Additive colour and the RGB colour model

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 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 imaginable colour if mixed in the right proportion.
  • Different colours are produced by varying the brightness of the component colours between fully off and fully on.
  • When fully saturated red, green and blue primary colours are combined in equal amounts, they produce white.
  • A fully saturated hue 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 implementations of RGB colour models can produce millions of colours by varying the brightness 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.
  • The RGB colour model does not define the exact hue of the three primary colours so the choice of wavelengths for each primary colour is the principal determinant of their admixture.
  • The RGB colour model can be made device-independent by specifying a colour profile such as sRGB or Adobe RGB (1998) which ensures consistent results regardless of the device used to output an image.

Adobe RGB colour space

About the Adobe RGB colour space
  • The Adobe RGB (1998) colour space is designed to encompass the colours that can be reproduced 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 Adobe RGB (1998) colour space was developed 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 cell functions

About amacrine cell functions

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.

Amplitude, brightness, colour brightness & intensity

About amplitude, brightness, colour brightness and intensity

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

Amplitude
Brightness
  • Brightness is related to how things appear from the point of view of an observer.
    • When something appears bright it seems to radiate or reflect more light or colour than something else.
    • Brightness may refer to a light source, an object, a surface, transparent or translucent medium.
    • The brightness of light depends on the intensity or the amount of light an object emits( eg. the Sun or a lightbulb).
    • The brightness of the colour of an object or surface depends on the intensity of light that falls on it and the amount it reflects.
    • The brightness of the colour of a transparent or translucent medium depends on the intensity of light that falls on it and the amount it transmits.
    • Because brightness is related to intensity, it is related to the amplitude of electromagnetic waves.
    • Brightness is influenced by the way the human eye responds to the colours associated with different wavelengths of light. For example, yellow appears relatively brighter than reds or blues to an observer.
Colour 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.
    • In a general sense, brightness is an attribute of visual perception and produces the impression that something is radiating or reflecting light and/or colour.
    • Colour brightness increases as lighting conditions improve, whilst the vitality of colours decreases when a surface is poorly lit.
    • Optical factors affecting colour brightness include:
    • Material properties affecting the colour brightness of a medium, object or surface include:
      • Chemical composition
      • Three-dimensional form
      • Texture
      • Reflectance
    • Perceptual factors affecting colour brightness include:
    Intensity
    • 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.

Bands of colour, spectral and non-spectral colours

About bands of colour, spectral and non-spectral colours
Bands of colour
  • Bands of colour are composed of a continuous range of wavelengths, so for example:
    • A continuous range of wavelengths between 750 – 620 nanometres (nm) typically appear red to an observer.
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    • Wavelengths between 590 – 570 nm will typically appear yellow.
    • A continuous range of wavelengths between 450 – 380 nm will typically appear violet.
Spectral colours
  • A spectral colour is a colour that is evoked by a single wavelength of light (or narrow band of wavelengths) within the visible spectrum.
  • Spectral colours are the colours red to violet.
  • Diagrams of the spectral colour model are linear and may show colours selected:
    • Using equal and incremental steps in wavelength.
    • According to equal and incremental steps in the appearance of colours.
Non-spectral colours
  • Non-spectral colours are produced by additive mixtures of wavelengths of light.
  • Examples of non-spectral colours produced by two spectral colours are:
    • Purple – produced by mixing wavelengths corresponding with red and violet. Red (740nm) and violet (400nm) are at the extreme limits of the visible spectrum.
    • Magenta –  produced by mixing red (660nm) and blue (490nm).
    • Mauve – produced by mixing orange (600nm) and blue (450nm).
    • Examples of non-spectral colours produced by three spectral colours are:
      • Tints
      • Greys
      • Shades
      • So all achromatic colours are non-spectral colours.
  • Whilst both spectral and non-spectral colours are produced by mixing a combination of colours corresponding with different wavelengths of light:
    • The RGB colour model produces a full gamut of colours by mixing red, green and blue primary colours in different proportions.
    • The CMY colour model produces a full gamut of colours by mixing cyan, magenta and yellow primary colours in different proportions.

Brightness

About brightness
  • In this resource, the term brightness is associated with the intensity of light an object such as the Sun or a lightbulb emits.
  • The brightness of a light is always determined by comparing it with the brightness of other light sources.
  • As light propagates through a vacuum it is invisible but its brightness becomes apparent when a light source shines directly into our eyes or it is reflected towards us.
  • The perceived brightness of a light source depends on how the photoreceptive rod and cone cells in the human retina respond to wavelengths of light (rather than the way that translates into the experience of colour).

Brightness and colour models

About brightness and colour models

Brightness, intensity & amplitude

About brightness, intensity and amplitude

In this resource:

Centre-surround antagonism

About 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.
  • 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.

Chromatic adaptation

About chromatic adaption
  • Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of objects.
  • Chromatic adaptation is responsible for the stable appearance of the colour of familiar objects despite the wide variation of lighting conditions.
  • Chromatic adaption means an observed colour stimulus such as a white surface is judged to remain white even as other projected or reflected colours fall upon it.
  • Chromatic adaption often becomes noticeable when comparing photographs of the same subject in changing lighting conditions. Cameras use white balance to compensate for changes in lighting but two photos taken only minutes apart may render the same subject matter differently.

