Dictionary tag: B

Bands of colour

An observer sees bands of colour when a continuous range of wavelengths within the visible spectrum appear to be a single colour.

In the presence of a rainbow, an observer will typically see six bands of colour (red, orange, yellow, green, blue and violet) rather than a unique colour corresponding with each wavelength.

  • The phenomenon of perceiving distinct colour bands is typically attributed to the characteristics of human colour vision, or as an artefact of human colour vision.
  • There is no property belonging to the visible part of the electromagnetic spectrum that that results in the appearance of bands of colour to an observer.
  • The visible spectrum is composed of a continuous range of wavelengths between red and violet that produce a continuous range of corresponding colours.
  • In experimental situations, human observers can distinguish between spectral colours corresponding with many hundreds of different wavelengths of light.
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.
About rainbows and bands of colour
  • There are several reasons why an observer looking at phenomena like rainbows perceives bands of colour.
    • The human perceptual system tends to simplify colour information rather than perceiving a smooth gradient across the spectrum.
    • Our eyes respond to colours based on their relative brightness and hue when presented with a portion or the entirety of the visible spectrum.
    • Observers tend to search for colours they are familiar with and can recognize and name.
    • Cone cells in our eyes are especially sensitive to red, green, and blue wavelengths due to the trichromatic nature (trichromacy) of human vision.
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.
Related diagrams

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

References

https://en.wikipedia.org/wiki/Rainbow

An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes between some colours better than others.

      • The human eye and brain together translate light into colour.
      • When sunlight is dispersed by rain and forms a rainbow, an observer often distinguishes red, orange, yellow, green, blue and violet bands of colour.
      • Although a rainbow contains electromagnetic waves with all possible wavelengths between red and violet, some ranges of wavelengths appear more intense to a human observer than others.

Bands of colour

When light separates into its component wavelengths, an observer perceives bands of colour due to the human eye’s sensitivity to different parts of the visible spectrum.

  • When sunlight is dispersed by rain and forms a rainbow, an observer often distinguishes red, orange, yellow, green, blue and violet bands of colour.
  • Although an atmospheric rainbow contains electromagnetic waves with all possible wavelengths between red and violet, our eyes encounter difficulties in distinguishing between colours within specific regions of this spectrum. For example, all wavelengths between 520 to 570 nanometers may appear to be exactly the same green to most observers.

Bands of colour

The fact that we see a few distinct bands of colour in a rainbow, rather than a smooth and continuous gradient of hues, is sometimes described as an artefact of human colour vision.

  • We see bands of colour because the human eye distinguishes between some ranges of wavelengths of visible light better than others.
  • It is the interrelationship between light in the world around us on one hand and our eyes on the other that produces the impression of different bands of colour.
  • The visible spectrum is made up of a smooth and continuous range of wavelengths that correspond with a smooth and continuous range of hues.
  • There is no property belonging to electromagnetic radiation that causes bands of colour to appear to a human observer.

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.
      Custom
    • 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.

Bipolar cells

Bipolar cells are the retinal interneurons that provide the primary pathway from photoreceptors (rod and cone cells) to ganglion cells. In addition to directly transmitting signals from photoreceptors to ganglion cells, they connect to amacrine cells that assist in integrating information and forming a comprehensive picture of an entire visual scene.

  • Bipolar cells are linked to the light-sensitive rod and cone cells in the retina of the human eye.
  • There are approximately a dozen types of bipolar cells, all of which serve as centres for integration.
  • Each type of bipolar cell acts as a dedicated channel for information about light, collected by either a single or a small group of rod or cone cells.
  • Each type of bipolar cell interprets and relays its own version of information gathered from photoreceptors to ganglion cells.
  • The signals relayed from bipolar cells to ganglion cells include not only the direct responses of bipolar cells to signals resulting from photo-transduction but also responses to signals indirectly received via information from amacrine cells.
  • We could envisage a type of bipolar cell that connects directly from a cone to a ganglion cell and simply conveys information about wavelength. The ganglion cell uses this information to discern whether a specific point in the observed scene is red or green.
  • Not all bipolar cells create synapses directly with a single ganglion cell. Some relay information sampled by various groups of ganglion cells. Others end at different locations within the retina’s complex network of interconnections, enabling them to deliver packets of information to a range of locations and cell types.
Related diagrams

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

References

https://en.wikipedia.org/wiki/Retina_bipolar_cell

Summary

Bipolar cells

Bipolar cells

Bipolar cells, a type of neuron found in the retina of the human eye connect with other types of nerve cells via synapses. They act, directly or indirectly, as conduits through which to transmit signals from photoreceptors (rods and cones) to ganglion cells.

There are around 12 types of bipolar cells and each one functions as an integrating centre for a different parsing of information extracted from the photoreceptors. So, each type transmits a different analysis and interpretation of the information it has gathered.

The output of bipolar cells onto ganglion cells includes both the direct response of the bipolar cell to signals derived from photo-transduction but also responses to those signals received indirectly from information provided by nearby amacrine cells that are also wired into the circuitry.

