Colour Management - Part Five

Kiran Prayagi, print technologist and chairman, Graphic Art Technology & Education demystifies colour management in a series of articles. In this fifth article, he discusses the problems in colour reproduction.

16 May 2013 | By Kiran Prayagi

Colour reproduction in any field has some inherent reproduction problems that make’s it impossible to match most colours, however, the technology available and the adaptability of human eye hides many of the colour matching faults as they exist in reproduction.

There are various reasons for colour changes in the reproduction processes and can be described as follows. In the reproduction systems the various components used, such as lenses, mirrors, dyes, digital camera sensors, colour monitors and television phosphors, printing inks, etc. have certain characteristics for light and colour transmission and response. To reproduce colours as human eye perceives it these various components should match the various corresponding parts in the human visual system, such as cornea, lens, iris, retina, cones, etc. This is not so.

Figure 1 above shows the sensitivity of the cones in the human eye to the visible colour spectrum in red, green, and blue arears. The red cone has the negative sensitivity in the blue-green area. This is like taking the red colour away from blue-green making these colours to appear highly saturated such as green leaves on the tree. The kind of colour saturation we experience in nature cannot be achieved in the reproduction processes as no negative values can be created like in the human eye. It is not possible to take out red colour from the base paper or television screen and in effect the blue-greens are reproduced as if some red is mixed in it and thus lower the saturation of these colours compared to what we see in nature. See Figure 2. This is the first step in faulty colour reproduction.

Secondly, human eye has a very good adaptability to the viewing conditions, whereas this is not the case with reproduced images either in soft or hard format. For example, when a particular scene, such as room interior, viewed in daylight or under artificial light it appears almost the same. Human eye adapts its spectral sensitivity to different lighting. But the reproduction systems cannot. This is clearly seen in the photographed images. See figure 2 and 3.

Figure 3 photograph is shot in bluish light and figure 4 photograph in yellowish light. Since photography cannot compensate its sensitivity like a human eye different types of films are designed for different lighting conditions. The same principle is applied in digital photography.

The reproduction system components colour production system is as follows.

Figure 5 shows the spectral transmission characteristics of filters and sensors used in the reproduction system. Figures 6, 7, and 8 are the colour measurement density curves of red, green, and blue filters against the ink and dyes reflection curves for which they are used. Figure 9 and 10 shows the spectral colour characteristics of CCDs and figure 11 of red, green, blue phosphors of television screen.

From all the colour separation and reproduction systems it is seen that none has the spectral characteristics matching the human eye. These characteristics pose serious problems in colour reproduction systems’ for good colour match and therefore calls for ‘colour correction’ during the process.

Any wavelength striking the eye gives rise to a visual stimulus. Within the overlapping region of human cones, see figures 1 and 2, these wavelengths stimulates both the cones unlike reproduction systems.

Each process has to be looked at separately, however the basic underlying principles are the same. These will be looked at in future articles.