{2}$$

$$\mathbf{Z} = \sum \mathbf{P}\_{\lambda} \mathbf{z}\_{\lambda} \mathbf{R}\_{\lambda} \tag{3}$$

where Pλ=Spectral power distribution of standard source, R<sup>λ</sup> <sup>=</sup> Spectral reflectance of substrate and xλ. yλ. zλ=colour coordinates/factor of standard observer for red, blue and green.

For ease of working, colours are redefined from TRISTIMULUS values to CIE Chromaticity coordinates (x, y and z instead of Capital X, Y, Z as tristimulus values), which can be plotted in two-dimensional plot. These new CIE chromatically coordinates (x, y, z) can be defined as follows.

$$\mathbf{x} = \frac{\mathbf{X}}{\mathbf{X} + \mathbf{Y} + \mathbf{Z}} \tag{4}$$

$$\mathbf{y} = \frac{\mathbf{y}}{\mathbf{X} + \mathbf{Y} + \mathbf{Z}} \tag{5}$$

$$\mathbf{z} = \frac{\mathbf{Z}}{\mathbf{X} + \mathbf{Y} + \mathbf{Z}} \tag{6}$$

and

$$\mathbf{x} + \mathbf{y} + \mathbf{z} = \mathbf{1} \tag{7}$$

From Eq. (22), i.e., x + y + z = 1, the value of anyone CIE chromaticity coordinate can be determined from the values of other two CIE chromaticity coordinates, i.e., the third one can be determined easily from first two.

Still, as Plot of Dye Concentrations Vs Reflectance (R) are non-linear and nonadditive, Tristimulus values X, Y, and Z are interdependent on one another, and CIE chromaticity coordinates are still two factors dependent variables to get third one. HUE, value chroma are also 3 coordinates based, it is difficult in practice to control all those multivariate/factors/colour parameters simultaneously to get a precision match of colour.

So, quantification of colour was finally made by Kubelka and Munk [2, 3, 6–8], where K/S value (surface colour strength) is defined as follows:

$$\text{Surface colour strength} \left(\frac{\text{K}}{\text{S}}\right) = \frac{\text{Co-efficient of absorption}}{\text{Co-efficient of scattering}} = \frac{\left(1 - \text{R}\_{\text{i}\text{max}}\right)^{2}}{2\text{R}\_{\text{i}\text{max}}} = \text{aC}\_{\text{D}}\tag{8}$$

where K is the coefficient of absorption; S, the coefficient of scattering; and Rʎmax, is the Reflectance value at maximum absorbance wavelength (λmax) and CD is the dye concentration and α is the constant. Moreover, K/S Vs Dye concentration plots are linear, and K/S is additive in nature.

For Additive nature of K/S value, for use of mixture of colourants/dyes at different concentrations c1, c2 and c3 respectively for dye1, dye2 and dye3 K/S values of resultant fabric may be written as:

$$(\mathbf{K}/\mathbf{S})\_{\text{MAX}} = (\mathbf{K}/\mathbf{S})\_{\text{subs}} + (\mathbf{K}/\mathbf{S})\_1 + (\mathbf{K}/\mathbf{S})\_2 + (\mathbf{K}/\mathbf{S})\_3 + \dots + \tag{9}$$

$$\mathbf{H} = (\mathbf{K}/\mathbf{S})\_{\text{subs}} + \lambda\_1 \mathbf{C}\_1 + \lambda\_2 \mathbf{C}\_2 + \lambda\_3 \mathbf{C}\_3 \tag{10}$$

Thus, handling of K/S values become much easy to match colour, as because K/S is treated as a single variable i.e., it operates on a single constant theory (scattering remaining constant for same fabric and dye sample) and K/S is directly proportional to dye concentration in linear and additive relationship.

