Combining Spectral Colours to Make White

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This diagram looks at three situations in which an observer will see white light reflected off a dark surface.


Three different light sources are shown in the diagram.

  • Each light source produces a different combination of wavelengths and each wavelength has the same intensity.
  • The light source at the top of the diagram contains three component colours: RGB (red, green, blue).
  • The light source on the left contains six component colours – ROYGBV (red, orange, yellow, green, blue and violet).
  • The light source on the right contains a continuous spectrum of wavelengths which means it contains many thousands of component colours.
  • In each case, the light source is projected onto a dark neutral coloured surface.

Description

Combining Spectral Colours to Make White

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
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.
Yes! When spectral colours are arranged in a diagram, the order in which they appear corresponds with their wavelength. The easiest way to show this is a band of colours of increasing wavelength with red at one end and violet at the other.
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.

About the diagram

About the diagram
  • This diagram looks at three situations in which an observer will see white light reflected off a dark surface.
  • Three different light sources are shown in the diagram.
  • Each light source produces a different combination of wavelengths and each wavelength has the same intensity.
  • The light source at the top of the diagram contains three component colours: RGB (red, green, blue).
  • The light source on the left contains six component colours – ROYGBV (red, orange, yellow, green, blue and violet).
  • The light source on the right contains a continuous spectrum of wavelengths which means it contains many thousands of component colours.
  • In each case, the light source is projected onto a dark neutral coloured surface.
Remember that:

The surface where the light sources are focused appear white:

  • When wavelengths corresponding with the three primary colours (RGB) are reflected off the surface towards an observer.
  • When wavelengths corresponding with ROYGBV are reflected off the surface towards an observer.
  • When all the wavelengths of the visible spectrum are reflected off the surface towards an observer.

Some key terms

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 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.

White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.

  • As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
  • As light travels through the air it is invisible to our eyes.
  • When we look around we see through the air because it is very transparent and light passes through it.
  • The term white light doesn’t mean light is white as it travels through the air.
  • One situation in which light becomes visible is when it reflects off the surface of an object.
  • When white light strikes a neutral coloured object and all wavelengths are reflected then it appears white to an observer.

ROYGBV is an acronym for the sequence of hues (colours) commonly described as making up a rainbow: red, orange, yellow, green, blue, and violet.

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.
  • 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.

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