Spectrophotometer

Computer colour matching (CCM)

Computer Color Matching System (CCMS): Computer Color Matching (CCM) is the instrumental color formulation based on recipe calculation using the spectrophotometric properties of dyestuff and fibers.

The basic three things are important in CCMS: 

• Color measurement Instrument (Spectrophotometers).

• Reflectance (R%) from a mixture of Dyes or Pigments applied in a specific way.

• Optical model of color vision to closeness of the color matching (CIE L*A*B).

Functions of Computer Color Matching System: The following works can be done by using CCMS • Color match prediction.• Color difference calculation.• Determine metamerism.• Pass/Fail option.• Color fastness rating.• Cost Comparison.• Strength evaluation of dyes.• Whiteness indices.• Reflectance curve and K/S curve.• Production of Shade library. • Color strength

4. Pass / Fail option: The sample which is dyed according to the recipe of the CCMS is it matches with the buyers sample that could be calculate by this system. If the dyed sample fulfill the requirements then CCMS gives pass decision and if can’t then it gives fail decision. So, pass-fail can be decided by CCMS.

5. Color Fastness Rating: • Color fastness can be calculates by CCMS. There is

different color fastness rating (1-5/1-8). CCMS analyze the color fastness and gives result.

6. Cost Comparison: – Cost of the produced sample can be compare with others. It

also helps to choose the right dyes for dyeing.

7. Strength Evaluation of Dyes: – It is important to evaluate the strength of the dyes which will

be used for production.

– All of the dyes have not same strength. Dyes strength effects the concentration of dyes which will be used for dyeing.

8. Whiteness Indices: – Whiteness Indices also maintained in CCMS.

9. Reflectance Curve and K/S Curve: – Reflectance curve also formed for specific shade by which we

can determine the reflection capability of that shade.

10. Production of Shade Library: 

– Computer color matching system also store the recipe of the dyeing for specific shade.

– This shade library helps to find out the different documents against that shade.

– It is done both for the shade of sample and bulk dyed sample.

11. Color Strength: 

– Computer color matching system also determine the color strength of the sample.

Working Procedure of Computer Color Matching Systems ( CCMS ):

• The working procedure of CCMS which is used for dyeing lab to match the shade of the products.

• Generally buyer gives a fabric sample swatch or Panton number of a specific shade to the producer.

• Producer gives the fabric sample to lab dip development department to match the shade of the fabric.

• After getting the sample they analyze the color of the sample manually.

• In the other hand they can take help from the computer color matching system.

At first it needs to fit the sample to the spectrophotometer which

analyzes the depth of the shade and it shows the results of the

color depth.

At the same time it needs to determine the color combination

by which you want to dye the fabric.

Then it will generate some dyeing recipe which is nearly same.

Here it needs to determine the amount of chemicals which you

want to use during dyeing.

After formation of dyeing recipe it needs to dye the sample with stock

solution.

Then sample should dye according to the dyeing procedure.

After finishing the sample dyeing it needs to compare the dyed sample

with the buyer sample.

For this reason dyed sample are entered to the spectrophotometer to

compare the sample with the buyer sample.

Then CCMS gives the pass fail results.

If the dyed sample match with the buyer sample than CCMS gives pass

results.

After that, dyed samples send to the customer or buyer.

After getting the approval from the buyer producer goes for the

bulk production.

If the dyed sample does not match with the buyer sample than

the CCMS analyses the color difference and correct the recipe.

Then another sample dyeing is carried out for matching the

shade of the sample.

Advantages of Computer Color Matching System (CCMS) :

• Customers get the exact shade wanted with his knowledge of degree of metamerism.

• Customers often have a choice of 10-20 formulation that will match color. By taking costing, availability of dyes, and auxiliaries into account, one can choose a best swatch.

• 3 to 300 times faster than manual color matching.

• Limited range of stock color needed. 

Spectrophotometer

It is a photometric device that measures

spectral transmittance,

spectral reflectance relative spectral emitance.

• It compares light leaving from the object with that

incident on it at each wavelength.

• According to Beer's law, the amount of light absorbed by

a medium is proportional to the concentration of the

absorbing material or solute present.

• Thus the concentration of a colored solute in a

solution may be determined in the lab by measuring

the absorbency of light at a given wavelength.

• Wavelength (often abbreviated as lambda) is

measured in nm.

• The spectrophotometer allows selection of a

wavelength pass through the solution.

• Usually, the wavelength chosen which corresponds

to the absorption maximum of the solute .

Absorption Spectroscopic methods of analysis rank among

the most widespread and powerful tools for quantitative

analysis.

The use of a spectrophotometer to determine the extent of

absorption of various wavelengths of visible light by a given

solution is commonly known as colorimetry.

This method is used to determine concentrations of various

chemicals which can give colours either directly or after

addition of some other chemicals.

The  Instrument of Spectrophotometer:All spectrophotometer instruments designed to measure

the absorption of radiant energy have the basic components as follows :

1. A stable source of radiant energy (Light); 2. A wavelength selector to isolate a desired

wavelength from the source (filter or monochromator);

3. Transparent container (cuvette) for the sample and the blank;

4. A radiation detector (phototube) to convert the radiant energy received to a measurable signal; and a readout device that displays the signal from the detector.

The energy source is to provide a stable source of light

radiation, whereas the wavelength selector permits separation

of radiation of the desired wavelength from other radiation.

Light radiation passes through a glass container with sample.

The detector measures the energy after it has passed through

the sample.

The readout device calculates the amount of light absorbed

by the sample displays the signal from the detector as

absorbance or transmission.

The spectrophotometers which are used for such measurements

may vary from simple and relatively inexpensive colorimeters

to highly sophisticated and expensive instruments that

automatically scan the ability of a solution to absorb radiation

over a wide range of wavelengths and record the results of these

measurements.

One instrument cannot be used to measure absorbance at all

wavelengths because a given energy source and energy detector is

suitable for use over only a limited range of wavelengths.

Both filters and monochromators are used to restrict the radiation wavelength.

Photometers make use of filters, which function by absorbing large portions of the spectrum while transmitting relatively limited wavelength regions.

Spectrophotometers are instruments equipped with monochromators that permit the continuous variation and selection of wavelength.

The effective bandwidth of a monochromator that is satisfactory for most applications is about from 1 to 5 nm.

• The sample containers, cells or cuvettes, must be fabricated from

material that is transparent to radiation in the spectral region of

interest.

• The commonly used materials for different wave length regions

are:

– Quartz or fused silica: UV to 2 mm in I R

– Silicate glass: Above 350 nm to 2 mm in I R

– Plastic: visible region

– Polished NaCI or AgCI: Wave lengths longer than 2mm

• Cuvettes or cells are provided in pairs that have been carefully matched to make possible the transmission through the solvent and the sample.

• Accurate spectrophotometric analysis requires the use of good quality, matched cells.

• These should be regularly checked against one another to detect differences that can arise from scratches, etching and wear.

• The most common cell path for UV-visible region is 1 cm.

• For reasons of economy, cylindrical cells are frequently used.

• Care must be taken to duplicate the position of such cells with respect to the light path;

General Measurement Procedures :

As explained above, the Beer-Lambert Law forms the basis of the measurement procedure.

The amount of light radiation absorbed by a compound is directly related to the concentration of the compound.

The general measurement procedure consists of 5 steps:

• Prepare samples to make colored compound

• Make series of standard solutions of known concentrations and treat them in the same manner as the sample for making colored compounds

• Set spectrophotometer to l of maximum light absorption

• Measure light absorbance of standards

• Plot standard curve: Absorbance vs. Concentration, 

Metamerism-Index

• The Metamerism-Index (MI) will show the probability that two samples will show the same color difference under two different illuminants (represented by the first and second illuminant)

• L*1, a*1, b*1 are the Delta CIE Lab* color coordinates between Standard and Sample for the first illuminant

• L*2, a*2 ,b*2are the Delta CIE Lab* color coordinates between Standard and Sample for the second illuminant

Color Inconsistency

• This attribute indicates a color change in the sample (without any reference to the standard) under different illuminants. This property is sometimes known as "flare."

CIE 1976 (L*, a*, b*) color space (CIELAB)

• CIE L*a*b* (CIELAB) is color space specified by the CIE International Commission on Illumination (French Commission internationale de l'éclairage).

• It describes all the colors visible to the human eye and was created to serve as a device independent model to be used as a reference.

• The three coordinates of CIELAB represent the lightness of the color (L* = 0 yields black and L* = 100 indicates diffuse white; specular white may be higher).

• Its position between red/magenta and green (a*, negative values indicate green while positive values indicate magenta) and

• Its position between yellow and blue (b*, negative values indicate blue and positive values indicate yellow).

• The asterisk (*) after L, a and b are part of the full name, since they represent L*, a* and b

• Since the L*a*b* model is a three-dimensional model, it can only be represented properly in a three-dimensional space.

• Because the red/green and yellow/blue opponent channels are computed as differences of lightness transformations of (putative) cone responses, CIELAB is a chromatic value color space.

CIE L*C*H*

• The L* axis represents Lightness.It ranges from L* = 0 yields black and L* = 100 indicates diffuse white.

• The C* axis represents Chroma or "saturation". This ranges from 0 at the centre of the circle, which is completely unsaturated (i.e. a neutral grey, black or white) to 100 or more at the edge of the circle for very high Chroma (saturation) or "color purity".

