About terms

A family of closely related terms underpins all the content at lightcolourvision.org and we aim for accuracy and consistency of usage as we demonstrate the connections between light, colour and vision.

We want our resources to be accessible to a general readership of students, teachers and researchers of all ages who want to build on their existing knowledge one step at a time.

Each term has its own page in the REFERENCES section and starts with a brief definition. Bullet points follow that aim to provide context.

Links embedded in the text of RESOURCES pages link to the definitions. Shorter summaries also appear on pages within the RESOURCES section entitled SOME KEY TERMS. The purpose of the summaries is to provide a quick reference stripped down to basics.


When light strikes an opaque material the wavelengths that are not reflected are absorbed and their energy is converted to heat.

  • When sunlight strikes an opaque object or material, it can be reflected or absorbed.
  • If the light is reflected it bounces off at the same wavelength.
  • If the light is absorbed, its energy is transferred to electrons which re-emit the energy as heat.
  • Of the light that reaches Earth’s surface from the Sun:
    • 54% is already heat (infrared wavelengths of light) before it reaches the earth’s surface.
    • 45% is visible light.
    • 1% is shorter wavelengths (ultraviolet).
  • Absorption of a particular wavelength of light into a material takes place when the frequency of the wave matches the frequency of electrons within it. As electrons within atoms absorb energy and heat up they vibrate more vigorously causing the atoms to collide with one another.
  • Because the electrons in different atoms have different frequencies they selectively absorb different frequencies of visible light.




Accommodation refers to the way the eye increases optical power by changing the shape of the lens. This is necessary to produce a clear, focused, image of an object when it is near to the eye.

  • The eye focuses on a given object by changing the shape of the eye lens through accommodation. This is controlled by the ciliary muscle, which surrounds the lens.
  • The distance between the retina (the detector) and the cornea (the refractor) is fixed in the human eyeball. The eye must be able to alter the focal length of the lens in order to focus images of both nearby and far away objects on the retinal surface. This is achieved by small muscles that alter the shape of the lens. The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance remains constant.
  • The ability of the eye to adjust its focal length is known as accommodation. The eye accommodates by assuming a lens shape that has a shorter focal length for nearby objects in which case the ciliary muscles squeeze the lens into a more convex shape. For distant objects, the ciliary muscles relax, and the lens adopts a flatter form with a longer focal length.



Any colour that lacks strong chromatic content is said to be achromatic, unsaturated, or near neutral. Pure achromatic colours include black, white and all greys.

  • Achromatic, unsaturated colours include browns, tans, pastels and darker colours.
  • Achromatic, unsaturated colours can be of any hue or lightness.
  • Achromatic, unsaturated colours are obtained by mixing pure colours with white, black or grey, or by mixing two complementary colours.
  • In colour theory, neutral colours achromatic, unsaturated colours are easily modified by adjacent more saturated colours and appear to take on the hue complementary to the saturated colour.  Next to a bright red couch, a grey wall will appear distinctly greenish.


Additive colour

Additive colour involves mixing different wavelengths of light to produce other colours.

  • Whilst additive colour is the method used to mix wavelengths of light, subtractive colour is the method used to mix dyes, inks and paints.
  • An additive approach to colour is used in the case of emission of light from the (black) screens of mobile phones, computers and televisions.
  • An additive approach to colour is used in the case of the reflection of light off-white, neutral or black surfaces by digital projectors.
  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce other 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.
  • RGB colour uses three light sources or beams. Each 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 fully saturated red, green and blue primary colours are combined, they produce white.
  • When any two fully saturated additive primaries are combined, they produce a secondary colour: yellow, cyan and magenta.
  • Some RGB colour models can produce over 16 million colours by varying the proportion and intensity of each of the three 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 see subtractive colour.


Adobe RGB colour space

The Adobe RGB (1998) colour space is an RGB colour space developed by Adobe Systems, Inc.

  • The Adobe RGB (1998) colour space was designed to encompass most of the colours that can be output by CMYK colour printers.
  • When using RGB primary colours on a modern computer screen, the Adobe RGB (1998) colour space encompasses roughly 50% of the visible colours specified by the CIELAB colour space.
  • The Adobe RGB (1998) colour space improves on the gamut of the sRGB colour space, primarily in cyan-green hues.


Amacrine cell

Amacrine cells are interneurons in the human retina that interact with retinal ganglion cells and/or bipolar cells.

  • Amacrine cells interact with bipolar cells and/or ganglion cells. They are another type of interneuron which in this case monitor and augment the stream of data through bipolar cells and then control and refines the response of ganglion cell subtypes.
  • Amacrine cells are located in a central but inaccessible region of the retinal circuitry. Most are without tale-like axons. Whilst they clearly have multiple connections to other neurons around them their precise inputs and outputs are difficult to find. They are driven by and feedback to the bipolar cells but also synapse on ganglion cells, and with each other.
  • Amacrine cells are known to serve narrowly task-specific visual functions including:
    • Efficient transmission of high fidelity visual information with a good signal-to-noise ratio.
    • Maintaining the circadian rhythm which keeps our lives tuned to the cycles of day and night and so helps to govern our lives throughout the year.
    • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism) which enables edge detection and contrast enhancement.
    • Object motion detection which provides an ability to distinguish between the true motion of an object across the field of view and our own eye movements.
  • Centre-surround antagonism refers to the way retinal neurons organize their receptive fields.  The centre component is primed to measure the sum-total of signals received from a small number of cones directly connected to a bipolar cell. The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point. The two signals are then compared to find the degree to which they disagree.



