Colour Management - Part Twenty

Kiran Prayagi, print technologist and chairman, Graphic Art Technology & Education demystifies colour management in a series of articles. In this twentieth article, he discussed CIE Systems.

17 Sep 2013 | By Kiran Prayagi

Last article 19 described the CIELab and CIELch systems of CIE Colorimetry in detail. How
it is applied for colour match or to give precise difference in two colours is discussed here.






Difference between two colours in each of the three values, i. e. L, a, b or L, c, h is the
arithmetic difference between the co-ordinates of the reference and sample colours, colours 1
and 2 in figures 1 and 2.
 
Δ L = L1 - L2
Δ a = a1 - a2
Δ b = b1 - b2
Δ c = c1 - c2
Δ h = h1 - h2



Delta E is the total difference between two colours. This puts together all the three parameters and work like a rubber band. When difference in one of the three value increases the tolerance in other one or two values decreases, see figure 3. 

The ellipses in figures 4, 5, 6, and 8 indicate that colour plot falling anywhere within the ellipse do not show any visual colour difference. The size and shape of ellipses change with different formulae of delta E. 

Delta E 1976 equations and figure 4
Δ Eab = ( Δ L2 + Δ a2 + Δ b2 ) ½
Δ Ech = ( Δ L2 + Δ c2 + Δ h2 ) ½


Delta E 1994 equation and figure 5
Δ E94 = { [ ( Δ L2 ) / ( KL SL ) ] + [ ( C ab 2 ) / ( Kc Sc ) ] + [ ( Δ H2 ) / ( KH SH ) ] } ½



CMC stands for Colour Matching Committee of the Society of Dyers and Colourists. This formula has two versions based on the human vision. Change in lightness of colour is less noticeable than hue and saturation, therefore, acceptability of any colour match has higher tolerance of lightness value.
 


L = c = h - This is known as CMC 1 : 1 where ratio of luminance to chrominance is same and is based on perceptibility of colour. 2 L = c = h - This is known as CMC 2 : 1 where ratio of luminance is twice that of chrominance and is based on acceptability of colour, see figure 7.