R G & B Lights on a Dark Surface

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This diagram shows the effect of projecting red, green and blue light onto a neutral coloured surface.

Understanding the diagram:

  • The diagrams illustrate how the RGB colour model works in practice.
  • Each torch emits light at the same intensity.
  • Each torch points towards a different area of the surface.
  • The light in each case is of a single wavelength so produces a spectral colour.
  • The selected wavelengths are: red = 660 nanometres (nm), green = 525 nm, blue = 460 nm.
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Description

R, G & B Lights on a Dark Surface

TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
What are the two primary colours of light that together make yellow?
Red and green are the two primary colours of light that together make yellow.
What are the two RGB primary colours that together make magenta?
Blue and red are the two primary RGB colours that together make magenta!
Why is light colourless
Although pure white light is perceived as colourless, it actually contains all colours in the visible spectrum.
Do mixtures of RGB primary colours produce spectral colours?
No! The colours produced by mixing primary colours are not spectral colours. Spectral colours are produced by a single wavelength of light.

About the diagram

About the diagram
  • This diagram shows the effect of projecting red, green and blue light onto a neutral coloured surface.
  • Imagine that the three circles of colour (red, green and blue) are produced by torches shining beams of light so they overlap one another.
  • Overlapping pairs of primary colours produce secondary colours.
  • Overlapping pairs of secondary colours produce primary colours. (No surprise there!)
  • The area where all three primary colours overlap is white.
Understanding the diagram
  • The diagrams illustrate how the RGB colour model works in practice.
  • Each torch emits light at the same intensity.
  • Each torch points towards a different area of the surface.
  • The light in each case is of a single wavelength so produces a spectral colour.
  • The selected wavelengths are: red = 660 nanometres (nm), green = 525 nm, blue = 460 nm.
About the RGB colour model
  • RGB colour is an additive colour model that combines wavelengths of light corresponding with the red, green and blue primary colours to produce other colours.
  • RGB colour is called a model because it is a method that can be followed to produce a full gamut of colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce all other colours.
  • Each of the three beams is called a component of the resulting colour.
  • Different colours are produced by varying the intensity of the component colours between fully off and fully on.
  • When any two fully saturated additive primaries are combined they produce a secondary colour: yellow, cyan and magenta.
  • When fully saturated red, green and blue primary colours are combined they produce white.
  • Some RGB colour models can produce over 16 million colours by varying the proportion and intensity of each of the three component primary colours.
  • The additive RGB colour model cannot be used for mixing different colours of pigments, paints, inks, dyes or powder. To understand these colourants find out about subtractive colour.

Some key terms

Secondary colour

A secondary colour is a colour made by mixing two primary colours in a given colour space. The colour space may be produced by an additive colour model that involves mixing different wavelengths of light or by a subtractive colour model that involves mixing pigments or dyes.

White light

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.
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.
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.
Light source
  • A light source is a natural or man-made object that emits one or more wavelengths of light.
  • Natural light sources include:
    • The Sun is the most important natural light source in our lives and emits every wavelength of light in the visible spectrum.
    • Celestial sources of light include other stars, comets and meteors.
    • Other natural sources of light include lightning, volcanoes and forest fires.
  • There are also bio-luminescent light sources including some species of fish and insects as well as types of bacteria and algae.
  • Man-made light sources include:
    • Man-made light sources of the most simple type include natural tars and resins, wax candles, lamps that burn oil, fats or paraffin and gas lamps
    • Tungsten lights: These are a type of incandescent source which means they radiate light when electricity is used to heat a filament inside a glass bulb.
    • Halogen bulbs: These are more efficient and long-lasting versions of incandescent tungsten lamps and produce a very uniform bright light throughout the bulb’s lifetime.
    • Fluorescent lights: These are non-incandescent sources of light. They generally work by passing electricity through a glass tube of gas such as mercury, neon, argon or xenon instead of a filament. Fluorescent lamps are very efficient at emitting visible light, produce less waste heat, and typically last much longer than incandescent lamps.
    • LED lights: An LED (Light Emitting Diode) is an electroluminescent light source. It produces light by passing an electrical charge across the junction of a semiconductor. An LED light typically emits a single colour of light which is composed of a very narrow range of wavelengths.
  • Made-made lights can emit a single wavelength, bands of wavelengths or combinations of wavelengths.
Colour wheel
  • A colour wheel is a circular diagram divided into segments, featuring primary colours, and used to visualize the result of colour mixing.
  • Colour wheels can enhance understanding of colour relationships and assist with the accurate selection and reproduction of colours.
  • A colour wheel starts with segments representing primary colours. Additional segments are added between them to explore the outcome of mixing adjacent primary colours.
  • By adding more segments between existing ones, further mixing of adjacent colours can be explored.
  • A colour wheel exploring the additive RGB colour model starts with red, green, and blue primary colours.
  • A colour wheel exploring the subtractive CMY colour model starts with cyan, magenta, and yellow primary colours.

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