The amplitude of a wave is a measurement of the distance from the top of a crest through the centre line  (the still position, zero-point, mid-point) to the bottom of a trough.

In the case of an electromagnetic wave, amplitude corresponds with the intensity of light and the brightness of colour perceived by an observer.

  • When the amplitude of an electromagnetic wave of a specific wavelength increases so does the overall distance between the peak and the trough of the wave.
  • The measurement taken between the topmost point on a wave (the peak) and the centre line of the wave is called peak amplitude.
  • Amplitude is measured in metres (m).
  • The greater the amplitude of a wave, the more energy it carries.
  • When amplitude increases so does the perceived vividness of a colour.
  • Whilst the perceived brightness of a colour depends on the amplitude of a light wave, brightness is also somewhat influenced by wavelength. So for example yellows tend to look brighter than reds or blues.


Analogous colours

Analogous colours are groups of colours that are adjacent to each other on a colour wheel, with one being the dominant colour.

  • The dominant colour amongst a group of analogous colours is usually a primary or secondary colour.
  • The immediately adjacent colours to secondary colours are tertiary colours.
  • An analogous colour scheme creates a rich, monochromatic look but is less vibrant than complementary schemes.
  • Red, reddish-orange, orange, yellow-orange is one example of a set of analogous colours.


Angle of incidence

The angle of incidence measures the angle at which incoming light strikes a surface.

  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.


Angle of reflection

The angle of reflection measures the angle at which reflected light bounces off a surface.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
  • Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction when referring to light passing across the boundary between two different transparent media, such as water, glass, or air.
  • In optics, the law is used in ray diagrams to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a medium.


An atom is the smallest particle that can be described as a chemical.

  • At the centre of an atom is a nucleus containing protons which are positively charged sub-atomic particles. The number of protons determines what chemical the atom is. A hydrogen atom has a single proton.
  • The nucleus also contains a number of neutrons, sub-atomic particles with no charge.
  • Surrounding the nucleus are very small negatively charged particles called electrons, held in place by their attraction to the positively charged nucleus.
  • Quantum mechanics is used to describe the orbital motion of electrons around a nucleus.
  • In an atom, the number of electrons always matches the number of protons.
  • If an electron is removed from an atom the result is a charged particle called an ion.
  • For a single atom, the number of protons and neutrons combined determines its atomic mass.
  • Atomic mass is a measure of the total mass of protons and neutrons in an atom and is used to order elements in the periodic table.


Attributes of visual perception

Attributes of visual perception are the innate abilities and the skills we develop over the course of a lifetime that enable us to make sense of what we see. They are evident in the diverse properties of the world we see around us.

  • Innate attributes of visual perception associated with the response of the human eye and brain to light include:
  • Colour perception: The ability to see colour in the presence of light including all the greys between black and white.
  • Visual attention: The ability to focus on important visual information and filter out unimportant background information.
  • Sensory processing: Accurate registration, interpretation and response through the coordination of visual information with other forms of sensory stimulation.
  • Visual discrimination: The ability to recognise differences or similarities in objects based on size, colour, shape, etc.
  • Spatial relationships: The ability to understand the relationships of objects, particularly their distance, direction of movement and position relative to an observer.
  • Figure-ground: The ability to locate something against a busy background.
  • Form constancy: The ability to know that a form or shape is the same, even if it becomes larger or smaller, or its orientation changes.
  • Visual closure: The ability to recognise a form or object when part of it is hidden or missing.
  • Visual memory: The ability to recall visual traits of a form or object.
  • Visual sequential memory: The ability to recall a sequence of objects in the correct order.

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.

Chemical bond

A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.

  • A chemical compound consists of two or more atoms from different elements chemically bonded together.
  • There are two types of chemical bond: covalent bonds and ionic bonds:
    • A covalent bond forms when two atoms share a pair of electrons.
    • Atoms can lose or gain electrons in chemical reactions. When they do this they form charged particles called ions.
  • Chemical bonds occur because opposite charges attract via the electromagnetic force.
  • Negatively charged electrons orbiting the nucleus of an atom and the positively charged protons in the nucleus attract each other.
  • An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond.
  • Due to the matter-wave nature of electrons and their smaller mass, they must occupy a much larger amount of volume compared with the nuclei, and this volume occupied by the electrons keeps the atomic nuclei in a bond relatively far apart, as compared with the size of the nuclei themselves.
  • The physical world is held together by chemical bonds, which dictate the structure and the bulk properties of matter.


Chromatic dispersion

Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.