#### *Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

For dyed textiles/clothes, it is pre-assumed that dyes on specific textile fabric do not add or substract, i.e., change scattering and K is the sum of absorption of dye stuff on dyed textiles and therefore, it is only dye absorption values of dyed textile substrate (if textile substrate remained unaltered/fixed). So, it may be considered that for dyed textiles, K/S directly varies with concentration of dyes linearly and scattering of dyed textile substrate is independent of dye concentration (which is not the case for pigments in paints for wall colours). So, in textile it is single constant theory of colourmatch prediction through K/S values, as most widely applicable colour parameter for colour quantification, measurement and colour matching of textiles. So, for the particular dyed textile sample (with same fibre material, yarn parameter and fabric construction/surface finish remain unaltered) scattering value is assumed to be constant.

Thus, higher is the K/S value, meant higher is the dye absorption in textiles, meant higher absorption value of dye thus signifying or indicating higher dye uptake, but this measurement is surface colour strength, not bulk dye uptake, which can only be determined by extraction of colour from dyed textile samples and then analysis of optical density or absorbance values in absorption spectrophotometric analysis of coloured liquid.

Dye Uniformity in terms of CV % of K/s values at minimum 10 different points may be expressed for deciding factor for level/unlevel dyeing. CV % of K/S values within 5% value is considered as acceptable for level dyeing and more than 5% values (CV % of K/S values) is considered as un-level dyeing leading to rejection of the sample.

#### **3.4 Measurement of colour differences by estimating DE, DL\*, Da\*, Db\*, DC and DH**

Colour attributes of a human perception consisting of any combination of chromatic and achromatic content in terms of differences in combination of red, blue and green sensation of human eye (as shown in **Figure 4**) alters change in predominating hue which can be described by chromatic hue names such as yellow, orange, brown, red, pink, green, blue, purple, etc., or by achromatic colour names such as white, grey, black, etc., and is associated with some other attributes like bright, light, dark etc., hence colour differences in between two samples arises by value of these attributes of human perception or instrumental measurements. Measurement of colour differences is important for judging two nearer coloured

**Figure 4.** *Red-Blue-Green perception of colour.*

samples as match with degree of matching or mismatch. It is Judged by differences in light and dark (ΔL\*), Redness or Greenness (Δa\*) and Blueness or yellowness (Δb\* ) as CIELab\* colour difference coordinates in CIE colour difference space diagram to determine the total colour difference values (in terms of Δ*E\**) from respective CIE-Lab equations following CIELab\* standard-1976, which are measurable in UV VIS Reflectance Spectrophotometer and Associated software attached to computer aided colour measurement and matching system.

Thus, according to CIE (Commission International de eclairase, Paris) 1976, total colour difference values (in terms of DE\* or Δ*E\**) as obtained from individual DL\* or ΔL\* (Light or dark), Da\* or Δa\* (Redness or greenness), and Db\* or Δb\* (Blueness and yellowness) values makes it easy to compare the colour difference values in between two nearer match samples (standards and produced) and this gives a degree of matching according to tolerances set for these attributes of colour as well as gives opportunity to correct shade by adding exactly required colour/dyes to improve less red, less green, less yellow or less blue sample to solve its light and dark by adding white or black as well during dyeing production.

The above said terms DE\* or Δ*E\* represent total colour difference,* DL\* or ΔL\* by 0–100 scale representing lightness and darkness, Da\* or Δa\*, if positive represents redness and if negative represent greenness and Db\* or Δb\* , if positive represents yellowness and if negative represent blueness by their positive and negative values respectively, as shown in Eqs. 5 to 8 and pictorially is shown in **Figure 5**.

The above said CIE colour differences equations are depicted below for ease of understanding:

$$
\Delta E = \left[ (\Delta L^\*)^2 + (\Delta a^\*)^2 + (\Delta L b^\*)^2 \right]^{1/2} \tag{11}
$$

where,

$$\mathbf{L}^\* = \mathbf{1} \mathbf{1} \mathbf{6} (\mathbf{Y}/\mathbf{Y}\_\mathbf{a})^{\dagger\_\circ} - \mathbf{1} \mathbf{6} \tag{12}$$

