About this dictionary

This DICTIONARY OF LIGHT, COLOUR & VISION contains a vocabulary of closely interrelated terms that underpin all the resources you will find here at lightcolourvision.org.

  • Each term has its own page in the DICTIONARY and starts with a DEFINITION.
  • Bullet points follow that provide both context and detail.
  • Links embedded in the text throughout the site (highlighted in blue) take you directly to DICTIONARY entries.
  • Shorter SUMMARIES of terms appear on DIAGRAM PAGES under the heading SOME KEY TERMS. These entries strip definitions back to basics and can be viewed without leaving the page.
Why a dictionary of light, colour & vision
  • One of the practical objectives of this website is to make the connections between the topics of light, colour and vision accessible to students and researchers of all ages.
  • Our DICTIONARY aims to avoid a problem faced by websites such as Wikipedia where articles are often composed by contributors with narrow specialisation and their own topic-specific vocabulary.
  • The layout of the DICTIONARY also aims to avoid situations where a single unknown word or phrase makes it difficult, if not impossible, for our visitors to find the information they need (as explained below).
Terms, definitions and explanations
  • All the terms we have selected for the DICTIONARY are widely used and are applied consistently across the topics of light, colour and vision.
  • The aim is to avoid definitions and explanations with different meanings in different fields.
  • As far as possible definitions contain no more than two short sentences.
  • The explanations that follow each definition are arranged as short bullet points that avoid paragraphs of information completely.
  • Each bullet makes a stand-alone point and is intended to deal with a single piece of information that we believe is likely to be important to our readership.
  • The writing style across all terms aims to be clear, accessible and engaging.
  • The idea is to enable our visitors to find and digest information quickly and to confirm facts one at a time.
  • Because our readership and their concerns are diverse, bullet points sometimes provide different perspectives on a single term or topic.


Show me the DIAGRAMS


When light strikes an object, some wavelengths are absorbed and their energy is converted to heat, others undergo reflection or transmission.

About absorption
  • When light is absorbed by an object or medium, its energy is transferred to electrons and emitted as heat.
  • Absorption of a particular wavelength of light into a material takes place when the frequency of the wave matches the frequency of electrons orbiting atomic nuclei.
  • Electrons selectively absorb photons with matching frequencies.
  • As electrons orbiting atomic nuclei absorb energy, they vibrate more vigorously causing atoms to collide with one which produces heat.
  • When light is reflected off a surface it bounces off at the same wavelength with little or no change in energy.
Related diagrams

Each diagram can be viewed on its own page with a full explanation.


Accommodation refers to the way our eyes keep things in focus by simultaneously changing the shape of each lens. The result is sharp images of the world regardless of whether things are close by or in the distance.

About accommodation
  • If you look into a mirror, the lens in each eye is located just behind the pupil. The lens shape is controlled by ciliary muscles.
  • The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance always remains the same.
  • Image distance is measured between the retina (the light-sensitive surface at the back of the eye) and the cornea (the transparent surface at the front of the eye) and is fixed in the case of the human eyeball.
  • Because the image distance is fixed, our eyes accommodate for this by using the ciliary muscle to alter the focal length of the lens. This enables images of both nearby and far away objects to be brought into sharp focus on the retinal surface.
  • The ciliary muscle forms a ring of flexible tissue around the edge of each lens in the eye’s middle vascular layer.
  • Our eyes accommodate nearby objects by forming each lens into a shape with a shorter focal length. In this case, the ciliary muscles squeeze the lens into a more convex form.
  • Our eyes accommodate distant objects, by relaxing the ciliary muscles, causing the lens to adopt a flatter and less convex shape with a longer focal length.


Achromatic means without colour so refers to surfaces or objects that appear white, grey or black. Achromatic colours can be described in terms of their apparent brightness but are without hue or saturation.

About achromatic colours
  • Near-neutral colours (very light tints or very dark tones) are also sometimes described as achromatic.
  • Unsaturated achromatic hues include browns, tans, pastel tints and equivalent darker tones.
  • When mixing paint, achromatic hues are produced by adding black and/or white until the original colour almost disappears.
  • Achromatic colours are produced on TV, computer and phone screens by mixing RGB colours in equal proportions. The RGB colour model produces a middle grey with the hexadecimal colour value #808080.
  • The way achromatic hues appear to an observer often depends on adjacent more saturated colours. So next to a bright red couch, a grey wall will appear distinctly greenish – green and red being complementary colours.
  • Saturated colours are colours produced by a single wavelength (or a small band of wavelengths) of light.

Additive colour

Additive colour refers to the way any two or more wavelengths of light can be combined to produce other colours.  The RGB colour model, HSB colour model and Spectral colour model use different methods to produce systematic ranges of colour.

