Device-dependent digital colour spaces
  • Device-dependent means the colours selected on-screen during production and editing are not matched to the specific equipment used to reproduce them.
  • Device-dependent means the digital colour space being used as part of a workflow does not know how an image will be used or what type of equipment it will be matched with.
  • Device-dependent colour spaces use basic colour notation such as RGB decimal and hexadecimal values without determining the exact colours that will be selected by the equipment it is reproduced on.
  • Common Device-dependent colour spaces include:
Device-independent digital colour space
  • A device-independent colour space is one where specified colours appear relatively the same regardless of the equipment used to reproduce them.
  • Device-independent colour spaces are used to ensure colours appear consistent throughout a workflow and that colours can be accurately reproduced at the end of the process.
  • An example of a device-independent colour space is the CIE Lab* colour space (known as CIELAB and based on the human visual system).
    • sRGB
    • Adobe 1998
    • CIE Lab*
    • Pantone
  • sRGB (standard Red Green Blue) is a widely used colour space or standard for displaying images and colours on electronic devices, such as computer monitors, smartphones, and TVs.
    • Colour Representation: In the sRGB colour space, each colour is represented by combining three primary colours: red, green, and blue. By varying the intensity of these three colours, a wide range of colours can be displayed.
    • Gamma Correction: sRGB uses a gamma correction curve to ensure that the colours displayed on screens appear more natural to the human eye. Gamma correction adjusts the brightness levels to match how our eyes perceive light.
    • Limited Gamut: While sRGB covers a broad range of colours, it has a relatively limited gamut compared to some other colour spaces. This means it may not accurately represent certain vibrant or intense colours found in the real world.
    • Default Colour Space: Most electronic devices are set to use the sRGB colour space as the default, ensuring that images and colours look consistent across different screens.
    • Compatibility: sRGB is widely supported by various software, web browsers, and devices, making it an excellent choice for sharing images and graphics online or for general purposes.
Adobe 1998
  • Adobe RGB (Adobe 1998 or Adobe RGB 1998) is another widely used colour space, commonly used in the professional photography and printing industry.
  • It was developed by Adobe Systems to offer a larger gamut than sRGB, making it more suitable for preserving and reproducing a broader range of colours, particularly vibrant and saturated colours found in some real-world scenes.
    • Larger Gamut: Adobe RGB has a wider gamut compared to sRGB, meaning it can represent more colours. This expanded gamut is especially beneficial for capturing and preserving the vibrant colours seen in nature, such as deep greens, rich reds, and intense blues.
    • Chromaticity Coordinates: The colour values in Adobe RGB are specified using chromaticity coordinates for red, green, and blue primaries. These coordinates determine the position of each colour in the colour space and define its hue and saturation.
    • Suitable for Printing: Adobe RGB is often preferred for professional printing because it can retain a more extensive range of colours during the printing process, resulting in more accurate and vibrant prints.
    • Less Common for Web and Display: While Adobe RGB is excellent for preserving colours in high-quality prints, it is less commonly used for web and digital display purposes. Some web browsers and software may not fully support Adobe RGB, leading to potential colour shifts if not handled correctly.
CIE Lab*
  • CIE Lab* (CIELAB) is a device-independent colour space developed by the International Commission on Illumination (CIE).
  • It is designed to represent all visible colours in a way that is consistent with human perception, making it an essential tool for colour-related applications, such as colour matching, colour conversion, and colour comparison.
  • CIE Lab* is based on the concept of perceptual uniformity, which means that the numerical differences between colours in this space correspond to the perceived differences in the human visual system. In simpler terms, two colours that have the same numerical distance in CIELAB are visually perceived as equally different.
  • CIE Lab* encompasses the entire range of human vision, covering all the visible colours, including highly saturated and vivid colours that cannot be fully represented in some other colour spaces.
  • The asterisk (*) used in the name CIE Lab* distinguishes it from an earlier 1976 version.
  • The Pantone colour system is widely used for mixing paint and defines a colour space by:
    • Matching an existing colour or set of colours to Pantone colour swatches, or
    • Choosing a set of colours from Pantone colour swatches
    • Calibrating a paint machine (or another type of equipment) to accurately reproduce the colour of each swatch.

Dictionary of light, colour & vision

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

  • 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 pop-up entries strip definitions back to basics and can be viewed without leaving the page.