Chromophores

About chromophores
  • Things appear to have colour because they absorb some wavelengths of light and reflect others.
  • Chromophores are the part of molecules responsible for the absorption and reflection of light.
  • A chromophore is formed by a group of atoms within a molecule and the electrons that orbit their nuclei.
  • The colour produced by an opaque object corresponds with the wavelengths not absorbed during the interaction of light with the chromophores of the molecules that form its surface.
  • Whether different wavelengths of light are absorbed or reflected by a chromophore depends on whether there is an energy difference between orbiting electrons.
  • If the energy difference between the electrons of a chromophore falls within the range of the visible spectrum (2 to 2.75 electron volts) then it produces the colour seen by an observer.

Colour and visual perception

About colour and visual perception
  • 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 when an observer notices a red ball they are probably seeing a range of similar wavelengths of light within the visual spectrum.
  • Perception of colour is a subjective process as our eyes respond to stimuli produced by incoming light but each of us responds differently.

Colour brightness

About colour brightness
  • In this resource, the term colour brightness is used to refer to how things appear to a human observer in terms of their perception of colour.
  • Colour is what humans see in the presence of radiated or reflected light.
  • The brightness of the colour of an object or surface (its colour brightness) depends on the wavelengths and intensity of light that falls on it and the amount it reflects.
  • The colour brightness of a transparent or translucent medium may depend on the wavelengths and intensity of light that falls on it and the amount it transmits or reflects.
  • Colour brightness often depends on 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.
  • The impression of colour brightness is also affected by hue because some hues appear brighter than others to human observers. So a fully saturated yellow may appear relatively brighter than a fully saturated red or blue.

Colour brightness & light intensity

About colour brightness & 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.
  • Every colour (eg. red, magenta or violet) can be defined in terms of its hue, saturation and brightness.
  • Saturated colours are produced by a single wavelength of light or a narrow band of wavelengths.
  • The perception of the brightness of a hue depends on the intensity of the light a light source emits (a coloured light bulb for example) or the amount of light reflected off a coloured surface.
  • The intensity of light, along with factors such as phase and interference, are directly related to the amplitude of an electromagnetic wave.
  • Amplitude measures the height of light waves from the centre-line of a waveform to its crest or to a corresponding trough.
  • Colour brightness, light intensity and amplitude of a light wave can all be thought of in terms of the volume of photons that strike the eye of an observer.
  • So 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.

Colour model

About colour models

A colour model is the how-to part of a 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 colour 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 models and colour wheels

About colour models and colour wheels

Colour space

About colour space
  • A colour space aims to accurately define the relationship between any selected colour within a colour model and how it will appear when it is reproduced by a specific device such as a digital display, printer or paint mixing machine.
  • When an artist chooses a limited number of tubes of oil paint to add to a palette they are already working within the RYB subtractive colour model and are establishing the colour space within which they plan to work.
  • A colour space may aim to restrict the number of colours or establish the widest possible gamut of reproducible colours.
  • Digital colour spaces are commonly used to accurately set the range of colours that can be output to and then displayed by digital screens and printers.
  • The Pantone colour collection which is widely used for mixing paint defines its colour space by:
    • Establishing a set of inter-related colour swatches that a user can choose from
    • Giving each swatch a name or code
    • Calibrating a paint machine (or another type of equipment) to accurately reproduce the colour of each swatch
  • When a colour space is to be matched with a specific digital device such as a projector or printer, a colour profile is loaded along with the image file to ensure accurate colour reproduction.
  • A colour profile is a program that allows a piece of equipment such as a digital printer to know how to handle and process the information it receives so that it can produce the intended colour output.

Colour theory

About colour theory

Colour theories underpin colour management by seeking to explain how human beings perceive colour and establish the rational basis for practical how-to methods for managing colour in different situations.

A system of colour management may be associated with:

The aspect of colour theory concerned with the human perception of colour aims to answer questions about:

  • How our eyes register colour when exposed to light.
  • The way our eyes and brains work together to produce the complex colour perceptions that make up the visible world.
  • The part of the electromagnetic spectrum that is related to colour and how our eyes respond to different wavelengths of light.
  • The fact that red, green and blue lights combined in different proportions can produce the impression of all the colours of the visible spectrum.
  • The way colours appear in different situations such as in low or bright light and under artificial lighting.
  • Human responses to different combinations of colour such as analogous, complementary and contrasting colours.
  • The differences between the scientific, technical and creative understandings and descriptions of colour.
  • Understanding the differences between:
    • The way our eyes see colour
    • Light and colour in the world around us
    • The colour of opaque objects and surfaces
    • The colour of transparent media
    • Colour on TVs, computers and phone screens
    • Colour in printed images

The aspect of colour theory concerned with how-to methods for managing colour in different situations aims to answer questions about:

  • The differences between mixing coloured lights, pigment or inks.
  • Mixing and managing ranges (gamuts) of colours in logical, predictable and repeatable ways.
  • Identifying and mixing particular colours in predictable and repeatable ways.
  • Specifying colours using names, codes, notation, equations etc.
  • The difference between additive and subtractive colour mixing.
  • Systems and rules for mixing different and applying them to different materials such as fabrics, interiors and vehicles.
  • Creating colour palettes, gamuts and colour guides.
  • Managing the consistent reproduction of digital colour from start to finish.

Distinct colour theories are evident in: