Colour Management - Part Thirteen

Kiran Prayagi, print technologist and chairman, Graphic Art Technology & Education demystifies colour management in a series of articles. In this tenth article, he discusses pigments and dyes - deficiency.

30 Jul 2013 | By Kiran Prayagi

Article five in this series outlined some basic problems in colour reproduction. Also explained
was the differences in the colour reproduction systems and the colour perception by the
human eye. That is the first stage where faulty colours began to appear in the reproduction
systems. The second most important problem comes from the pigments and dyes used. All
suffer from some deficiencies that result in faulty colours unless some corrections are applied
in the process. In analogue photography and the printing processes CMY and CMYK
colorants are used, respectively. The ideal colorants are as shown in figure 1.



Looking at the visual colour spectrum and divided onto 3 sections of roughly 100 nanometers
each, the first one third appears blue, the middle one third green, and the last one third red.
These correspond to the basic colours of additive principles. When two thirds of the visible
spectrum is perceived the two basic colours combine to give one secondary colour that
correspond to cyan, magenta or yellow. The concept of ‘grey error’ and ‘hue error’ was
explained in article 10. With ideal colorants shown in figure 1 the reflecting colours are have
full energy and are reflected 100 percent and remaining one third is fully absorbed. The same
is indicated in figure 2 in a different way. Figure 2 shows full reflection of red, green, blue
colours from white surface. The same amount of reflection of two primary colours is obtained
from ideal cyan, magenta, yellow colorants.

If all colorants behave in ideal fashion the problems in colour reproduction would be solved to a great extent. Unfortunately, this is not the case. In real world the colorants behave the way shown in figures 3 and 4.



Figure 3 indicates the deficiencies of cyan, magenta, yellow colorants as ‘hue error’ and ‘grey error’. To understand this better the following explanation may help. Compare figures 1 and 2 with 3 and 4.


Yellow colorant absorbs blue light, magenta and yellow absorb green and blue, respectively.
Absorption of this one third of the spectrum makes the colorant appeared coloured it shows. If
the ideal colorant cyan is added with some quantity of ideal magenta and a little quantity of
ideal yellow then in this combination cyan colorant will absorb red, some quantity of magenta
will absorb green and a little quantity of yellow will absorb blue. The available cyan colorant
behaves exactly in this fashion as if it has contamination of magenta and yellow colorants.
If the ideal colorant magenta is added with some quantity of ideal yellow then in this
combination magenta colorant will absorb green and some quantity of yellow will absorb
blue. The available magenta colorant behaves exactly in this fashion as if it has contamination
of yellow colorants.

Yellow colorant is much better and very close to ideal yellow, hence deficiency can be disregarded.
 
In actual fact, each of the three inks is contaminated with the other two, but at this stage to
keep explanation understandable only major faults are highlighted. So when only available
cyan colorant is deposited on the white surface it is as if cyan, magenta, and yellow are
deposited as follows.
 
Available cyan = C m y

Similarly, for available magenta and yellow colorants
Available magenta = My
Available yellow = Y
 
But the matter is more complicated and is actually as follows.
Available magenta = Mcy
Available yellow = Ycm
 
Due to these ink deficiencies when equal quantity of available cyan, magenta, yellow
colorants is mixed it works as follows.
cyan (Cmy) + magenta (My) + Y = CMmYyy
 
With equal quantities the combination has maximum percentage of yellow and a minimum
percentage of cyan. This excess of yellow and then magenta makes combination appear
reddish brown rather than grey. To convert this in to grey a higher quantity of cyan dye is
used in the paper print and transparency photographs and a bigger dot of cyan in the printing
processes compared to magenta and yellow.
 
The explanation above relates mostly to ‘hue error’ of colorants. The ‘grey error’ make
colours dirty to some extent and makes it difficult to obtain clean colours.
 
To calculate the deficiencies precisely and apply correction mathematical analysis using
measurement instruments is possible and has been used. See article 10. To successfully apply
the rule of four as mentioned in article 11 colour correction is applied in the process.