We might imagine one type of bipolar cell connecting directly from a cone to a ganglion cell that simply compares signals based on differences in wavelength. The ganglion cell might then use 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 so enabling them to carry packets of information to an array of different locations and cell types.

Blackbody

An object that absorbs all radiation falling on it, at all wavelengths, is called a blackbody.

  • A blackbody is a theoretical concept for an object that completely absorbs all electromagnetic radiation, regardless of factors such as angle of incidence, wavelength, frequency, or amplitude.
  • A perfect blackbody doesn’t exist in reality. However, certain objects and materials, such as stars and carbon in soot or graphite behave almost like blackbodies.
  • When a blackbody emits electromagnetic radiation, the spectral distribution of the emissions is dependent on its temperature.
  • The radiation emitted by a black body is known as blackbody radiation.
  • Blackbody radiation is the type of electromagnetic radiation released by a body in thermodynamic equilibrium with its surroundings. This means that the body emitting the radiation is in a state where there is no net exchange of energy between the body and its environment.
  • If enough heat is applied to a blackbody, it will begin to appear orange at a certain point, and as the temperature increases, it changes from white to pale blue and then to light blue.
  • The study of blackbody radiation has practical applications in the development and testing of materials for lighting, heating, and thermal imaging equipment.
  • Blackbody radiation is a cornerstone in the study of quantum mechanics.
Related diagrams

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

References

Brightness

In this resource, the terms brightness and colour brightness have distinct meanings. The first refers to a property of light, the second to a property of colour as detailed below.

  • In the first instance, brightness (as opposed to colour brightness) is used to refer to a property of light.
  • Colour brightness is used to refer to how much colour something appears to emit or reflect towards an observer.
  • When brightness is used in connection with the HSB colour model it is used alongside hue and saturation and refers to the method of selecting and adjusting colours in software applications such as Adobe Illustrator.
  • The HSB colour model is a representation of colours that combines hue, saturation, and brightness components.
  • In the HSB brightness represents the intensity or lightness of a colour, with higher values indicating brighter or lighter colours.
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.
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.
About brightness & colour models
About the HSB colour model and colour brightness

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

  • The RGB and HSB colour models differ only in the way colours are represented in terms of colour notation and handled in software and applications.
  • Both the HSB and RGB colour models involve mixing red, green, and blue light to produce other colours.
  • HSB is popular because it offers an intuitive method for selecting and adjusting colours within applications like Adobe Creative Cloud, which is commonly used in design, photography, and web development.
  • The HSB colour model can be used to describe any colour on a TV, computer or phone screen.

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 a colour wheel and expressed in degrees between 00 and 2590.
  • Saturation refers to the perceived difference between one colour and another in terms of purity.
    • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour that appears dull and washed out, eventually reaching a monochromatic grey tone (0%).
    • A fully saturated colour is produced by a single wavelength or a narrow band of wavelengths of light.
    • On HSB colour wheels, saturation is typically shown to increase from the centre to the circumference.
  • Brightness (colour brightness) refers to the difference between a hue that appears bold and vivid at maximum brightness (100%) and then appears progressively darker in tone until it appears black at minimum brightness(0%).
  • Colour brightness is often evident in the distinction between how a colour appears to an observer under well-lit conditions compared to its more subdued appearance when in shadow or under poor illumination.
Related diagrams

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

References

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.
  • 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

Brightness: HSB colour model

This entry discusses colour brightness in relation to the HSB colour model, where H represents hue, S represents saturation, and B represents brightness.

Colour brightness can be understood as the variation in how a colour is perceived by an observer under well-lit conditions compared to its more muted appearance when in shadow or under poor illumination.

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.
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.
About brightness & colour models

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:

Colour theory and human perception

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
Colour theory and colour management

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.
Where to find colour theories

About the 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).

  • The main difference between the HSB colour model and the RGB colour model is how colours are represented and managed in software and applications.
  • The HSB model represents colours based on hue, saturation, and brightness, whereas the RGB model mixes red, green, and blue light to create colours.
  • HSB is popular because it provides a user-friendly way to select and modify colours when using applications like Adobe Creative Cloud for design, photography, or web development.
  • On HSB colour wheels, saturation typically increases from the centre towards the edge.

In the HSB colour model:

  • Hue refers to the perceived difference between colours and is usually described using names such as red, yellow, green, or blue.
    • Hue can be measured as a location on an HSB colour wheel and expressed as a degree between 0 and 360.
  • Saturation refers to the vividness of a colour compared to an unsaturated colour.
    • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour (0%)that appear either:
      • Dull and washed out until all colour disappears, leaving only a monochromatic grey tone (0%).
      • Misty or milky the nearer they are to white.
    • On many HSB colour wheels, saturation decreases from the edge to the centre.
  • Brightness refers to the perceived difference in the appearance of colours under ideal sunlit conditions compared to poor lighting conditions where a hue’s vitality is lost.
    • Brightness can be measured as a percentage from 100% to 0%.
    • As the brightness of a fully saturated hue decreases, it appears progressively darker and achromatic.

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About colour brightness & light intensity
Related diagrams

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

References