Spectral Colours & RGB Colours

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This diagram shows the six spectral colours associated with the colours of the rainbow.

  • The top half of the diagram shows that each of these spectral colours can be produced by a single wavelength of light.
  • The bottom half of the diagram shows that three spectral colours (red, green and blue) can be combined in pairs to produce orange, yellow and violet.

Remember that:

  • Spectral colours are produced by a single wavelength of light. So each individual wavelength of visible light produces a different spectral colour.
  • Rainbows are produced by spectral colours as sunlight is refracted and dispersed by raindrops.
  • Red, orange, yellow, green, blue and violet are familiar spectral colours.
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Description

Spectral Colours & RGB Colours

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
What are spectral colours?
Spectral colours are all the colours between red and violet that can be produced by a single wavelength of light. Sunlight is composed of spectral colours.
Can orange, yellow and violet be produced by combining pairs of the spectral colours red, green and blue?
Yes! The spectral colours red, green and blue can be combined in pairs to produce orange, yellow and violet when using an additive colour model.
What are the three primary colours in the RGB colour model?
_x000D_ _x000D_ Red, green and blue are the three primary colours in the RGB colour model.
What are non-spectral colours?
A non-spectral colour is any colour that is not be produced by a single wavelength of visible light.

About the diagram

About the diagram
  • This diagram shows the six spectral colours associated with the colours of the rainbow.
  • The top half of the diagram shows that each of these spectral colours can be produced by a single wavelength of light.
  • The bottom half of the diagram shows that three spectral colours (red, green and blue) can be combined in pairs to produce orange, yellow and violet.
Remember that:
  • Spectral colours are produced by a single wavelength of light. So each individual wavelength of visible light produces a different spectral colour.
  • Rainbows are produced by spectral colours as sunlight is refracted and dispersed by raindrops.
  • Red, orange, yellow, green, blue and violet are familiar spectral colours.
  • The visible spectrum contains a continuum of spectral colours.
  • The wavelength corresponding with each colour is shown in nanometres (nm). The wavelengths selected for each colour are for illustration only.
  • In practice, the choice of wavelengths for primary colours usually depends on factors such as the colour model being used, the gamut of colours that a display device can produce and the context in which colours are to be viewed.
Remember also:
  • The visible spectrum is a small part of the electromagnetic spectrum.
  • Spectral colours should not be confused with RGB colours.
  • Spectral colours relate to the visible spectrum.
  • RGB colours are produced by mixing wavelengths using the three primary colours, red, green and blue.
  • A demonstration of RGB colour is often presented in the form of a colour wheel.
  • In a continuous spectrum of wavelengths, separate colours are indistinguishable to the human eye.
  • The fact that we see distinct bands of colour in a rainbow is an artefact of human colour vision.

Some key terms

RGB colour model
  • To be clear about the RGB colour model it is useful to remember first that:
    • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
    • A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
    • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
    • RGB colour is an entirely different approach to producing and managing colour.
  • RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
  • Except for the three primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
  • RGB colour provides the basis for a wide range of technologies used to reproduce digital colour.
  • RGB colour provides the basis for reproducing colour in ways that are well aligned with human perception.
  • When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue coloured lights they are able to create a match for a very extensive range of colours.
  • When looking at any modern display device such as a computer screen, mobile phone or projector we are looking at RGB colour.
  • Magenta is an RGB colour for which there is no equivalent spectral colour.
Electromagnetic spectrum

The electromagnetic spectrum includes electromagnetic waves with all possible wavelengths of electromagnetic radiation, ranging from low energy radio waves through visible light to high energy gamma rays.

Primary colour

Primary colours are a set of colours from which others can be produced by mixing (pigments, dyes etc.) or overlapping (coloured lights).

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to the electromagnetic spectrum that results in the perception of colour.
  • A set of primary colours is a set of pigmented media or coloured lights that can be combined in varying amounts to produce a wide range of colour.
  • This process of combining colours to produce other colours is used in applications intended to cause a human observer to experience a particular range of colours when represented by electronic displays and colour printing.
  • Additive and subtractive models have been developed that predict how wavelengths of visible light, pigments and media interact.
  • RGB colour is a technology used to reproduce colour in ways that match human perception.
  • The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998) and sRGB are the result of an extensive investigation of the relationship between visible light and human colour vision.
Wavelength

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

Visible spectrum

The visible part of the electromagnetic spectrum is called the visible spectrum.

  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • As light travels through the air it is invisible to our eyes.
  • Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and colours produced when different wavelengths are combined.
  • The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength.
  • The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
  • Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.

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