Colour perception depends on the colour background of the sample. Also structure, gloss and metallic effects influence the perceived colour, asserts Vijaya Shanbhag.

Colour is something which makes the object more appealing, attractive and gives the pleasure of observation. The Committee on Colorimetry of Optical Society of America defines colour as visual sensation arising from the stimulation of retina of the eye. Thus it is defined as a psychophysical- psychological response to a physical stimulus.

Colour may have different meanings to different people. To a chemist it may be a chemical compound, a dye or a pigment; To a physicist it is scattering and absorption of light or reflectance spectra of the object; To a physiologist it is a measurable activity of nerves; To a psychologist it is a complex process in brain of interpreting the nerve signal. To the artist and others it is a means to create sensation in the mind of the observer. For eg, the colours as red and yellow create the sensation of warmth. Green and blue are associated with the feeling of coolness. Colour harmony in wall paints, curtains and furniture make room cheerful, and comfortable. Perception of colour includes source of light, object that is illuminated and eye and brain that perceive the colour. A source of light is characterised by the energy that is radiated at different wavelengths, ie, by its spectral power distribution.

The modification of incident radiations depends on the nature of colourants in object. This is related to chemistry of dyes. The radiant energy reflected is absorbed by photosensitive pigments in retina of the eye. The photosensitive detectors on retina are called as rods and cones from their shape. The rods detect the light but have no ability to specify the colour. The colour is detected by cones.

The colourists in today's commercial environment has to choose from a vast range of available colourants, the most effective recipe to achieve a commercially acceptable match with the correct physical qualities to satisfy his customers requirements. He has also to ensure that subsequent batches are within acceptable tolerances for depth, shade, and hue. Computer colour matching systems can be used to achieve success in all the above areas and will help increase productivity and reduce costs.

Theory and Concept of Colour

From times immemorial colour has been used to express feelings and emotions and to decorate. We are conditioned from birth to react to colour in an emotional and psychological way. We are taught to obey certain colours -- red means stop, green means go. We also use colour to give warnings; Blue means cold, red means hot. Colour can describe our moods -- green with envy, feeling blue, seeing red, our state of health we say in the pink of health, or even our character.

Industry exploits colour in many ways to attract consumer's interest and merchandise.Thousands of those in apparel and textiles fields have spent countless amounts of time getting colour right, so that the product on the selling floor projects the right message to boost consumer sales. Colour is a perceived psychophysical sensation depending on both physical stimuli in eye and message transfer to brain when visible radiation (380 - 760 nm) falls on the eye.

Radio                             soft    vacuum near visible near               fundamental                         Far
micro
Y-Rays       X-Ray        X-Ray    UV                    UV                IR                              IR
                     IR             waves
 ______|____|___|_____|_______|______|_____|___________|______|_______|__
1A0    10A0  100A0    200nm    400nm    800nm 2.5µm           25µm    400µm     25cm

A coloured surface appears coloured because the light is reflected from it in that colour's wavelength only. All other wavelengths predominantly are absorbed by it. A blue object reflects blue light but absorbs most of the red, orange, yellow, green and violet. Black and white are slightly different. They are not, strictly speaking colours. White surfaces reflect all or nearly all colours and black surfaces absorb them totally.

The mechanism of perception of colour is based on Colour Mixing Laws, ie, Additive and Subtractive Colour Mixing.

How the Computers see Colour

The following components work together to create colour on your computer.

1) The computer hardware on the mother board: In the simplest terms deep inside your computer is a "Brain". It may or may not be able to see and recreate accurate colours.

2) Graphics cards or videos cards/boards: You may have a graphic card or video card/board installed.

3) Your monitor: Cheap monitors deliver terrible colour. You get what you pay for. If you have an old monitor or PC that costs less than 300$ (US), you probably have very little accuracy for viewing the world web.

To sum it up the monitor can be the major cause of good or bad colour or the monitor can be part of good combination of several components that create good or bad colour. In other words if you have a good video card, good operating system software and good application software a bad monitor can still create inaccurate colours. And even if you have a fantastic monitor, the other components can still create bad colours. So you are looking at a case-by-case kind of situation.

