Additive & RGB colour

About additive & RGB colour

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 on TV, computer and phone screens by creating arrays of red, green and blue pixels (picture elements) in different proportions.
  • Red, green and blue are called additive primary colours in an RGB colour model because just these three component colours alone can produce any conceivable colour if blended in the correct proportion.
  • Different colours are produced by varying the brightness of the component colours between completely off and fully on.
  • When fully saturated red, green and blue primary colours are mixed 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 mixed, 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 important if it is to be used as part of a colour-managed workflow.
  • 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.

Additive & subtractive colour models

About additive and subtractive colour models

There are two main types of colour models, additive and subtractive.

Additive Colour Models
Subtractive Colour Models
  • Subtractive colour models are used when working with pigments, inks and dyes.
  • The primary colours for most subtractive colour models are cyan, magenta, and yellow (CMY).
  • When combined cyan, magenta, and yellow produce black.
  • The subtractive CMY colour model and CMYK colour model are central to printing technologies.
  • In practice, the CMY colours often can’t produce a perfect black when mixed due to impurities in the pigments or inks, so a fourth ‘Key’ component (represented as K) is often used in printing to produce a true black.

Adobe RGB, sRGB & ProPhoto

About Adobe RGB, ProPhoto RGB & sRGB

The most common colour profiles in photography are sRGB, Adobe RGB (1998), and ProPhoto RGB.

  • Adobe RGB, developed in 1998, consists of the same red green blue colours as sRGB but the colour space has a larger gamut.
    • It was developed to communicate with standard CMYK multi-function and inkjet printers and is commonly used for printing on fine art papers.
    • 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.
  • sRGB stands for standard red green blue and has the smallest colour space.
    • It was developed by HP and Microsoft in 1996 for use with monitors, printers, and the World Wide Web.
    • It is the most commonly used colour profile today because of its consistent reproduction of colours across different platforms.
  • ProPhoto RGB has the largest colour space with a gamut that covers a significant part of the perceptual colour space of the human eye.
    • ProPhoto RGB is used in high-end photography and editing workflows to preserve a wider range of colours and maintain the quality of the original image during processing.

Amacrine cell functions

About amacrine cell functions

Amacrine cells are a type of neuron found in the retina, the light-sensitive tissue lining the back of the human eye. They play a critical role in the processing of visual signals before these signals are sent to the brain.

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

  • Spatial Contrast Enhancement: Amacrine cells contribute to a process called lateral inhibition, which helps to enhance the contrast between light and dark areas in a visual scene, thereby improving our ability to see edges and borders.
  • Temporal Contrast Enhancement: Amacrine cells play a role in detecting changes in light intensity over time, which helps us to perceive motion and changes in a visual scene.
  • Direction Selectivity: Certain types of amacrine cells are involved in detecting the direction of moving objects. These are known as starburst amacrine cells.
  • Centre-surround antagonism: Amacrine cells interact with both bipolar cells and retinal ganglion cells to contribute to the centre-surround antagonistic structure of ganglion cell receptive fields.
  • Complex Visual Processing: Amacrine cells form connections with multiple types of retinal cells, including bipolar cells and ganglion cells. This allows them to participate in complex processing and integration of visual information.
  • Inhibitory Signalling: Many amacrine cells arstae inhibitory interneurons, which means they can inhibit the activity of other neurons. This inhibitory function plays a role in shaping the output of retinal ganglion cells, which send visual information to the brain.
  • Regulation of Circadian Rhythm: Some amacrine cells release a pigment called melanopsin and are involved in non-image-forming visual functions, such as the regulation of circadian rhythms and the pupillary light reflex.
  • Neurotransmitter Release: Amacrine cells can release a variety of neurotransmitters, including GABA, glycine, dopamine, and others, allowing them to modulate the activity of various neural circuits in the retina.

Amplitude, brightness, colour brightness & intensity

About amplitude, brightness, colour brightness and intensity

The terms amplitude, brightness, colour brightness and intensity are easily confused. In this resource:

  • Brightness refers to a property of light, to how strong a light source or light reflected off an object appears to be.
  • 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 how colours appear to a human observer in terms of the lightness or darkness of colours.