$$
\Delta \mathbf{L}^\* = \mathbf{L}\_1^\* - \mathbf{L}\_2^\* \tag{13}
$$

$$\mathbf{a}^\* = \mathbf{500} \left[ (\mathbf{X}/\mathbf{X\_a})^{\natural\_\circ} - (\mathbf{Y}/\mathbf{Y\_a})^{\natural\_\circ} \right. \tag{14}$$

$$
\Delta \mathbf{a}^\* = \mathbf{a}\_1^\* - \mathbf{a}\_2^\* \tag{15}
$$

**Figure 5.** *CIE L\*a\*b\* colour difference space diagram. By human eye (wavelength vs. intensity).*

*Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

$$\mathbf{b}^\* = \mathbf{200} \left[ (\mathbf{Y}/\mathbf{Y\_a})^{\dagger \circ} - (\mathbf{Z}/\mathbf{Z\_a})^{\dagger \circ} \right. \tag{16}$$

$$
\Delta \mathbf{b}^\* = \mathbf{b}\_1^\* - \mathbf{b}\_2^\* \tag{17}
$$

Chroma, (psychometric chroma) values in CIELAB colour space can be calculated as follows:

$$\mathbf{C}\_{\text{(ab)}}^{\*} = \left(\mathbf{a}^{\*2} + \mathbf{b}^{\*2}\right)^{\natural\_{\Omega}} \tag{18}$$

$$
\Delta \mathbf{C}^\* = \mathbf{C}\_{1(\text{ab})}^\* - \mathbf{C}\_{2(\text{ab})}^\* \tag{19}
$$

(21)

where, C<sup>∗</sup> 1 ab ð Þ and C<sup>∗</sup> 2 ab ð Þ are the chroma values for standard and produced sample. CIE 1976 metric Hue-Difference (ΔH) for CIELAB system can be calculated as follows:

$$
\Delta \mathbf{H}\_{\rm ab} = \left[ \left( \Delta \mathbf{E}\_{\rm ab}^{\*} \right)^{2} - \left( \Delta \mathbf{L}^{\*} \right)^{2} - \left( \mathbf{C}\_{\rm ab}^{\*} \right)^{2} \right]^{\natural\_{\rm \natural}} \tag{20}
$$

Moreover, Brightness is another additional colour attribute associated with perception of colour differences. This attribute of visual sensation of colour gives an additional visual perception that appears to be more or less intense or luminescence i.e., this visual stimulus appears to emit more or less light from specific hue of colour and differs from one another.

Brightness Index (BI) as per ISO-2469/2470–1977 method [11] can be calculated by following ISO formula for this:

$$\text{Brightness Index} = \frac{\text{Reflectance Value of the Sample at 457 nm}}{\text{Reflectance Value of the Standard white diffusivity} (\text{white tiles}) \text{at } 457 \text{ nm}}$$

$$\times 100$$

Application of fluorescent brightening agents to white textiles show an additional higher reflectance value more than 100 and up to 150. Though the sample appears to be still whiter as usual, there is emitting of more reflectance of incident light in the bluer zone and the appearance thus changes its chroma towards blue increasing its more whiteness and brightness, where brightness value may be represented or expressed in quantitative term by ISO standard method. Conversely, yellowing of white textiles by chemical treatment or by heat scorching or by any type of degradation by exposure to light or by gas fading etc. can blur the brightness value of the white or dyed sample. Thus, along with colour differences like DE, DL, Da, Db, this Brightness index (BI) as another additional colour attributes related to surface appearance properties of textiles have immense important role and simply high or low BI values an important colour surface appearance parameter too in defining the colour quality of any textile fabric.

A recent newer concept of defining colour differences by Colour Difference Index (CDI) values as a measure of dispersion of colour values at different points from all angle of instrumental measured variation, depending on dyeing process variables, to understand the combined effects of different dyeing process variables by a single parameter, is defined [12] taking only the magnitudes of the respective Δ*E*, Δ*C*, Δ*H* and MI values (irrespective of their sign and direction), to calculate CDI values using the following empirical formula (Eq. 22).

$$\text{Color Difference Index (CDI)} = \frac{\Delta E \text{ X\Delta H}}{\Delta C \text{ X MI}} \tag{22}$$


#### **Table 2.**

*Effect of Mordant concentration on Colour Strength and Colour Differences for dyeing silk fabric with tesu (containing butein) extract as natural colourant.*

Higher the differences in between maximum and minimum CDI values, higher is the dispersion of colour values at different points i.e., colour values are more widely dispersed, and that variable become critical for reproducibility for such dyeing. So, lower the differences in between maximum and minimum CDI value in one set of dyeing for particular dyeing process variables or use of mixture of same set of binary mixture of dyes, better is the match with lower dye dispersion in such cases of colour match CDI value below 5 is acceptable and good and below 1.0 is considered as excellent.

CASE STUDY 3:

The above shown data in **Table 2** on colour parameters, obtained in a study on use of different mordant concentration yields different surface colour strength(K/ S) showing reasonable differences of Colour values in terms of Δ*E,* Δ*L,* Δ*a,* Δ*b,* Δ*C,* Δ*H,* MI (LABD) and CDI values indicating the inter-dependence of colour strength and other colour interaction parameters of tesu dyed silk fabric, clearly showing the role of increasing mordant concentrations up to 15% for higher K/S values, with maximum Δ*E,* and medium CDI, while increase of Mordant concentration beyond 15–25%, gradually reduces colour strength but increases colour dispersion with lowering of CDI.

#### **3.5 Computer aided colour match prediction of textiles and others by using UV: VIS reflectance spectrophotometer and colour measuring/matching software for producing any standard shades**

Colour matching of two samples are considered as fully satisfactory, if any one of the following 3 conditions are achieved with plus-minus mutually accepted tolerances values of their colour differences in CIELab attributes as follows:

Thus, to become colour of produced sample = colour of given standard sample, following should be the conditions be satisfied - i.e., below given conditions (1)–(3).


*Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

3rd Conditions are easy to check and achieve, as it is additive in nature and Dye Concentration vs. K/s values plot are linear and is predictable from sample database by computerised algorithm.

For computer aided colour matching theory [2, 6–8], for a shade from mixture of multiple colourants (say 3 colourants), following three equations are to be solved as a function of dye concentrations of the colourants (1, 2,3 or n) and to be checked by measuring tristimulus values or reflectance values or K/s values with measurement of DE\*, DL\*, Da\* and Db\* values under different standard illuminants.

$$\begin{aligned} \mathbf{f}(\mathbf{c}\_1, \mathbf{c}\_2, \mathbf{c}\_3) &= \mathbf{x} \\ \mathbf{f}(\mathbf{c}\_1, \mathbf{c}\_2, \mathbf{c}\_3) &= \mathbf{y} \\ \mathbf{f}(\mathbf{c}\_1, \mathbf{c}\_2, \mathbf{c}\_3) &= \mathbf{z} \end{aligned}$$

where x, y, z tristimulus values of standard given sample are to be matched with the matched dyed textile sample to be produced, by using say 3 different dyes with respective concentrations of those 3 selective dyes indicated by c1, c2 and c3. For determining/predicting these selective concentrations of specific dyes to get a specific match of colour, In practice, the reflectance values of standard sample at 400 to 700 nm are initially measured from standard dyed textile substrate and those reflectance data are processed through computer aided software to generate matched K/S Values, within tolerance set for specific L, a and b colour difference parameters and DE total colour difference parameter to match for the predicted/ produced sample. As K/S values vs. concentration of dyes is linear & additive, so this is used as basic data for handling colour match prediction by computer aided colour measuring cum matching instrument from different companies with application software in built in the system.

Colour matching is always associated with Some practicable values of DE\*, DL\*, Da\* and Db\* values, within acceptable tolerances, but is also associated another factor/term called metamerism index (MI), due to measurement of colour values under different conditions of measuring colour values i.e. within varying illuminates or varying observers or varying instruments etc. [2, 6–8].

Thus, only colour difference values do not represent true differences of perceived colour in human eye due to observer's metamerism or even instrumental metamerism or illuminate metamerism etc. An ideal or perfect colour match is called isomeric match i.e., which are always match under all illuminates or under all observers or under all instruments in all the ranges of wavelength values in visible region and then that ideal match is called true isomeric match. While Most of the given standard of colour and produced samples are not at all show isomeric match, there is always some differences in their colour difference results at different wavelength range or otherwise i.e. when two coloured sample (standard and produced sample for colour matching) show match under one illuminant/one observer or one instrument but do not match under any other illuminant/other observer or other instrument at different wave length values is termed as a metameric match. So, it is a challenge to produce a Least metameric match instead of ideal isomeric match. A general metamerism index (MI) value can be calculated using Eq. 23, as follows:

$$\text{General attenuation Index} = \frac{\sum \left(\Delta \mathbf{R} \overline{\mathbf{x}}\right)^2}{\mathbf{X}^2} + \frac{\sum \left(\Delta \mathbf{R} \overline{\mathbf{y}}\right)^2}{\mathbf{Y}^2} + \frac{\sum \left(\Delta \mathbf{R} \overline{\mathbf{z}}\right)^2}{\mathbf{Z}^2},\tag{23}$$

where ΔR = Difference in reflectance between pair of metamer samples; *x*, *y*, *z* = CIE standard observer colour function X, Y, Z = CIE tristimulus value normally

taken for illuminate C. It is average value of colour differences of two specimens under two different measuring conditions.

The Metamerism-Index (MI) indicate the probability of any two near match or matched two samples when show the different colour difference values under changed conditions of measurements like if measured under two different illuminants (represented by the first and second illuminant) or under two different make reflectance spectrophotometer instruments or under any other two different conditions of measuring colour parameters of the said two specific samples by calculating. CIE LAB i.e., LABD metamerism index [2, 6–8], which is represented below in Eq. 24:

$$\mathbf{MI}\_{\text{(LAB)}} = \left[ \left( \Delta L\_1^\* - \Delta L\_2^\* \right)^2 + \left( \Delta a\_1^\* - \left( \Delta a\_2^\* \right)^2 + \left( \Delta b\_1^\* - \left( \Delta b\_2^\* \right)^2 \right)^{\natural\_{\zeta}} \right. \tag{24} \right. \tag{25}$$

Δ*L1\**, Δ*a1\**, and Δ*b1\**are the Delta CIELab\* colour coordinates between standard and sample for the first illuminate and Δ*L2\**, Δ*a2\**, and Δ*b2\** are the Delta CIELab\* colour coordinates between standard and sample for the second illuminate interpretation:

If MI is low, the colour difference between the sample pair is the closer and more similar for different conditions of measurement, even under different illuminates or observers or instruments. So, matching of two-coloured samples produced at comparable conditions are to always to minimize to obtain least metameric match for control of colour by using computer aided colour measuring and matching system [7, 13].

CASE STUDY 4: Computer aided colour match prediction for dyeing of textiles: as an Example

Practical Guideline for Colour Match prediction: it is necessary to prepare Company wise Dye Class type and Sample type (Substrate fibre type) database by calibration dyeing [7, 14, 15] of 0.25, 0.50, 0.75, 1.00, 1,25, 1.50, 1.75. and 2, 2.5, 3 percent dyed sample of specific fabric (based on type of fibre) i.e., say- bleached cotton fabric and their reflectance or X, Y and Z Data are to be measured and to be saved as library of database for use for formulation prediction of dye weight % required for colour matching from time to time for given standard sample.

Colour matching tolerances against Standard daylight D65 illuminate, Artificial Tube light -TL84 (A) and fluorescent light (F) are to be set as maximum 1.00 for each light or to be mutually fixed between buyers and sellers in order agreement. If dye cost from lot to lot regular purchase is updated in this system, cost of dyes for different formulations are also calculated and available at fingertips, other dyeing process and utility cost remaining same. Not only it helps to reduce dye inventory and it saves matching time for lab to production trial time with reasonable known combination of dyes and cost involved along with average predicted dE\*, dL\*, da\*, db\* values to know the degree of precision of colour matching, below is the example of one colour match prediction formulation using computer aided colour matching system with database for different class of textile dyes already fed in (the present example is colour matching formulation of cotton fabric with reactive dyes database, as given in **Table 3**.

