The Human Eye and RGB Colour




To find out more about the diagram above . . . . read on!

The Human Eye and RGB Colour

Look carefully at the diagram at the top of the page. Now check out the following questions (and answers)!

  1. What are spectral colours?
  2. What are RGB colours?
  3. What is a cone cell?
  4. Does the peak response of cone cells in the human eye correspond directly with RGB?
  5. Why do objects appear to be different colours to an observer?

About the Diagram

Introducing the diagram! Read back and forward between the image at the top of the page and the explanation below!

This diagram is about the perception of colour. It explores how our eyes respond to wavelengths of light corresponding with red, green and blue.

  • As we look at the world on a sunny day a vast range of wavelengths of visible light enters our eyes. Each wavelength corresponds with the perception of a different spectral colour.
  • When wavelengths of light corresponding with only red, green and blue enter the eye in different proportions, our eyes are still able to see a full gamut of colours.
  • The colours we see when wavelengths corresponding with just red, green and blue light are called RGB colours.
  • Because RGB colours are produced by mixing red, green and blue light in different proportions they are not spectral colours.
  • The RGB colour model does not correspond directly with the light sensitivity of the three different types of cone cells in the human eye. Cone cells work together and cross-reference the information they receive to deduce colour.
  • RGB colours include magenta which is produced by mixing red and blue in different proportions in the absence of green.

Some basic facts:

  • Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space.
  • Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
  • The experience of colour is a feature of human vision that depends first of all on the construction of our eyes and then on the particular wavelength, frequency and amplitude of visible light that strikes the retina at the back of each eye at any particular moment.
  • Light enters the eye through the cornea, through the pupil and then through the lens. The lens shape is changed for near focus and controlled by the ciliary muscles.
  • Photons of light falling on the light-sensitive cells of the retina (cones and rods) are converted into electrical signals that are transmitted to the brain by the optic nerve and results in sight and vision.

Now a summary:

Things have colour because light has properties that are visible to the human eye. But what does this mean?

  • The colour of objects depends firstly on the light source and the wavelengths it emits.
  • The way any object appears to an observer depends on the material it is made from and what happens when light strikes (or is emitted from) its surface.
  • Light striking an object may undergo absorption, dispersion, reflection, refraction, scattering or transmission.
  • The appearance of an object to an observer also depends on the mental processes that lead to colour perception.
  • Spectral colours are produced by a single wavelength of light or by a band of similar wavelengths.
  • RGB colour is an additive colour model in which red, green and blue light is added together 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.

Remember that:

When light strikes an object at least one of the following things happens:

  • Absorption. When light strikes an opaque medium the wavelengths that are not reflected are absorbed and their energy is converted to heat.
  • Dispersion. Chromatic dispersion refers to the way that light separates into its component wavelengths and the colours corresponding with each wavelength become visible.
  • Reflection. The term reflection refers to a situation where light strikes the surface of an object and some wavelengths are obstructed and bounce off. If the surface is smooth, light is reflected away at the same angle as it hits the surface. The term reflection refers then to what happens to wavelengths of light that are neither absorbed (by an opaque medium) nor transmitted (through a transparent medium).
  • Refraction. The term refraction refers to the way a light wave changes direction and speed as it travels from one medium to another.
  • Scattering. Scattering takes place when light waves are reflected in random directions at the boundary between two media. Scattering can also take place when light strikes particles or other irregularities within a medium through which light propagates.
  • Transmission. In optics, transmission refers to the passage of electromagnetic radiation through a medium.

Follow the blue links for definitions . . . . or check the summaries of key terms below!

Some Key Terms

Move to the next level! Check out the following terms.

Cone cell

Cone cells, or cones, are one of three types of photoreceptor cells (neurons) in the retina of the human eye ...
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A human observer is a person who engages in observation by watching things. In the presence of visible light, an ...
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Primary colour

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

To be clear about RGB colour it is useful to remember first that: The visible spectrum is the range of ...
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Spectral colour

A spectral colour is a colour evoked in normal human vision by a single wavelength of visible light, or by ...
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