So colour brightness can refer to the difference between how a colour appears to an observer in well-lit conditions and 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:
    • The angle at which incidence light approaches a medium, object or surface
    • The composition of incident light in terms of wavelength and frequency
    • The polarization of incident light
  • 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 refers to the amount of light produced by a light source or the amount of light that falls on a particular area of the object.
    • So 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.
      • Light intensity 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.
    • 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:


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.
  • In everyday experience, we often gauge the brightness of a light source subjectively, by comparing it with the brightness of other known light sources.
  • The brightness of a light can also be measured objectively using units like lumens or candela.
  • Light travelling through a vacuum is not visible until it interacts with something such as our eyes or an object that reflects the light towards us, enabling us to perceive its brightness.
  • The perceived brightness of a light source depends on the intensity and wavelength of the light and how the photoreceptive rod and cone cells in the human retina respond.
  • Brightness, when used in this way, is the same as luminance.
  • Luminance is a measure of the amount of light emitted, transmitted, or reflected from a particular area in a specific direction. It is used to quantify the intensity of light that is perceived by the human eye from a particular direction.
  • Our eye’s photoreceptors, especially the rod cells which are more sensitive to light intensity, play a crucial role in our perception of brightness. Rods are more abundant and distributed throughout the retina, and they function mainly in low light conditions to help us perceive the brightness or lightness of an object, but they can’t distinguish colour.
  • On the other hand, our perception of colour is based on how different wavelengths of light stimulate the three types of cone cells in our eyes. These cone cells are sensitive to short (S, which corresponds to blue), medium (M, corresponds to green), and long (L, corresponds to red) wavelengths of light. The combination of signals from these three types of cone cells allows us to perceive a broad spectrum of colours. Colour perception depends not just on the light’s intensity, but on its spectral composition – what mix of wavelengths it contains.

Brightness & colour models

About brightness & colour models

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 refers to the ability of our visual system to adjust to changes in lighting conditions, helping to keep the perceived colour of objects relatively stable.
  • Chromatic adaptation helps us perceive the colours of familiar objects as constant, even under widely varying 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 try to mimic chromatic adaption through white balance adjustments, but differences in lighting conditions can still result in two photos of the same subject appearing different in colour.


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.

CMY colour model & colour perception

About the CMY colour model and colour perception

CMY colour printing

About CMY colour printing
  • CMY printing involves mixtures of three primary colours of dyes or inks – cyan (C), magenta (M) and yellow (Y).
  • There are two distinct types of CMY digital printing, one involves using solid areas of translucent colour, and the other involves halftoning.
    • CMY colour printing using solid areas of translucent colour applies each of the CMY inks to paper in separate layers of solid colour, creating the appearance of different colours and shades by varying the amount of each ink that is applied.
    • Halftoning involves dividing each image into a grid of tiny dots and printing each dot in a single colour (typically CMYK) at a fixed size and spacing to create the appearance of different shades and colours.
  • CMY printing using solid areas of translucent colour can produce less intense or vibrant colours than would be obtained with opaque ink because the translucent inks allow some of the white paper to show through.
  • Halftoning is the most common method of colour printing used in modern printers, as it allows for high-quality, photo-realistic images to be printed with relatively simple equipment.
  • In practice, black ink is often added to the CMY inks to improve the depth and clarity of dark areas in the image. This combination of CMYK inks is often used in printing to produce full-colour images with accurate colour reproduction.
  • Some effects can not be produced using the CMY colour model or CMYK printing.
  • Screen printing, for example, can use a wide variety of ink types, including spot colours, metallic inks, and special effects inks to achieve results that are unachievable using the standard CMY colour model.
  • In screen printing, each colour layer is printed separately, and this method often uses spot colours (premixed inks of a specific hue) instead of relying on CMY colour mixing. This allows for more accurate colour matching and vibrant, solid colours.
  • The use of spot colours can be when only a few colours are needed, as it reduces the number of screens and printing passes required compared to using CMYK colour separation.

