8.3. COLOR QUALITY PROCESSING 257
factors, metamerism, and texture and composition of the material (substrate),
has made visual coordination an acceptable methodology to obtain a repeatable
finish and color quality. Subjective assessment in color matching, especially in
colors that closely resemble one another,leads to laborious and time-consuming
adjustments that have to be performed to obtain the right concentration of the
colorants.
In the visual examination of colored objects, the standard ñ a fully char-
acterized substrate with known colorant concentrations ñ is compared with a
sample ñ a substrate whose characteristics are unknown, but with the same
colorant concentrations as the standard. These are usually placed side by side
and viewed at the same time. This visual observation is performed using stan-
dardized light sources to arrive at a consensus of whether or not the sample
and the standard match in their color representations. If the sample and the
standard do not match, the most important issue in this analysis that needs
consideration is by how much the sample and the standard deviate from one
another. In other words, it is not sufficient to know that the sample and the
standard do not match. We need to know the changes in the colorant concen-
trations to be applied to the sample substrate in order to ultimately match the
standard. This gives rise to an area of quantitative judgment in which the eye
is less adept than at judging whether two or more objects are identical in their
color, and therefore the situation requires the use of measuring instruments such
as spectrophotometers for color evaluation [7, 82].
Another important aspect in color evaluation is the issue ofmetamerism.
This is a phenomenon in which two or more colored objects viewed under one
illuminant may appear to match, while they do not match under another illu-
minant. This implies that the objects have different reflectance characteristics.
Therefore, a pair of objects having different spectral curves but the same color
coordinates are called metameric objects. They are said to exhibit metamerism.
Colorant evaluation is based on the computation of tristimulus values [82].
The tristimulus values, generally referred to by theX,Y,andZcoefficients, are
based upon the spectral energy and reflectance characteristics of the colorant
mixture (a mixture of primary colors) and the substrate (textile, paper, metal,
wood, plastic, etc.). The problem in color matching can therefore be stated as
follows: Given a standard comprising a colorant mixture of known concentra-
tions, sayC 1 ,C 2 ,C 3 , with tristimulus valuesX 0 ,Y 0 ,andZ 0 ,theobjective
is to determine the changes inC 1 ,C 2 ,C 3 such that the new colorant mixture
when applied to a different substrate sample will result in the desired tristimulus
valuesX 0 ,Y 0 ,andZ 0.
Suppose a colorant mixture comprising30%ofC 1 ,40%ofC 2 ,and30%of
C 3 when applied to a specific standard substrate produces a reference set of
tristimulus valuesX 0 =3. 0 ,Y 0 =4. 0 ,andZ 0 =5. 0 .If the same colorant mix-
ture were then applied to a sample fabric, the resulting tristimulus values would
be different due to the absorbance and reflectance characteristics of the new
substrate. To obtain the same tristimulus values as the reference, the colorant
concentrations have to be modified appropriately. Present technology involves
the solution of complex mathematical formulas only to obtain an approximate