Thus, the above predicted 2 formulations indicate that formaulation#1 is less metameric as understood from comparison of their dE\*, dL\*, da\*, db\* values, and cost wise Formulation#2 is least cost match,

#### **3.6 Estimation of compatibility between two colorants to use for compound binary shades**

Compatibility between any two same class of dyes can be judged by different methods, such as (i) comparative subjective visual assessment of the degree of on-



**Table 3.**

*Example of a colour match predicted from the database of direct dye for cotton.*

tone build up by carrying out a series of dyeing for both dyes to same substrate and checking gradual colour build up by visual assessment, (ii) theoretical prediction of compatibility [16] by comparison of rates of dye by rate of diffusion of dyes by determining diffusion coefficients or by determining time of half dyeing for each individual dye at comparable dyeing conditions (iii) by quantitative assessment of change in hue angle(ΔH) for increasing dyeing time and temperature or increasing dye concentrations [16] under two sets of dyeing for colour built up on specific textile substrate (iv) by comparing the nature of plots of ΔC vs. ΔL or K/S vs. ΔL values for two sets of progressive built up shades as said in point -no (iii) obtained by dyeing with varying dye concentration and also with varing dyeing time and temperature as said in point no-3 using 50:50 of two dyes [17] and (v) quantitative compatibility rating for the mixtures of more than two dyes by colorimetric analysis of actual colour strength developed (not on the basis of dye absorbed) for mixture dyeing in different proportions following Relative compatibility rating (RCR) method [12] by calculating differences of CDI (Colour difference Index) values [17, 18] as a newer empirical index of overall colour differences for dyeing different proportions of two dyes of different pairs of synthetic or natural dyes applied on any textiles.

CASE STUDY 5: Comparison of compatibility of two dyes by comparing the nature of plots of ΔC vs. ΔL or K/S vs. ΔL values for two sets of progressive built up shades by dyeing with variation of dye concentrations (SET-1) and dyeing with variation of Time and temperature (SET-2) using 50:50 of two dyes as well as also Determining compatibility of 2 dyes by Relative compatibility rating (RCR) method by calculating differences of CDI values for dyeing different proportions of any two dyes.

Dyes Selected are: Direct dyestuffs (make: Atul Ltd. (Tuladir)) of four different colours, i.e., Direct Turquoise blue (CI Direct Blue 199), Direct Red (CI Direct Red 31), Direct Yellow (CI Direct yellow 44), Direct Green (CI Direct Green 513).

Dyeing carried out for Conventional methods of determining compatibility, for obtaining plots of ΔC vs. ΔL or K/S vs. ΔL values for two sets of progressive built up shades following selected binary pairs (50:50) of synthetic direct dyes were applied on the 6% H2O2 (50%) bleached Jute fine hessian fabric using three pair of following combination of binary pair of direct dyes such as M-11 -Direct Red + Direct Green, b) M-12-Direct Red + Direct Yellow and c) M-13-Direct Red + Direct T. Blue taken in 50:50 ratio in two sets.

In Set I, the progressive depth of colour was gradually built up by varying dyeing time and temperature profile for each pair of dyes (M11, M-12 and M13), three jute fabric samples were dyed laboratory beaker dyeing machine with temperature controller for 10–60 min varying dyeing time period. The said dyed fabric samples were one by one taken out from the respective dye bath at equal interval of 10 min from dyeing temperature of 60°C onwards up to 100°C, maintaining the constant heating rate of 2–5°C/min. The final and ultimate dyed sample was taken out from dye bath after 60 min dyeing time at 100°C dyeing temperature.