CMY colour printing in practice

About subtractive colour printing in practice
CMY on a white sheet of paper
  • Cyan ink is painted onto the paper to create a circular shape.
  • The paper seen through the cyan ink appears cyan to an observer because:
    • The ink has absorbed or transmitted all wavelengths of light except those around 500 nanometres (cyan).
    • The wavelengths of light around 500 nanometres reflected off the ink, making it look cyan.
    • Some transmitted wavelengths passed straight through the ink, reflected off the paper below, passed back through the ink, and added to the intensity of the colour seen by the observer.
  • Matching patches of magenta and yellow are now painted onto the paper so that areas of each of the three colours overlap.
  • As already established,  the paper seen through the yellow ink alone appears yellow because it has absorbed all wavelengths of light other than those around 500 nanometres (cyan).
  • Whilst the paper seen through the magenta ink alone appears magenta because it has absorbed all wavelengths of light other than those around 700 nanometres (red).
  • And the paper seen through the yellow ink alone appears yellow because it has absorbed all wavelengths of light other than those around 580 nanometres (yellow).
  • Where cyan and magenta ink overlap, the paper appears blue. This is because the cyan ink absorbs red light and allows blue light to pass through, while the magenta ink absorbs green light.
  • Where magenta and yellow ink overlap, the paper appears red. This happens because the magenta ink absorbs green light and lets red and blue light pass through, while the yellow ink absorbs blue light, leaving only the red light.
  • Where yellow and cyan ink overlap, the paper appears green. This occurs because the yellow ink absorbs blue light and allows green and red light to pass through, while the cyan ink absorbs red light, leaving only the green light.
  • Where all three inks overlap the paper appears dark brown.
  • Remember that in practice, a fourth ink, black (K), is often added to the CMY model to create the CMYK model, which provides better depth and detail in dark areas and helps save ink.
  • CMYK is commonly used in printing processes like inkjet and laser printing, as well as offset printing for large-scale projects.

Colour & visual perception

About colour & visual perception
  • Colour is not a property of electromagnetic radiation, but rather a characteristic of visual perception.
  • The human eye, and therefore human perception, is sensitive to the range of light wavelengths that constitute the visible spectrum, including the corresponding spectral colours from red to 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 describe how things appear to a human observer in terms of their perception of colour.
  • Colour is what humans perceive in the presence of radiated or reflected light.
  • The brightness of the colour of an object or surface (colour brightness) depends on the wavelengths and intensity of light that illuminate it and the amount of light it reflects.
  • The colour brightness of a transparent or translucent medium may be influenced by the wavelengths and intensity of light that pass through or reflect off it and the amount it transmits or reflects.
  • Colour brightness is frequently influenced by the contrast between how a colour appears to an observer under well-lit conditions and its more subdued appearance when in shadow or under poor illumination.
  • The perception of colour brightness is also influenced by hue, as certain hues appear brighter than others to human observers. For example, 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 depends on the wavelengths that reach an observer’s eyes. Red has a longer wavelength, while violet has a shorter wavelength.
  • Any colour (e.g. red, magenta, or violet) can be defined by its hue, saturation, and brightness.
  • Saturated colours are produced by a single wavelength of light or a narrow band of wavelengths.
  • The brightness of a colour depends on the intensity of the light emitted by a light source (e.g., a coloured light bulb) and the amount of light reflected from a coloured surface.
    • So, for example, the texture of a surface can affect brightness even when the intensity of the light source remains constant.
  • 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 the amplitude of a light wave can all be thought of in terms of the number of photons that strike the eye of an observer.
    • Therefore, increasing the amplitude of a wavelength of light will increase the number of photons falling on an object, making it appear brighter to an observer.

Colour management & photographic workflow

About colour management & photographic workflow
  • In photography, the main goal of colour management is to control the accurate capture of original colours and ensure consistent reproduction of specific colours or entire gamuts throughout the creative process.
  • When producing a photo, colour management is used to ensure consistent output across various devices, including digital cameras, scanners, monitors, TV screens, computer printers, and offset printing presses.
  • Colour management compensates for the differences in technologies, devices, and media all of which may have distinct capacities for reproducing gamuts and intensities of colour, potentially leading to unintended shifts in appearance.
  • At the consumer level, all operating systems include built-in colour management by default.
  • Most hardware and software related to visual design and image reproduction offer colour management options that can be set by default or require configuration based on specific purposes.
  • The International Colour Consortium’s (ICC) colour management system serves as a comprehensive industrial standard for cross-platform colour management.