About additive colour
  • Additive colour is the method used to mix wavelengths of light, whilst subtractive colour is the method used when mixing pigments such as dyes, inks and paints.
  • An additive approach to colour is used to control the emission of light by television, computer and phones screens.
  • A colour model can be thought of as a theory of colour whilst additive colour and subtractive colour refer to the method used in practice.
About additive colour and the RGB colour model

The RGB colour model used by TV, computer and phone screens involves additive colour mixing. The RGB colour model produces all the colours seen by an observer simply by combining the light emitted by arrays of red, green and blue pixels (picture elements) in different proportions.

  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce all other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours can produce any other colour if mixed in the right proportion.
  • 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.
  • A fully saturated colour is produced by a single wavelength (or narrow band of wavelengths) of light.
  • When any two fully saturated additive primary colours are combined, they produce a secondary colour: yellow, cyan or magenta.
  • Some RGB colour models can produce over 16 million colours by varying the intensity of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing pigments such as paints, inks, dyes or powders.

Adobe RGB colour space

The aim of a colour space is to ensure that a colour produced by a colour model is accurately reproduced when displayed on-screen, printed or made into paint. The Adobe RGB (1998) colour space is an RGB colour space developed by Adobe Systems, Inc.

About colour spaces
  • A colour space aims to accurately define the relationship between any selected colour and how it will be perceived by the human eye when it is reproduced by a specific digital display, printer or paint mixing machine.
  • The Pantone colour collection defines its colour space by:
    • Establishing a set of inter-related colour swatches
    • Giving each swatch a name or code
    • Calibrating a paint machine (or other types of equipment) to accurately reproduce the colour of each swatch.
  • When an artist chooses a limited number of oil paints to add to their palette they establish a colour space within which they plan to work.
  • Digital colour spaces are often used to define the range (gamut) of colours that will be produced by a particular device or file type.
  • Each digital colour space is programmatically defined to produce a specific gamut of colours and accurately display them when paired with a type of equipment or when paired with a specific colour profile.
  • A colour profile is a program that allows a piece of equipment to know how to handle and process the information it receives so that it can produce the intended colour output.
About the Adobe RGB colour space
  • The Adobe RGB (1998) colour space is designed to encompass the colours that can be output by CMYK colour printers.
  • When the RGB colour model is used on a modern computer screen, the Adobe RGB (1998) colour space aims to reproduce roughly 50% of the range of colours that an observer is capable of seeing in ideal conditions.
  • The purpose of the  Adobe RGB (1998) colour space was to improve on the gamut of colours that could be produced by the earlier sRGB colour space, primarily in the reproduction of cyan-green hues.

Amacrine cells

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

About amacrine cells
  • Amacrine cells are a type of interneuron within the human retina.
  • Amacrine cells are embedded in the retinal circuitry.
  • Amacrine cells are activated by, and feedback to, bipolar cells. They also have junctions with ganglion cells, as well as with each other.
  • Amacrine cells are known to add information to the stream of data travelling through bipolar cells and then to control and refine the way ganglion cells (and their subtypes) respond to it.
  • Most amacrine cells don’t have tale-like axons. But whilst they clearly have multiple connections to other neurons around them, research into their precise inputs and outputs is ongoing.
  • Axons are the part of neurons that transmit electrical impulses to other neurons.
  • Neurons are the nerve cells that the human central nervous system is composed of.
About the functions of amacrine cells

Amacrine cells are known to contribute to narrowly task-specific visual functions such as:

  • 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 helps to govern our lives throughout the year.
  • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism), so enabling edge detection and contrast enhancement.
  • Motion detection and the ability to distinguish between the movement of things across the field of view and our own eye movements.
Centre-surround antagonism

Centre-surround antagonism refers to the way retinal neurons organize their receptive fields.

  • Centre-surround antagonism refers to the way that light striking the human retina is processed by groups of light-sensitive cone cells in the retina.
  • The centre component is primed to measure the sum-total of signals received from a small number of cone cells 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 an electromagnetic wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough. The greater the distance the more energy the wave carries.

About amplitude
  • In any particular situation, the relative amplitude of an electromagnetic wave correlates with the relative intensity of light falling on a surface and the relative brightness of the colour perceived by an observer.
  • As the amplitude of an electromagnetic wave increases so does the overall distance between the peak and a corresponding trough.
  • The greater the amplitude of a wave, the more energy it carries.
  • The energy carried by an electromagnetic wave is proportional to the amplitude squared.
  • The amplitude of the electric field of an electromagnetic wave is measured in volts per metre and the magnetic field in amperes per metre.
About amplitude, brightness, colour brightness and intensity

The terms amplitude, intensity and colour brightness are sometimes confused.

Amplitude is a feature of electromagnetic waves. Other features include:

Brightness is used alongside hue and saturation in the HSB colour model.

Colour brightness depends on spatial context: the same stimulus can appear light or dark depending on what surrounds it.

Intensity measures the energy carried by a light wave or stream of photons:

  • When light is modelled as a wave, intensity is directly related to amplitude.
  • When light is modelled as a particle, intensity is directly related to the number of photons present at any given point in time.
  • The intensity of light falls exponentially as the distance from a point light source increases.
  • Light intensity at any given distance from a light source is directly related to its power per unit area (when the area is measured on a plane perpendicular to the direction of propagation of light).
  • The power of a light source describes the rate at which light energy is emitted and is measured in watts.
  • The intensity of light is measured in watts per square meter (W/m2).
  • Cameras use a light meter to measure the light intensity within an environment or reflected off a surface.



Analogous colours

Analogous colours are colours that are very similar to one another and appear next to each other on a colour wheel.

About analogous colours
  • Analogous colours are colours of similar hue.
  • An example of a set of analogous colours is red, reddish-orange, orange, yellow-orange.
  • An analogous colour scheme creates a rich, smooth look but is less vibrant than a complementary colour scheme.
  • Increasing the number of segments on a colour wheel shows analogous colours more clearly as the gradation between colours becomes finer.

Angle of incidence

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

About the angle of incidence
  • 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 (at 900 to), the boundary between two media.
  • If the boundary between two media is curved then the normal is drawn perpendicular to the boundary.


Angle of reflection

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

About the angle of reflection
  • 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 (at 900 to), the boundary between two media.
  • If the boundary between two 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.

About the angle of refraction
  • 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 (at 900 to), the boundary between two media.
  • If the boundary between two 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 the angle of refraction when dealing with light crossing the boundary between two different transparent media, such as air and water.
  • In optics, Snell’s law is used when drawing 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.

About atoms
  • At the centre of an atom is a nucleus containing protons which are positively charged sub-atomic particles. The number of protons determines the chemical element associated with the atom. A hydrogen atom has a single proton.
  • The nucleus of an atom also contains neutrons, sub-atomic particles with a mass slightly greater than that of a proton but no positive or negative charge.
  • Surrounding the nucleus are negatively charged particles called electrons which are held in place by their attraction to the positively charged protons in the nucleus.
  • In an atom, the number of electrons matches the number of protons.
  • When an electron is removed from an atom it forms 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.
  • In atomic theory and quantum mechanics, an atomic orbital is a mathematical function describing the location and motion of electrons around the nucleus of atoms.
Related diagrams

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.

About attributes of visual perception

Innate attributes of visual perception associated with the response of the human eye and brain to light include:

  • Colour perception: The ability to see differences in 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.
  • https://en.wikipedia.org/wiki/Visual_perception
  • https://www.seevividly.com/info/Binocular_Vision/Visual_Skills


Aurora (also known as the polar lights) are a natural display of curtains, rays, spirals, and flickering patterns of light in the northern polar latitudes (Aurora Borealis) and southern polar latitudes (Aurora Australis). They are most prominent after dark.

About Aurora
  • Auroras are the result of charged particles (electrons) produced by the Sun (solar wind) interacting with Earth’s magnetosphere.
  • The magnetosphere accelerates electrons as they plunge into the atmosphere during their final few 10,000 km journeys from the Sun.
  • The colour and pattern of aurora depend partly upon the amount of acceleration imparted to the precipitating particles.

Bands of colour

Bands of colour are associated with an observer seeing a continuous range of spectral colours and distinguishing colours that correspond with a band of wavelengths within the visible spectrum.

About seeing in colour
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • The human eye, and so human perception, is tuned to the range of wavelengths of light that make up the visible spectrum and so to the corresponding spectral colours between red and violet.
  • Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a spread of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
  • An observer’s perception of colour is a subjective process as the eyes and brain respond together to stimuli produced when incoming light reacts with light-sensitive cells within the retina at the back of their eye.
About bands of colour
  • The visible spectrum forms a smooth and continuous range of wavelengths that produce a continuous range of colours.
  • In experimental situations human observers can distinguish between spectral colours corresponding with many thousands of wavelengths of light in the visible spectrum.
  • 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.
  • It is the human brain that draws lines between different bands of colour when an observer looks at a rainbow for example.
  • A rainbow may appear to contain bands of colour because:
    • The human eye responds to colours that appear brighter than others when presented with part or the whole of the visible spectrum.
    • Observer look for colours they know and recognise..
    • Cone cells in our eyes are particularly sensitive to red, green and blue wavelengths. This results from the trichromatic nature (trichromacy) of human vision.
  • Combinations of wavelengths from different areas of the visible spectrum produce non-spectral colours.

Bipolar cells

Bipolar cells are the retinal interneurons that provide the principal pathway from photoreceptors (rod and cone cells) to ganglion cells. As well as acting directly to transmit signals from the photoreceptors to the ganglion cells.

  • Bipolar cells are connected to rod and cone cells by synapses.
  • There are around 12 types of bipolar cells and they all function as integrating centres.
  • Each type acts as a specialised conduit for information about light that has struck and single, or small group of rod and cone cells.
  • So, each type of bipolar cell transmits its own interpretation of information extracted from photoreceptors and passes this on to ganglion cells.
  • The output of bipolar cells onto ganglion cells includes the direct response of the bipolar cell to signals derived from phototransduction and also responses to signals received indirectly from information 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.


Brightness: HSB colour model

This entry deals with the term brightness as used in the HSB colour model, where H = hue, S = saturation and B = brightness.

Brightness (colour brightness) refers to the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.

About brightness
  • The term brightness is best understood when associated with a specific colour model.
  • Examples of colour models include:
  • The HSB colour model uses the term brightness alongside hue and saturation.
  • Some colour models don’t use the term brightness at all.
  • When we change from one colour model to another, it’s best to change our terminology as well.
About colour models

Colour models are the how-to part of colour theory. Together they establish terms and definitions, rules or conventions and a system of notation for encoding colours and their relationships with one another.

A colour model is a way to:

  • Make sense of colour in relation to human vision, to the world around us and to different media and technologies.
  • Understand the relationship of colours to one another.
  • Understand how to mix a particular colour from other colours to produce predictable results.
  • Specify colours using names, codes, notation, equations etc.
  • Organise and use colours for different purposes.
  • Use colours in predictable and repeatable ways.
  • Work out systems and rules for mixing and using different media (light, pigments, inks).
  • Create colour palettes, gamuts and colour guides.
Colour theory
  • When an observer asks themselves about the colour of something, they will often unconsciously think in terms of a particular colour theory associated with:
    • Spectral colours with names associated with atmospheric rainbows
    • Pigments, where powders are mixed with water, oil or acrylic to produce different colours
    • Objects and surfaces which transmit, reflect and absorb wavelengths of light in different proportions
  • A broader vocabulary of names can be used to describe colours such as dark red, vermilion, golden yellow, lemon yellow, pale yellow, greenish-yellow, chartreuse, leaf green or light green.
  • A colour model derived from a theory of colour allows for a more exact and reproducible approach to colour.
HSB colour model

The HSB colour model is an additive colour model used to mix light. Subtractive colour models are used to mix pigments and inks.

  • HSB is an alternative to using the RGB colour model in so far as both deal with mixing red, green and blue light to produce other colours.
  • HSB is popular because it provides an intuitive way to select and adjust colours when using applications such as Adobe Creative Cloud for design, photography or web development.
  • The HSB colour model can be used to describe any colour on a TV, computer or phone.
  • In the HSB colour model:
    • Hue refers to the perceived difference between one colour and another and accounts for colour names such as red, yellow, green or blue.
      • Hue can be measured as a location on the standard colour wheel and expressed in degrees between 0 and 360.
    • Saturation refers to the perceived difference between one colour and another in terms of vividness.
      • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out until all colour disappears leaving only a monochromatic grey tone (0%).
      • A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths.
      • On HSB colour wheels, saturation increases from the centre to the edge.
    • Brightness (colour brightness) refers to the difference between the way a colour appears to an observer in well-lit conditions compared with its subdued appearance when in shadow or when poorly illuminated.
Brightness, Intensity, amplitude

In this dictionary:

    • Brightness is used in connection with the perception of colour.
    • Intensity is used in connection with the amount of light that is produced by or falls on an object.
    • Amplitude is used in connection with the properties of electromagnetic waves.
Colour brightness and light intensity
  • The perception of colour in the world around us depends on the spread of wavelengths that reach the eyes of an observer. Red has a long wavelength, violet has a short wavelength.
  • The perception of the brightness of a colour depends on the intensity of the light an object emits (a light source) or reflects (a surface).
  • The intensity of light depends on the amplitude of the light wave that produces it.
  • Amplitude measures the height of light waves from trough to peak.
  • The amplitude of a light wave can be thought of in terms of the volume of photons that it carries.
  • Increasing the amplitude of a wavelength of light means the volume of photons falling on an object will increase its apparent brightness to an observer.

Chemical bond

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.