Colour Management - Part Nine

In colour reproduction, measurement of colour is very important at every stage to make sure the ultimate objective of reproducing right colours is achieved efficiently. There are various methods of measuring colour as follows. 

1. Human memory

2. Colour charts

3. Colour reference catalogues

4. Densitometers

5. Colorimeters

6. Spectrophotometers

Human memory

From childhood we observe the nature with its various colours and develop strong bonds with natural surroundings. We are also told that water is blue, grass is green, human skin is brown, etc.

Blue sky or water has variations of blues from cyan-blue to greenish-blue and when its reproduction meets anywhere between these variations our memory colour accept it. Similar is the case with grass and foliage, which ranges from bluish-green to yellowish-green and skin tones from light pink to dark brown. See figure 1.

Colour charts

Colour charts are the combinations of basic colours (red-green-blue) and secondary colours, e.g. printing primaries (cyan-magenta-yellow-black).

These charts are useful where reproduction is carried out using these basic colours. RGB charts are useful to work on colour computers for soft viewing whereas the CMY and CMYK charts are useful in printing processes with ink-on-paper.

RGB charts are defined and measured in terms of ‘grey values’. Grey value of ‘0’ indicates absence of colour and ‘255’ indicates full strength colour. In between values give various shades of the same colour. Thus there are three colour scales, one each for red, green, and blue. Its different combination values gives various colours.

CMY or CMYK charts are defined and measured in terms of ‘dot percentages’. Dot percent of ‘0’ indicates base unprinted substrates and ‘100’ indicates full strength colour. In between values give various shades of the same colour. Thus there are three or four colour scales, one each for cyan, magenta, yellow, and black. Its different combination values in dot percentages give various colours.

 

Customer sample is compared with the closest match in the colour chart. Colour values of that particular patch in the colour chart is read and assigned to the colour in question in the reproduction. This method has been followed in the printing processes for the last almost over 70 years and has been working satisfactory.

However, the problems are:

a. Substrates used in production rarely match substrates used for colour charts

b. Printing inks used in production may be different than used for colour charts

c. Printing ink densities in production may be different than colour charts

d. Dot gain in production may not match the one in colour charts

e. Colour ink sequence in production may be different than colour charts

f. Since this is a visual colour measurement system viewing light is very important

Colour reference catalogues

Colour reference catalogues go beyond the basic three- or four-colours and its combinations. For more accurate colour reproduction of company logos, product colours, etc. special colour inks are necessary. Colour reference catalogues are useful for this purpose. See figure 4.

 

 

Various colour combinations of basic 14 or so ink colours at the ink mixing stage give lots of colours that can be directly applied on the substrates through the printing processes. This is applied as 100 percent dot, i. e. solid colour for good colour match.

These catalogues are available from Pantone, Focoltone, Toyo, DIC, ANPA, HKS, TruMach, etc.

However, many of the problems mentioned under ‘colour charts’ above also apply here.

Densitometers, colorimeters, and spectrophotometers are the instrument measurement systems that need special explaination, instrument structure, mathematics involved, and so on. The next article will deal with densitometry.