Colour difference equations and the human eye

Since colours could be measured various colour difference equations were used to describe the difference between two colours. Unfortunately the numbers generated from the various equations were not interchangeable and each number had a different meaning depending on which difference equation was used.

This was rectified by the introduction of the CIE DE equation in 1976. A DE of 1.0 was deemed to be the smallest colour difference perceivable by the human eye. It was soon discovered that the DE equation did not take into consideration that the human eye is more sensitive in some regions of the visible spectrum then in others. This means that a DE of 1.0 could be a small visible difference in one area of the visible spectrum (i.e. dark blue colours) and a large difference in another area (i.e light pastel type colours).

In 1984 the CMC (Colour Measurement Committee of the Society of Dyes and Colourists of Great Britain) developed an equation that is based on the L*C*h*-notation of colours that takes the various different colour sensitivities of the human visual system into consideration and a DE under CMC of 1.0 gives the same visual difference in all regions of the spectrum.

In 1995 the CIE introduced a colour difference equations that is similar to the CMC equation and takes also into consideration the sensitivities of the human visual system. This equation also has its weak points, which lead to the introduction of the CIEDE2000 equation. CIEDE2000 takes also the varying sensitivity in regards to Lightness into consideration to determine the difference between two colours.

Now there are 4 colour difference equations available. The fastest and easiest way to calculate colour difference is the DE76 formula, but it has its drawbacks. Most widely used in the graphic arts field are probably the DE94 and/or DECMC formula.

The question of this study was, is one formula better then the other and after which numerical difference is it not important which equation is being used, since the difference is quite large. People from various walks of live with mostly untrained eyes were asked to sort 41 colour chips that went from dark colour to pastel colour. It was tried to equally distance the colours in colour space. The results of this study will be presented in this paper.