Diffraction of electromagnetic radiation refers to various phenomena that occur when an electromagnetic wave encounters an obstacle or passes through an opening.

  • Diffraction and interference are phenomena associated with all kinds of waves. Electromagnetic waves are a special case however because of their unique behaviour.
  • Diffraction of electromagnetic waves deals with the way light bends around the edges of obstacles into regions that would otherwise be in shadow.
  • Interference deals with the way that electromagnetic waves behave during the diffraction process.
  • Diffraction can be produced by the edges or by a hole (aperture) in any opaque surface or object.
  • Diffraction causes a propagating electromagnetic wave to produce a distinctive pattern as waves interfere with one another. The resulting pattern becomes visible if diffracted light subsequently strikes a surface.
  • Diffraction produces a circular pattern of concentric bands when a narrow beam of electromagnetic waves passes through a small circular aperture and then strikes a flat surface.
  • Diffraction and interference of electromagnetic waves are not limited to visible light but occur across the entire electromagnetic spectrum, including radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays.
Diffraction in classical physics
  • In classical physics, an explanation of the diffraction of electromagnetic waves treats each point at which a propagating wavefront encounters the edge of an obstacle as a site at which a new spherical wavelet is generated, which modifies the original waveform.
  • Separate spherical wavelets bend independently of one another beyond the site at which an obstacle is encountered. However, interference between them alters the way they bend and the distance they must travel before striking a surface.
  • Explanations that describe the process of diffraction and interference patterns belong to Wave Theory and are the result of more than two centuries of study in the field of optics.
Diffraction in quantum mechanics
  • In modern quantum mechanics, diffraction is explained by referring to the wave function and probability distribution of each photon of light as it encounters the corner of an obstacle or the edge of an aperture.
  • Wave functions and probability distributions are part of mathematical formulations of the outcomes of all possible measurements of a photon’s behaviour in the course of diffraction.

Diffuse reflection

Diffuse reflections occur when light scatters off rough or irregular surfaces, such as matte or textured surfaces. The scattered light reflects in various directions, leading to a lack of clear images or sharp details.

  • Most objects produce diffuse reflections as light scatters off their surfaces in random directions. It is often almost impossible to pick out the shape or colour of objects in a diffuse refection.
  • All objects obey the law of reflection on a microscopic level.
  • If the irregularities on the surface of an object are larger than the wavelengths of the incident light, light reflects in all directions and produces diffuse reflections.
  • A diffuse reflection is easily distinguished from the mirror-like qualities of a specular reflection.


In the field of optics, diffusion refers to situations that cause parallel rays of light to spread out more widely.  When light undergoes diffusion it becomes less concentrated.

  • Diffusion results from any medium or material that scatters light during refraction, reflection or transmission.
  • Scattering takes place when streams of photons (or waves of light) are deflected in different directions.
  • Diffusion of light is a common phenomenon encountered in various real-world scenarios, such as when light passes through frosted glass, paper, or a cloudy sky.
  • Diffusion is responsible for softening shadows and reducing harsh contrasts in photography, making it particularly useful in portrait and product photography.
  • All objects obey the law of reflection on a microscopic level. If the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion.
  • Transparent and translucent materials transmit diffuse light unless their surfaces are perfectly flat and their interiors are free of any foreign material.
  • When diffuse light falls on an object or scene it produces softened effects without sharp detail.
  • A mirror-like reflection free of the effects of diffusion is called a specular reflection.

Digital printing

Digital printing uses the CMYK colour model to enable cyan, magenta, yellow and black inks to be used to output digital files onto paper and other sheet materials.

  • Digital printers typically overlay highly reflective white paper with cyan, magenta, yellow and black inks or toner.
  • CMYK is a subtractive colour model suited to working with semi-transparent inks.
  • Printing has a smaller gamut than TV, computer and phone screens which rely on light emission, rather than reflection of light off sheets of paper.
  • Digital displays produce comparatively brighter colours than printers because the amplitude of each wavelength of light is larger than can be achieved by a printer.
  • Digital printers produce dull and less intense colours than displays because the amplitude of each wavelength of light is smaller when light is reflected off paper through inks.
  • A display device, such as a computer screen, starts off dark and emits red, green and blue light to produce colour.
  • CMYK inks are the standard for colour printing because they have a larger gamut than RGB inks.
  • Highlights are produced on digital printers by printing without black to allow the maximum amount of light possible to shine through and reflect off the paper.
  • Mid tones rely on the brightness and transparency of the inks and the reflectivity of the paper to produce fully saturated colours.
  • Shadows are produced by adding black to both saturated and desaturated hues.
  • All modern printers have built-in colour profiles that allow both device-dependent and device-independent RGB colour spaces to be converted to CMYK.