In Set II, the progressive depth of shade was obtained by varying total concentration of dye mixture in 50:50 ratio but varying percent application from 20–100% of 1% shade for each pair of dyes, for 3 separate samples of jute fabrics, which were dyed at the at the increments of 20% points of dye concentration at pre-fixed dyeing conditions at 100°C for 60 min. Taking two dyes in equal proportions (50:50).

The colour difference values in terms of ΔE\* and ΔL\*, Δa\*, Δb\* and ΔC\* for all the above said dyed fabrics using Set I and Set II conditions, against undyed fabric sample as standard for reference, were obtained by individually separate measurement of the colour difference parameters Using UV–VIS reflectance spectrophotometer within built software and computer attached. The compatibility of a selected pair of dyes was judged [16–19] from the degree of closeness and overlapping of two curves ΔC vs. ΔL or K/S vs. ΔL observed using the two sets of dyeing (Set I and Set II) as shown in **Figure 6**.

**For Relative Compatibility Rating Newer method of Determining Compatibility of two dyes, 6**% H2O2 (50%) bleached jute fabric samples were dyed with four direct dyes taken from Atul direct dye of either single or selected binary pairs of direct dyes in varying proportions (100:0, 75:25,50:50,25:75 and 0:100) under specific fixed and comparable dyeing conditions. The results are shown in **Table 4**.

Thus, both of these methods show a similar results, while the method 2 of RCR compatibility rating method is easier and less time consuming and hence has advantages over plotting of K/S Vs DL.

#### **3.7 Optimization of dyeing process variables for dyeing textiles with any synthetic or natural dyes**

Dyeing of any textiles, say cotton or jute or any other fibres to be dyed with specific class of synthetic dyes like reactive dye (or even for any natural dyes) need *Colorimetric Evaluations and Characterization of Natural and Synthetic Dyes/Pigments… DOI: http://dx.doi.org/10.5772/intechopen.104774*

#### **Figure 6.**

*Plots showing K/S Vs ΔL curves of (a) M11-D Red: D Green (b)M-12 -D Red: D yellow and (c)M-!3 -D Red: D T. Blue for two sets of each showing M-12 -D Red: D yellow combination has good compatibility, while M11- D Red: D Green combination ahs not so good compatibility or has fair compatibility and M-!3 -D Red: D T. Blue has more or less average compatibility at higher time (Table 4).*

to be optimised [14, 15] to derive standard dyeing conditions to obtain maximum surface colour strength (K/S values).

So, it need to have experiments on varying dyeing time, temperature, dye concentration, salt concentrations, MLR and pH etc., so that reproduced and uniform dyeing can be achieved easily.

However, for reactive dyes, Dyeing time has two type -Dye exhaustion time and Dyeing fixation time and similarly dyeing temperature has two dimensions, i.e., Dye exhaustion temperature and Dye Fixation temperature and also for last stage of alkali fixation of reactive dye, addition of soda ash is to be considered, also, besides addition of salt for exhaustion as evident from earlier references [20].

UV VIS reflectance spectrophotometer thus helps by colorimetric analysis of Surface colour strength and other colour parameters, for dyeing of any fibre with specific class of dye by varying conditions of dyeing.


Fabric used: 3% H2O2 bleached fine hessian jute fabric having 215 tex jute yarns as warp and 285 tex jute yarns as weft, 64 ends/dm and 58 picks/dm, fabric area density 320 g/m2 and fabric thickness 0.70 mm, obtained from M/s Gloster Jute Mills Ltd., Bauria, Howrah, was used.

Dyes Selected: (i) Hot brand Reactive Green HE4BD (CI Reactive Green 19), (ii) Hot brand Reactive Orange CN (C.I. Reactive Orange 84) and (iii) Cold brand Magenta (C.I. Reactive Red 11) were used.

Measurement of Colour Parameter: K/S values of differently dyed jute fabrics under varying conditions of dyeing were determined by using computer-aided UV VIS Reflectance spectrophotometer [Premier Colour Scan Instrument Ltd. Mumbai

