Colour Management - Part Seven

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 importance of tonal separation.

03 Jun 2013 | By Kiran Prayagi

The second most important criteria, after ‘grey balance’ is ‘tone reproduction’. Tone reproduction affects how the details in the picture are reproduced. For good details it is very important that all tonal values or shades in the picture are properly separated.
Any perception in the human eye of the outside world is produced by various intensities of light entering in the eye. Light shades reflects more intense light compared to dark shades, and thus different intensities produce different shades and consequently the picture details. A ‘grey scale’ used by the photographers or in the printing processes helps to indicate proper rendition of tones. Similarly, on colour televisions or computer monitors ‘brightness’ and ‘contrast control’ settings are used to set tonal values right.
To illustrate it is best to understand single colour black and white reproduction first. Any two identical pictures reflecting same amount of light from any point of the picture will appear exactly the same as far as its visual reception is concerned. This will mean original and reproduction is exact match. This can be represented by a 450 curve on a graph. See figure 1.

Horizontal ‘X’ axis and vertical ‘Y’ axis can be any of the following.
1. Original scene vs television
2. Original scene vs photograph
3. Television vs photograph
4. Television vs printing process
5. Photograph vs printing process
Note : Photograph includes artist drawings and television includes computer monitor.

Figure 1
In black and white reproduction one is dealing with a single colour curve to achieve exact match. But in case of colour, there are three or four colour curves and when combined there combined intensities should match the single colour black and white curve, in terms of greys as well as tonal values. See figure 2, RGB process and figure 3, CMY process.

Figure 2 is for television, computer monitors, and digital cameras (all RGB reproduction systems) and figure 3 is for analogue photography and printing processes (CMY reproduction systems). In both the systems to achieve 450 line reproduction the individual three colour curves are required to be set something like indicated for good details in the picture. This is due to certain faults in the dyes and pigments and setting this way to achieve ‘grey balance’. As mentioned in the earlier articles due to binary nature of the printing processes ‘black’ ink is required to convert brownish tone in the deep shadows as well to improve its details. See figure 4. Also see figures 5, 12, 13. This is not necessary in the photographic processes as proportion three colour dyes can be changed to print grey in the shadow areas. The same is sometimes done in gravure printing processes.

Figures 5, 6, 7, 8 shows the effect on colour pictures when the tone curve changes. Figure 5 is
original picture, figure 6 is eliminates all tones between highlight and shadow. Figure 7 is similar but extreme highlight and shadow details are lost and midtones are more contrasty. Figure 8 has dirty highlights and lighter shadows.
Figures 9, 10, 11 have only one of the colours has change in tone. Figure 9 has cyan tone
change, figure 10 magenta tone change, and figure 11 yellow tone change. For special effects, sometimes tone value changes are effected deliberately or to provide pleasing effects or to improve on bad orgination.