Introduction
The basis of visual and computerized
processes for determining the dye concentrations to use in color matching is a
series of colorant dyeing commonly known as “primaries”. The accurate
production and evaluation of these primaries are essential parts of ensuring
accurate results from the color matching process selected. Utilization of
poorly developed primaries, especially in computer color matching is very
confusing for the result of dyeing. Our analysis will present basic guidelines
for preparing and evaluating dye primaries that will lead to success in
computer color matching.
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 equipment used
·
the dyeing process
·
dye and chemical selection
·
fabric selection and
·
weighting 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 color
matching system will be difficult at best.
Ø 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 can lead to dyeing variability.
Ø Dyeing Process
Regardless of the equipment used, the
actual dyeing process must produce samples that correlate well with production
equipment. Colors approved from lab dyeing that can be dyed in production with
minimal variation will mean fewer adds on new shades. Correlation can be
increased by proper control of 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 today repeatability. In cases where correlation is not possible
special compensation factors can be used to increase the accuracy of the
software’s shade predictions.
Ø Dye and Chemical Selection
As mentioned previously, chemical
selection and the amounts used in the lab 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 strength variability. Dye selection
typically depends upon the type of material being dyed and on end-use
requirements.
Ø 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. If a common material is not available, select an alternative material
with sufficient quantity available to dye all primaries that are to be loaded into
the software.
Ø 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.
Preparing the Primaries
After the process variables have been defined and modified to
produce repeatable and accurate dyeing, colorant primaries can be prepared for
all dyes to be used in the formulation system as follows:
Ø Blank Dyeing
An
essential part of developing a colorant database is the blank dyeing. A blank
dyeing or mock dyeing as it is often called as 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 color change that results from the process can then be accounted for
when the blank dyeing is entered into the database.
Ø Selection of Dye Concentrations
There
are a few guidelines to follow when determining the concentrations to select
when preparing dye primaries, but there are no specific rules. Typically, eight
to twelve levels are selected that accurately characterize the build behavior
of each dye, or at least cover the range used in production. The number of
samples dyed will also be dependent upon the number of primaries the software
will accept.
Storing the Primary Data
After
the primaries have been prepared, the next step is to store them into the color
system’s database. Proper techniques in sample measurement must be used or all
of the effort put into accurately dyeing the primaries will be lost.
Ø Instrument Considerations
Before
measuring samples on the spectrophotometer, perform diagnostic tests to check
the accuracy of the measurements. These tests should include a drift test to
check for read to read repeatability, a diagnostic tile test to ensure long
term repeatability, and a standardization or calibration with a white tile.
Only the white tile calibration is required daily while the other diagnostic
tests can be performed on a weekly basis. Any poor test results should be
resolved prior to measuring any of the primary samples. Long term stability of the
instrument is critical due to the fact that primaries are often used for many
years. Spectrophotometers make use of one of two light sources like a tungsten
filament bulb or a xenon flash bulb.
Ø Measurement Technique
All samples should be measured multiple times with
the largest area view available on the
spectrophotometer being used. A repeatable
measurement technique based on the number of layers of material used and sample
rotation should be developed prior to measuring the primaries.
Now the numerical measurement of computer color
matching system are describing below.
The Mathematics of Computer Color Matching
For
a computer formulation program to work effectively there must be a mathematical
relationship between the concentrations of each dye used to produce a sample
and the sample’s color. It is not the purpose of this paper to discuss in
complete detail the mathematics of color matching, but a brief description will
follow. More detailed information may be obtained from a large assortment of
textbooks and articles .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 theratios 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 ¹:
(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
²:
K/S = ( 1 - R ) ² / 2R
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 / 2(0.2) = 1.6
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.
After this mathematical evaluation we need
some graphical analysis for primaries used inn K/S and this graphs are looking
like as shown as bellows:
And
finally the confirmation of these tests are made by particular options of data
color software and visual expression.
Conclusions
A
computer color matching system is not a “black box” that magically produces the
exact formula for a sample on demand. The accurate preparation of dye primaries
and the careful evaluation of the stored data are essential if a color matching
system is to be able to predict formulas with any consistency. As this is
accomplished, significant reductions in the number of dyeing required to match
new shades are made possible.
References
- Principles of color technology by Billmeyer & Saltsman
- Datacolor international NC.
Prepared by,
Bakhtiar Rana
Editor - In - Chiep
Textile Researchers
No comments:
Post a Comment