Bands of colour

An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes between some colours better than others.

  • The human eye and brain together translate light into colour.
  • When sunlight is dispersed by rain and forms a rainbow, an observer often distinguishes red, orange, yellow, green, blue and violet bands of colour.
  • Although a rainbow contains electromagnetic waves with all possible wavelengths between red and violet, some ranges of wavelengths appear more intense to a human observer than others.

Bands of colour

An observer perceives bands of colour because:

  • The human eye is able to distinguish between some ranges of wavelengths of visible light better than others.
  • Some ranges of wavelengths appear more intense to a human observer than others.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • It is the human brain that draws lines between different bands of colour when an observer looks at a rainbow for example.
  • A human observer can distinguish between colours corresponding with many thousands of single wavelengths of light in the visible spectrum. These colours are called spectral colours.
  • Combinations of wavelengths from different areas of the visible spectrum produce other colours when perceived by a human observer which are called non-spectral colours.
  • There is no property belonging to electromagnetic radiation that causes bands of colour to appear to an observer. The fact that we do see distinct bands is often described as an artefact of human colour vision.
  • The visible spectrum is formed of a smooth and continuous range of wavelengths that can be demonstrated to produce a smooth and continuous range of colours.
  • Cone cells in our eyes are particularly sensitive to red, green and blue wavelengths.
  • Our brains process information received from the eye to produce all the colours of the visible spectrum.

Bipolar cells

Bipolar cells are a type of neuron found in the retina of the human eye. They are located between photoreceptors (rod cells and cone cells) and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.

  • Bipolar cells are connected to rod and cone cells by synapses. These cells are located within the retina between these photoreceptors and ganglion cells.
  • There are around 12 types of bipolar cells that function as integrating centres. Each type acts directly or indirectly, as a conduit from a photoreceptor to ganglion cells and each carries a different parsing of its output. So, each type of bipolar cell that contacts a given rod or cone transmits a different analysis and interpretation of information extracted from its output.
  • The output of bipolar cells onto ganglion cells includes both the direct response of the bipolar cell to signals derived from phototransduction but also responses to those signals received indirectly from information and actions provided by amacrine cells.
  • We might imagine a type of bipolar cell that connects directly from a cone to a ganglion cell and simply compares signals on the basis of what is known of their wavelength. The ganglion cell uses the information to determine whether a certain point is a scene is red or green.
  • Not all bipolar cells synapse directly with a single ganglion cell. Some channel information that is sampled by different sets of ganglion cells. Others terminate elsewhere within the complex lattices of interconnections within the retina enabling them to carry packets of information to an array of different locations and cell types.

Black body

An object that absorbs all radiation falling on it, at all wavelengths, is called a black body.


The brightness (luminance, brilliance) of an object refers to the quality, apart from hue and saturation that an observer uses to determine the comparative brightness of another object.

In terms of tonal differences, pure white has the maximum brightness, and pure black the minimum brightness.

In terms of colour, a pure yellow may appear to be much brighter than a pure blue.

  • Brightness is a colour coordinate in the HSB colour model.
  • Brightness is the perception elicited by the luminance of a visual target.
  • Brightness should not be confused with lightness.
  • In general terms, brightness is an attribute of visual perception used to refer to whether one object appears to be radiating or reflecting more or less light than another.
  • The field of photometry, the science of the measurement of light, recognises that the human eye is not equally sensitive to all wavelengths of visible light.