Digital screen

A digital screen (or digital display) is an output device for the presentation visual of information. RGB digital screens are used in TVs, computers, phones and projectors.

  • Digital screens use the RGB (Red, Green, Blue) colour model to represent and display information.
  • The range of colours that different types of screen can display depends on their technology and specifications.
  • Many RGB digital screens include light-emitting diodes (LEDs) that are able to directly or indirectly adjust the intensity of red, green and blue light within each addressable component of the screen to produce pixels of colour that together produce an image.
  • LEDs are typically used to backlight LCD (liquid crystal display )screens.  Different colours are created by colour filters and by adjusting the amount and the polarization of light that is allowed to pass through the crystal sub-pixels that make up each pixel on the screen.
  • In an OLED displays, each pixel provides its own illumination. The organic materials in the OLED emit light when an electric current is applied. Because each pixel can be turned on or off individually, OLED displays are able to achieve deeper blacks (by completely turning off pixels) and a higher contrast ratio compared to LED-backlit LCD screens.
  • Fully saturated hues (colours) are produced when pixels in an area of the screen are at maximum intensity (brightness).
  • Darker tones of any hue are produced by decreasing the intensity of light produced by each pixel.
  • Some digital screens use technologies other than RGB to create images. For example, some E-ink screens used in e-readers can only display shades of grey.


In the field of optics, dispersion is shorthand for chromatic dispersion which refers to the way that light, under certain conditions, separates into its component wavelengths, enabling the colours corresponding with each wavelength to become visible to a human observer.

Dispersion (or 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.

  • Dispersion is the result of the relationship between refractive index and wavelength.
  • Every wavelength of light is affected to a different degree by the refractive index of a medium. The result is that each wavelength changes direction and speed by a different amount.
  • In the case of white light, the separate wavelengths span out with red at one end and violet at the other.
  • A familiar example of dispersion is when white light strikes raindrops and a rainbow of colours becomes visible to an observer.
  • As the light first enters and then exits a droplet it separates into its component wavelengths which the observer perceives as colour.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception experienced by an observer.


Dispersion (or 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.

Distance to, size and duration of rainbows

Distance to a rainbow

Rainbows are formed from the millions of individual raindrops that happen to be in exactly the right place at the right time, so it is difficult to be precise about how far away a rainbow is.

  • Because a rainbow is a trick of the light rather than a solid material object set in the landscape it has no fixed position and is at no fixed distance from an observer. Instead, rainbows move as the Sun and the observer move or as curtains of rain cross the landscape.
  • Because a rainbow is composed of light reflecting off and refracting in millions of individual raindrops it might be fair to say that the distance to a rainbow is the distance to the location of the greatest concentration of raindrops diverting photons towards an observer.
  • An observer cannot easily estimate the distance to a raindrop or a curtain of rain along their line of sight but the position of clouds or objects in the landscape can help to determine where rain is falling.
  • The position of a rainbow is primarily determined by angles. The angles are constants and result from the physical properties of light and water droplets, not least the laws of reflection and refraction.
  • As an observer moves, the rainbow they see moves with them and the angles are preserved.
Size of a rainbow
  • Just as the visual impression of the size of the moon depends on how near it is to the horizon, the apparent diameter of a rainbow is also affected by other features in the landscape.
Duration of a rainbow
  • A rainbow may be visible for minutes on end before receding slowly into the distance. In other situations, a rainbow may appear one moment and be gone the next.


Radiation is energy that comes from a source and travels through space at the speed of light.

  • Radiant energy has an electric field, and a magnetic field and may be described in terms of electromagnetic waves or in terms of bundles of photons travelling in a wave-like pattern.
  • Visible light is a form of radiation often described in terms of either electromagnetic waves or photons.
  • Types of radiation with the highest energy include ultraviolet radiation, x-rays, and gamma rays.
  • When x-rays or gamma-rays interact with atoms, they can remove electrons which destabilises them and make them radioactive.
  • Radioactivity is the spontaneous release of energy from an unstable atom as it returns to a stable state.
  • Ionizing Radiation is the energy that comes out of a radioactive atom.