The principal components of a colour management system include:

Colour management in practice

About colour management in practice

A typical colour management workflow starts by ensuring that colours seen through a camera viewfinder are captured and digitally recorded. Editing software such as Adobe CC allows extensive choices to be made about the appearance of images. When the workflow demands it, the calibration of monitors ensures information is accurately reproduced when viewed on screen. A successful outcome is one where all the decisions made during the editing process are accurately rendered in the resulting image.

A. Image capture   B. Image editing   C. Monitoring images   D. Image output


A. Image Capture
  • Digital cameras provide settings to allow colour profiles to be selected that affect how colours are recorded, these deal with:
    • White balance
    • Photo style setting includes control over sharpness, depth of field, contrast, saturation and tone (including monochrome) etc.
  • Digital file formats enable control over the quantity and types of information stored about an image:
    • Raw file formats store all the recorded information without compression.
    • JPEG, TIFF and PNG use algorithms to produce a balance between file size and image quality.
B. Image Editing
  • Software suites such as Adobe CC allow for almost limitless choices when editing visual material.
  • Applications within Adobe CC such as Photoshop and Illustrator allow workspaces to be selected prior to editing.
  • A workspace in Adobe apps is an intermediate colour model-related colour space used during the editing process.
  • A global setting for the colour mode of a workspace in Illustrator can be selected in the Document colour mode dialogue box during the set-up of a new document.
  • Workspaces can also be temporarily switched between CMYK, HSB, RGB, Greyscale and Websafe RGB in the Colour Settings dialogue box without affecting the Document colour mode.
C. Monitoring Images
  • Monitor profiles control the translation of data within image files into a monitor’s colour space.
  • On-screen controls may include:
  • Monitor calibration tools ensure accurate colour across the visible spectrum and fine tonal adjustment. Professional monitor calibration packages include:
    • Datacolor SpyderX
    • Calibrite ColorChecker
    • Wacom Colour Manager
    • SpectraCal Colorimeter
D. Image Output
  • Colour management systems use output device profiles to prepare and translate the data in edited documents to match the capabilities of an output device and ensure the best possible match.
  • To ensure consistency across applications, Adobe CC provides options to be selected in the Colour Settings dialogue box that ensures all applications are synchronized to use the same device-independent colour space.
    • RGB Colour Settings options include:
      • Adobe RGB (1998)
      • Prophoto RGB
      • sRGB
    • An extensive range of CMYK colour space options are also available.

A. Lab colour space (entire visible spectrum)   B. Documents (working space)   C. Devices 

This diagram illustrates the generic colour gamuts of different types of devices and documents.


Colour models

About colour models

A colour model derived from colour theory enables a more precise and reproducible method of representing and working with colour.

  • Colour models are a practical application of colour theory that establish terms, definitions, rules or conventions, and systems of notation for encoding colours and their relationships to one another.
  • These days, the most practical colour models are built into applications such as Adobe Creative Cloud and allow seamless digital output to TVs, computers, phones, or printing onto paper and other surfaces.
  • Understanding colour models and utilizing them effectively can contribute to maintaining consistent and accurate colour reproduction across various media.
  • Widely used colour models include:
  • In addition to the colour models mentioned above, numerous other models are used in specific contexts, such as the Lab colour model employed in printing or the LCH colour model used in digital image processing.

A colour model is a framework that allows us to:

  • Make sense of colour in relation to human vision, the surrounding world, and various media and technologies.
  • Understand the relationship between different colours and their properties.
  • Mix specific colours from other colours to achieve predictable and desired results.
  • Specify colours using names, codes, notations, equations, and other forms of representation.
  • Organise and utilize colour for different purposes, such as design, visual arts, or scientific applications.
  • Use colours in consistent and repeatable ways across different platforms and media.
  • Develop systems and rules for blending and using different media, such as light, pigments, or inks.
  • Create colour palettes, define gamuts, and establish colour guides to guide artistic or design decisions.