perception depends on the colour background of the sample. Also structure,
gloss and metallic effects influence the perceived colour, asserts Vijaya
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
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.
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.
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.
and Concept of Colour
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
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.
soft vacuum near visible
1A0 10A0 100A0
200nm 400nm 800nm
25µm 400µm 25cm
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.
mechanism of perception of colour is based on Colour Mixing Laws, ie,
Additive and Subtractive Colour Mixing.
the Computers see Colour
following components work together to create colour on your computer.
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.
cards or videos cards/boards: You may have a graphic card or video
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.
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.
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.
management Practical colour management (colour matching)
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.
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.
Correlation of computer predicted recipe to the target shade in the
2) Convert into reality or
to get the exact target shade at the first attempt right on time.
mathematics of computer colour matching
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.
Database Development for Controlled Colouration for CCM (Computer Colour
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".
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
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:
The dyeing process.
Dye and chemical selection.
Fabric selection and weighing
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.
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:
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.
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.
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.
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
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
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