Visit Us:  
Find:  

FAQ & Tips - Light and Colour

We live in a colorful world. With the help of colours we brighten up our surroundings to make us feel good. Interior design and colour schemes directly influence our impressions and feelings. Colours that go well together create a harmonious balance, putting us in a good mood. The printing industry also uses colours to make presentations more effective. The quality demands on printed materials supplied to customers are increasing all the time. To meet these new demands, new standards of quality have to be introduced. In order to judge colours we have to “see” them. For this purpose we need light.

The sun emits light – it is a primary light source.

Most objects in our environment, however, do not emit light themselves. They are so-called secondary light source. We can perceive them and their colours only if they are illuminated by light.

Light is radiation which propagates very quickly – at a speed of 300,000 kilometers per second. Strictly speaking, light consists of electromagnetic oscillations spreading out from their source like waves. Like a water wave, each light wave consists of a crest and a trough.

 

 

Waves are classified on the basis of either their length or the number of oscillations they perform per second. Wavelengths are given in units such as kilometers, meters, centimeters, millimeters, nanometers or picometers. The number of oscillations per second – the frequency – is measured in Hertz.

Waves of different lengths possess different properties. X-rays, for examples, are used in medicine for diagnostic purposes, while many households are already equipped with microwaves ovens. Other kinds of waves serve to transmit telephones calls as well as radio and television programs.

Only a very small range of electromagnetic waves is seen by us as coloured light. The visible portion of the waves spectrum lies between 380 nm (ultraviolet light) and 780 nm (infrared light). By means of a prism, light can be broke up into its colour components, White lights, being composed of all the colours of the spectrum, is broken up into all the colours of the rainbow.

The following illustrations shows how the wavelengths from red to green to blue become shorter and shorter.
 

Visual perception of colour

It is only in conjunction with light that colours become “visible” – but why?

Colour can not be regarded as a characteristic feature of an object, as can its shape. Yet it is a property of objects to either absorb or reflect light of certain wavelengths. We can only perceive colours that corresponds to the reflected wavelengths.

If white light reaches an object, one of the following may occur:

  • All the light is absorbed. In this case we perceive the object as black.
  • All the light is reflected. In this case the object appears as white.
  • All the light is let through the object. In this case the colour of the light does not change.
  • Part of the light is absorbed, the rest is reflected. We perceive a colour whose hue depends on which wavelengths are reflected and which are absorbed.
  • Part of the light is absorbed, the rest is transmitted. We see a colour whose hue depends on which wavelengths are absorbed and which are transmitted.
  • Part of the light is reflected, the rest is transmitted. Under these circumstances the colour of the reflected and that of the transmitted light changes.

The properties of the illuminated object determine which of the above-mentioned effects is likely to occur.

Light reflected or transmitted by an object is received by our eyes and transformed into nervous impulses, which trigger the colour sensation in our brain.

 
 

The retina of the human eye contains light-sensitive cells. There are two kinds of cells: rods and cones. The rods distinguish between bright and dark., whereas the cones react to colours. There are three kinds of cones, each of which is sensitive to certain wavelengths. Part of them reacts to light within a range of 400 to 500 nm and is therefore sensitive to blue light. Other cones can “see” only within a range of 500 to 600 nm, i.e. only green light. The third kind of cone is receptive to red light, which is lies within a range of 600 to 700 nm.

This composition of rods and cones renders the human eye so sensitive that it is capable of perceiving and distinguishing millions of colours.

Colour mixture

A. Additive colour mixture

An additive mixture of colours is a superimposition of light composed of different colours. If all the colours of the spectrum are added together, the colour white results.

Red, green and blue are the additive primary colours. They are so-called one-third colours because each represents one third of the visible spectrum. The principle of the additive colours mixture can be illustrated very well with three diascopes, each of which produces a light spot on a screen in one of the three additive primary colours.

green + red = yellow

green + blue = cyan

blue + red = magenta

blue + red + green = white

no light = black

*** Within the overlapping areas of the three light spots the following secondary colours emerge:

The principle of additive colour mixture is used in colour TV and in the theater to produce all the colours of the visible spectrum.

B. Subtractive colour mixture

For subtractive colour mixture individual colours components are taken from white light. If all the colour components are removed, the colour black results.

Cyan, magenta and yellow are the subtractive primary colours. They are two-third colours because each represents two thirds of the visible spectrum.

They can be produced by subtracting an additive primary colour from white light (for example by means of a filter), or by superimposing the light of two additive primary colours.

Printing inks are transparent substances that function like colour filters. Which colour results if a blue-absorbing substance is printed on white paper?

Blue is removed from white light; the other components (green and red) are reflected. The additive superimposition of these two colours produces yellow. This is the colour we perceive. The printing ink has thus subtracted one third (i.e. blue) from the white light (consisting of red, green and blue).

 
Let us assume that two transparent substances are printed one upon another, for example, the printing inks “yellow” and “cyan”. The substances successively filter the blue and red portion from the white light. As a result, we perceive green light. Together, the printing inks have subtracted two third of the colour components.  
 
When cyan, magenta and yellow are printed in layers one upon another the incident light is totally absorbed, (i.e. there is no reflection); we perceive the colour black.  
 

cyan + yellow = green

yellow + magenta = red

magenta + cyan = blue

cyan + magenta + yellow = black

no color = white

*** In subtractive colour mixture the following secondary colours will result when cyan, magenta and yellow are printed one upon another.

 

C. Autotypical colour mixture

Colour images are printed using the four printing inks cyan, magenta, yellow and black The black printing ink improves the sharpness and depth of pictures. This is because, due to the properties of the pigments of chromatic colours, the black colour subtractively mixed from cyan, magenta and yellow, is never really dark black as such.

In offset printing the size of the dots depends on the desired hue. When printed, the dots of the individual colours are partly juxtaposed or partly or totally printed one upon another. If we look at the dots through a magnifying glass, we perceive colours which – except for the white of the paper – are the result of subtractive colour mixture. However, without a magnifying glass and from a normal distance, the human eye cannot discern the individual dots. In this case the printed colours are mixed additively.

The composition of additive and subtractive colour mixture is called autotypical colour mixture.
© Heidelberger Druckmashinen AG