At the present time there are only 216 colours that are common to all computers and web browsers. Each browser sees 256 colours but only 216 of them are common to all web browsers. By contrast, the human eye can see 10,000,000 colours. If the image contains colours that do not exist, the eyes of computer/browser try to mix the colour from the 256 colours available. It tries to patch tiny dots of its colours together to make it look like the colour it does not have in its vocabulary. This is called dithering. Website designers should design for the target audience. Use caution when selecting background colours and text colours. Some computer systems may read these backgrounds as so speckled that text is unreadable. Also, anytime you want a solid fill colour, select it from browser palette. Link to see the 216 colour palette and examples of dithering.

Colour management Practical colour management (colour matching)

Colour perception depends on the colour background of the sample. Also structure, gloss and metallic effects influence the perceived colour. Therefore it is difficult to match two samples that differ in structure - velvet and silk, broad cloth and corduroy, or leather and fabric. Visual judgement as well as measurement depends on how the two samples are arranged and how the light falls on the samples. This is also true when a glossy and matty samples are to be matched. Such difference in appearance influences the description of colours with numbers.

Computer Colour Matching is not only used as a most powerful tool to realising the controlled colouration but also an asset to total QUALITY COLOUR MANAGEMENT.

There are two stages of RFT (right first time) concept in Colour Management.

1) Correlation of computer predicted recipe to the target shade in the laboratory.

2) Convert into reality or to get the exact target shade at the first attempt right on time.

The mathematics of computer colour matching

For a computer formulation program, the mathematical basis for all colourmatching software is the Kubelka-Munk series of equations. These equations state that for opaque such as textiles 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 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 as R K/S = (1-R)2/2R If the K/S of a target colour is measured at several wavelengths, the concentration of each dye can be calculated by trial and error from primary dyeings to achieve the closest match. A computer colour matching program is capable of performing hundreds of iterations in a short period of time to produce initial dye concentrations.

A) Database Development for Controlled Colouration for CCM (Computer Colour Matching System)

The basis of visual and computerised processes for determining the dye concentrations to use in colour matching is a series of colourant dyeing commonly known as "primaries". The accurate production and evaluation of these primaries are essential parts of ensuring accurate results from the colour matching process selected. Utilisation of poorly developed primaries, especially in computer colour matching will lead to dyeing that do not match the desired target, hence "What you see is what you get".

The CCM technique requires the following data input:

• Reflectance reading of standard shade to be matched.

• Reflectance reading of residual print paste shade.

• Residual print paste quantity in kgs.

• Total quantity of print paste required.

• Printing methods.

• Substrate.

• Dyestuff inventories. Computer computes 'Recipes' for residual print paste shade with the cost and then calculate the quantity of dyes to be added to the residual print paste to match the final shade.

Dye Process Variables

The first step in preparing a set of dye primaries is to identify and evaluate the variables used in the production of the dyeing. These variables include:

• Equipment used.

• The dyeing process.

• Dye and chemical selection.

• Fabric selection and weighing technique.

• Before preparing the first set of primaries, test each of these items to ensure repeatability. If the process of dyeing the primaries is not consistent, generating accurate formulas from a computer colour matching system will be difficult.

Dyeing equipment

Most primary dyeing will be prepared on lab-scale equipment heated by glycol, infrared energy, glass beads, or some other heating medium. Equipment problems that can lead to dyeing variability if not resolved include: Leaking gaskets, uneven heating in the case of IR equipment, glycol diluted with water from leaking beakers or water lines, and poor temperature calibration. Dye contamination from previous cycles will also lead to poor quality primaries and can be controlled by proper cleaning of the dye beakers. Evaluation of various designs and suppliers is always recommended before any equipment purchase to ensure compatibility with the processes and materials used. Dyeing Process

Regardless of equipment used, the actual dyeing process must produce samples that correlate well with production equipment. Colours approved from lab dyeing that can be dyed in production with minimal variation will mean fewer adds on new shades. Correlations can be increased by:

• Proper control temperature.

• Liquor ratio.

• Heating and cooling cycles.

• Auxiliary selection.

• And sample size.

It may be necessary to modify one or more of these factors to increase lab to production correlation as well as to ensure day-to-day repeatability. In cases where correlation is not possible, special compensation factors can be used to increase the accuracy of the software's shade predictions.

Dye and chemical selection

Chemical selection and the amounts used in the labs should be based upon the processes required to ensure good repeatability and lab to production correlation. This means that there may be occasions where lab and production formulas use different amounts of some auxiliaries. A program of quality checks for incoming chemicals should be established to minimize the strength variability. Dye selection typically depends upon the type of material being dyed and on end-use requirements.

After a set of primaries has been dyed, strength variability of future lots of dye can have a significant effect on the performance of the computer colour matching system. One of the most important facts to remember about a computer colour matching system is that it calculates new dye formulas based on the lots of dye that were used to dye primaries. If a new lot of dye is brought into the dyeing facility and it is 10% stronger than the original lot, the samples produced from the software's formula prediction will have ten per cent more of this colour than is required. If several dyes vary in strength significant shade variation will occur.

Fabric Selection

The initial set of primaries must all be done on the same type of material to ensure continuity. Select a material that is used often in the dyeing facility and has exhibited good shade repeatability. Generate dye formulas for a beige, a medium grey, and a rose colour using the same three dyes. Use dyes that are in common use and round the formulas to the nearest tenth.

1) Use the split-bath techniques to produce a series of dyeings on all materials to be tested. As an example, if the weight of each sample is ten grams, then the total weight of all material will be one-hundred grams. If the bath volume is 200 ml, then the total volume for all baths will be 2,000 ml. Using the formulas from step one, mix a 2,000 ml solution based on one-hundred grams of fabric. After mixing the solution, divide it equally among ten beakers and perform the dyeing with the technique normally used to dye each material.

2) Use the computer software's strength calculation option to determine the strength of each material relative to the standard material.

3) Use the calculated strength as a factor to adjust the computer's shade prediction when a formula is required on one of the alternate materials. Again many software systems allow the entry of a factor based on the strength to automatically compensate for differences in depth of shade. If several materials exhibit the same relative strength, they may be grouped together during the formulation process. In addition to calculating the strength difference mentioned above, be sure visually evaluate each material to be sure there is no hue variation between the alternate material and the material used to dye the primaries.

Weighing Technique

Precision in weighing of fabric, dyes, and chemicals is essential in producing accurate, repeatable dye primaries. Calibrated scales that measure to two or three decimal places for fabric and analytical scales for weighing of powdered dye are required. The most accurate technique for weighing dyes and chemicals is to pipette from a stock solution using a calibrated glassware. An alternative to manual pipetting is automatic dispensing using a modern lab dispensing machine and stock machine if available.

Preparing the primaries

After the process variables have been defined and modified to produce repeatable and accurate dyeings, colourant primaries can be prepared for all dyes to be used in the formulation system.

Blank Dyeing

An essential part in developing a colourant database is the blank dyeing. A blank dyeing or mock-dyeing as is often called is simply a piece of the material used to dye the primaries that has been exposed to a complete dyeing process without any dye. The blank dyeing should include the same auxiliaries, dye cycle, and finish process if used. Any colour change that results from the process can then be accounted for when the blank dyeing is entered into the database.

Steps involved in database preparation:

• Define the class of dyes.

• Select the levels of dyeing.

• Select the subtrate of good length.

• Complete calibration dyeing for all the dyes on the same subtrate.

• Feed the database to the CCM.

• Verify the plots of conc vs K/S & K/S vs conc.

• Conform By recipe prediction

• Repeat the defective dyeing if any

• Store the database very clean and levelled.

Evaluation of primary data

Once the primaries are accurately measured and stored in the database, the data must be evaluated for consistency using specialised numeric and graphic calculations.

• Artist manipulates colour.

• Scientist analyses colour.

• Industrialists exploit colour.

Vijaya Shanbhag. Textile consultant Goregaon, Mumbai.