• The h* describes the hue angle. It ranges from 0 to 360 – h=0° = red / h=90° = yellow / h=180°=green / h=270° = blue

CIE Lab* Color AttributesL* Represents a standard or sample's position on the lightness axis in either

CIELAB or CIELCH color space. This attribute is also available in Strength Adjusted form.

a* Represents a standard or sample's position on the green/red axis in CIELAB color space, green being in the negative direction and red being in the positive direction. This attribute is also available in Strength Adjusted form.

b* Represents a standard or sample's position on the blue/yellow axis inCIELAB color space, blue being in the negative direction and yellow being in the positive direction. This attribute is also available in StrengthAdjusted form

C* Represents a standard or sample's chroma value in CIELCH color space.This attribute is also available in Strength Adjusted form.

h* Represents a standard or sample's hue value in CIELCH color space. Thisattribute is also available in Strength Adjusted form.

CIE Lab* Color Difference Attributes

DL*The delta value for the L* attribute. This attribute is also available in Strength Adjusted form.

Da*The delta value for the a* attribute. This attribute is also available in Strength Adjusted form

Db*The delta value for the b* attribute. This attribute is also available in Strength Adjusted form.

DC*The delta value for the C* attribute. This attribute is also available in Strength Adjusted form

Dh*The delta value for the h* attribute. This attribute is also available in Strength Adjusted form.

DE*The distance a sample falls from the standard in CIE* color space using a simple, straight-line calculation. This attribute is also available in Strength Adjusted form.

CIE94

• The 1976 definition was extended to address perceptual non-uniformities, whileretaining the L*a*b* color space, by the introduction of application-specific weights derived from an automotive paint test's tolerance data.

• ΔE (1994) is defined in the L*C*h* color space with differences in lightness, chroma and hue calculated from L*a*b* coordinates.

• Given a reference color L*1,a*1, b*1 and another color L*2,a*2,b*2 , the difference is:

• Where the K-values depend on the application

Hunter Lab

• The Hunter Lab color scale was developed in the 50´s and 60’s. There were several permutations of the Hunter Lab color scale until the current formulas were released in 1966.

• The Hunter Lab color space is organized in a cube form.

• The L axis runs from the top to the bottom. The maximum for L is 100 (for a perfect reflecting diffuser) while the minimum is 0.

• The a and b axes have no specific numeric limits.

• Positive a is red and negative a is green.

• Positive b is yellow and negative b is blue.

Hunter Lab Color Space attributes

L Represents a standard or sample's position on the lightness axis in Hunter color space. This attribute is also available in Strength Adjusted form.

a Represents a standard or sample's position on the green/red axis in Hunter color space. This attribute is also available in Strength Adjusted form

b Represents a standard or sample's position on the blue/yellow axis in Hunter color space. This attribute is also available in Strength Adjusted form.

Hunter Lab Color Difference attributes

DL The delta value for the L component of Hunter color space.

Da The delta value for the a component of Hunter color space.

Db The delta value for the b component of Hunter color space

DEh The distance a sample falls from the standard in Hunter color space

Chromaticity Coordinates

• To simplify , the coordinates X,Y,Z, could be divided by X+Y+Z to get CHROMATICIRY COORDINATES (x,y,z) which adds up to one.

x = X / (X+Y+Z ) ,

y = Y / (X+Y+Z) and

z = Z/ ((X+Y+Z )

x + y + z = 1

Hence it is sufficient to know x and y.

Instrumental Match Prediction

Kubelka-Munk equations• The mathematical basis for all color matching software

is the Kubelka-Munk series of equations. • These equations state that for opaque samples such as

textile materials, the ratio of total light absorbed and scattered by a mixture of dyes is equal to the sum of the ratios of light absorbed and scattered by the dyes measured separately.

• Where absorption is defined as "K" and scattering is defined as "S", Kubelka-Munk states that:

• (K/S) mixture = (K/S) dye 1 + (K/S) dye 2 + (K/S) dye 3 + ...

• K/S is not a readily measurable quantity, but it can be calculated from the reflectance of a sample "R" by the Kubelka-Munk equation that states

K/S = ( 1 - R ) _ / 2R

• If the K/S of a target color is measured at several wavelengths, the concentrations of each dye can be calculated by trial and error from primary dyeing to achieve the closest match.

• A computer color matching program is capable of performing hundreds of iterations in a short period of time to produce the initial dye concentrations.

• As an example, if a sample has a reflectance of 20% at a wavelength of 500nm, then the K/S can be calculated as:

K/S = ( 1 - 0.2) _ / 2(0.2) = 1.6

Delta E (ΔE)• Delta E is defined as the difference between two colors in an

L*a*b* color space.

• As the values determined are based on a mathematical formula, it is important that the type of color formula is taken into account when comparing the values.

• In Color Verifier alone, there are three different formulas to choose from, each producing different results.

• The CIE L*a*b* formula used in the proofing market calculates the Euclidian distance, i.e. purely the distance between two points in a three-dimensional color space.

• The actual position of the points themselves is irrelevant.

• The following delta E values are valid universally:

E value Meaning

0 - 1 A normally invisible difference

1 - 2Very small difference, only obvious to a trained eye

2 - 3.5Medium difference, also obvious to an untrained eye

3.5 - 5 An obvious difference

> 6 A very obvious difference