Colour Management - Part Twenty Four

Kiran Prayagi, print technologist and chairman, Graphic Art Technology & Education demystifies colour management in a series of articles. In this twenty fourth article, he discusses Digital Colour Imaging.

22 Oct 2013 | By Kiran Prayagi

In the reproduction systems most analogue methods have given way to the digital technology, whether in photography, printing on various substrates or textile, television, or computer graphics. Analogue methods refer to all manual methods where no computers are used but with the advent of digital computers the term analogue is also used for analogue methods of computing data. Analogue signal varies continuously according to the information, therefore, the data is represented in a continuous form. A device converting this continuous representation in mathematical calculations using electrical voltages is called analogue computer. The final output is in continuous form analogue signal. Digital information is based on digits representing data in physical quantities of binary digits of 0 and 1. Digital signal is a discrete or discontinuous electrical signal.

Digital imaging fundamentals
The basic raw materials which are building blocks of our world, such as carbon, aluminium, copper, etc. are known as elements and are the most basic form of material difficult to break down any further. If elements are broken down then the smallest possible part of an element is called an atom. Atoms are very fine, eg: one gram of copper contains 10,000,000,000,000,000,000,000 atoms which would take 15,748 men year to count @ 5 atoms per second. Further breakdown of atom give electrons, protons, and neutrons. Electrons carry negative charge while orbiting around the nucleus. Nucleus consist of protons which carry positive charge and neutrons are neutral. Atom is normally in a neutral state as negatively charged electrons and positively charged protons balance out each other. See figure 1.

When electromotive force, called voltage, is applied electron from the outermost orbit is dislodged and travels to the next atom. In this process the atom from which electron is dislodged becomes positively charged due to loss of some negative charge and the next atom that gains dislodged electron becomes negatively charged due to extra electron. In this process atoms have natural tendency to become electrically neutral and, therefore, the flow of electrons, called current, continue as long as voltage is applied. See figure 2.

In digital computers only two digits 0 or 1 rule the world of computers. 0 is assigned when no current flows and 1 is assigned when current flows. Various combinations of 0 and 1 constitute a signal and because the information is represented in two digits it is called binary digits or in short bits. For meaningful information minimum of eight digits or bits, called byte, are required.

0 = no current flows
1 = current flows
A = 01000001 a = 01100001
B = 01000010 b = 01100010
1 = 00110001 2 = 00110010
The same principle when applied to pictures and graphics it works as follows. One bit information can have two shades, i. e. when current flows the impression obtained is white and when no current flow the impression is black. With one bit the possible combinations are two shades, two bits four shades, three bits eight shades, four bits 16 shades and so on. 24 bits give 16.7 million and 32 bits give 4.2 billion colours. These shades are called grey levels. See figures 3 and 4. Grey levels affect the separation of tones and its smoothness in the picture. For good smoothness of tones without banding, i. e. harsh jump of tones, 256 grey levels are required. See figure 5.

Figure 3 - Bits and grey levels, result of possible on-off conditions


When it comes to colour reproduction there are two possibilities. In computer and television graphics three colours, red, green, blue are used, whereas in printed pictures cyan, magenta, yellow are used in analogue photography and cyan, magenta, yellow, black are used in photomechanical printing processes. In such cases 256 grey levels of each of the component colours are necessary for good tonal effect. See figures 6 and 7.

In RGB pictures the number of bits are:
8 bit red + 8 bit green + 8 bit blue = 24 Bits

and grey levels obtained are :
256 red x 256 green x 256 blue = 16.7 million
In CMYK pictures the number of bits are:
8 bit cyan + 8 bit magenta + 8 bit yellow + 8 bit black = 32 Bits

and grey levels obtained are :
256 cyan x 256 magenta x 256 yellow + 256 black = 4.2 billion
RGB pictures reproduction principles outlined above apply equally well to image capturing by colour scanners and digital cameras. CMYK principles are also applicable to colour scanners where capturing is done in CMYK like earlier special purpose graphic arts scanners supplied by Crosfield Electronics, Dr. Hell, Itek Graphics, Dianippon Screen, PDI, Scitex. File size naturally changes depending on the number of component colours and it is always 25 percent more in CMYK compared to RGB pictures.

Digital measurements
As explained earlier bit (binary digit) is a single digit giving only two grey levels. This situation exist in single colour text or line drawings where various shades or tones of the same colour are absent. See figure 8. 

A quick brown fox jumps over the lazy dog

However, to produce these images for meaningful representation the digital computer needs
eight bit signal as explained. This eight bit signal is called one byte. Digital measurements are
as follows.
One BYTE is eight BITS and represents one character, like A, B, C, D, ..........

One KILOBYTE is 1,000 BYTES or 8,000 BITS represent 1,000 characters.

One MEGABYTE is 1,000 KBYTES, or 1,000,000 BYTES, or 8,000,000 BITS. This represents 1,000,000 characters or single colour 11 square inches picture at an average resolution of 300 pixels per inch.

One GIGABYTE is 1,000 MBYTES or 1,000,000 KBYTES or 1,000,000,000 BYTES or
8,000,000,000 BITS. This represents 1,000,000,000 characters or single colour 111 A4
size or 27 four colour A4 size pictures at an average resolution of 300 pixels per inch.