Dictionary tag: B

Bands of colour

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

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.

  • When sunlight is dispersed by rain and forms a rainbow, an observer will typically 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.
  • 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.
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Related diagrams

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

References

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

Bands of colour

An observer perceives bands of colour when visible light separates into its component wavelengths and the human eye distinguishes a series of distinct adjacent colours.

  • The human eye and brain together translate light into colour.
  • When rain disperses sunlight and forms a rainbow, an observer will typically distinguish 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

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.
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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.

Bioelectroluminescence

Bioelectroluminescence is a type of chemiluminescence where the emission of light is specifically produced by a biological process within a living organism.

  • Bioelectroluminescence is a type of chemiluminescence where the emission of light is specifically produced by a biological process within a living organism.
  • It is found in many marine organisms (fireflies, jellyfish, anglerfish), as well as some terrestrial species like fungi.
  • It involves specialized biochemical reactions within the organism. Typically, a molecule called luciferin reacts with oxygen in the presence of an enzyme called luciferase.
  • The reaction releases energy in the form of visible light, creating the bioluminescent glow.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms		Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars		Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles		Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests		Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs		ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks		Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters		Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights		ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives		TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests		PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

Related diagrams

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

References
  • Bioelectroluminescence is a type of chemiluminescence where the emission of light is specifically produced by a biological process within a living organism.
  • It is found in many marine organisms (fireflies, jellyfish, anglerfish), as well as some terrestrial species like fungi.
  • It involves specialized biochemical reactions within the organism. Typically, a molecule called luciferin reacts with oxygen in the presence of an enzyme called luciferase.
  • The reaction releases energy in the form of visible light, creating the bioluminescent glow.

Bioluminescence

Bioluminescence is a type of luminescence resulting from the production and emission of electromagnetic radiation by living organisms. Bioluminescence, meaning “living light,” occurs in a wide variety of creatures, from bacteria and fungi to fish, and insects. Unlike artificial light sources, bioluminescence doesn’t involve heat generation but involves a chemical process within the organisms themselves.

  • The chemistry behind bioluminescence lies in a special molecule called luciferin. When luciferin reacts with another molecule, usually an enzyme called luciferase, in the presence of oxygen, energy is released. This energy takes the form of visible light, creating the characteristic glow. Different luciferins and luciferases determine the emitted light’s colour and intensity.
  • Marine organisms including single-cell dinoflagellates, some jellyfish and the deep sea anglerfish use the luciferin-luciferase reaction to release energy as blue-green light.
  • Terrestrial Organisms including fireflies and glowworms also use the luciferin-luciferase reaction to create light.
    • Fireflies have a specialized light organ that contains luciferin and luciferase. The reaction involves adenosine triphosphate to provide the energy to excite an electron in the luciferin, which then emits a photon upon returning to its ground state.

    • Glowworm larvae of various fly and beetle species possess luciferin and luciferase and produce a similar light-emitting reaction.

Materials Involved in Bioluminescence
  • Luciferin: This small organic molecule acts as the fuel, with different variations depending on the organism. It’s the source of the chemical energy that gets converted into light.
  • Luciferase: This enzyme acts as the catalyst, speeding up the reaction between luciferin and oxygen. Different luciferases determine the colour and intensity of the emitted light.
  • Oxygen: Although not universally used, oxygen serves as the final electron acceptor in most bioluminescent reactions, stabilizing the energy released and enabling light emission.
  • Adenosine Triphosphate (ATP): In some systems like fireflies, ATP provides the initial energy boost to excite the electron in luciferin, kicking off the bioluminescent reaction. However, in these systems, the excited electron ultimately interacts with oxygen as the final electron acceptor, similar to systems that use oxygen directly.
The Bioluminescent Process
  • Excitation: The luciferase enzyme interacts with luciferin, either directly using energy from ATP (fireflies) or indirectly drawing energy from sunlight or chemical reactions with the organism (dinoflagellates). This interaction excites an electron in the luciferin molecule, raising its energy state.
  • Electron Transfer to oxygen: In most bioluminescent reactions, the excited electron in the luciferin molecule doesn’t directly emit light. Instead, it transfers its energy through a series of steps, ultimately reaching oxygen as the final electron acceptor.
  • Energy Release: During this transfer, some of the released energy is channelled into a specific form suitable for light emission. This specific energy then triggers the luciferin molecule to emit a photon (light).
  • Light Emission: Upon releasing this converted energy as a photon (light), the excited molecule returns to its ground state.
  • Variations: While the basic principle remains similar, specific molecules, reaction pathways, and light colours can vary significantly depending on the organism and its unique bioluminescent system.
Electron Excitation in Bioluminescence
  • Imagine an electron within the luciferin molecule orbiting its nucleus, like a ball at the bottom of a hill. This is the ground state, where the electron has its lowest energy.
  • During the bioluminescent reaction, the luciferin molecule receives energy from different sources depending on the organism:
    • Fireflies: The luciferase enzyme uses ATP to directly excite the electron, pushing it to a higher energy level.
    • Dinoflagellates: Light or chemical reactions interact with luciferin, indirectly causing electron excitation.
    • Once energized, the excited electron jumps to a higher energy level within the luciferin molecule. This excited state is unstable, and the electron wants to return to its ground state.
    • However, it doesn’t simply drop back down. Instead, in most bioluminescent organisms, it transfers its energy through a series of steps to oxygen, the final electron acceptor. The excited electron doesn’t physically jump to oxygen but interacts with it indirectly through the molecule chain, ultimately transferring its energy.
    • During this transfer, some of the released energy gets converted into a specific form suitable for light emission. Finally, this converted energy triggers the emission of a photon from the excited molecule (often luciferin itself), releasing the remaining energy and bringing the electron back to its ground state.
Light sources
Emission mechanism DescriptionExamples
LIGHT-EMITTING PROCESS
LuminescenceLight emission due to the excitation of electrons in a material.Electrons within a material gain energy and then release light as they return to a lower energy state.Bioelectroluminescence
Electroluminescence
Photoluminescence
- Fluorescence
- Phosphorescence
Sonoluminescence
Thermoluminescence
Blackbody radiation (Type of thermal radiation)Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.Electromagnetic radiation (including visible light) emitted by any object with a temperature above absolute zero.All objects above temperature of absolute zero.
ChemiluminescenceLight from natural and artificial chemical reactions.Light from natural and artificial chemical reactions.Bioluminescence
Chemiluminescent reactions:
- Luminol reactions
- Ruthenium chemiluminescence
Nuclear reactionLight emission as a byproduct of nuclear reactions (fusion or fission).Light emitted as a byproduct of nuclear reactions.Nuclear reactors
Stars undergoing fusion
Thermal radiationLight emission due to the thermal excitation of atoms and molecules at high temperatures.Light emission due to the thermal excitation of atoms and molecules.Sun
Stars
Incandescent light bulbs
TriboluminescenceLight emission due to mechanical stress applied to a material.Light emission due to the mechanical stress applied to a material, causing the movement of electric charges and subsequent light emission.Sugar crystals cracking
Adhesive tape peeling
Quartz crystals fracturing.
Natural light source
Fireflies
Deep-sea creatures
Glowing mushrooms		Bioluminescence Light emission from biological organisms.Involves the luciferase enzyme.
Sun
Stars		Nuclear FusionLight emission as a byproduct of nuclear fusion reactions in stars.Electromagnetic spectrum (visible light, infrared, ultraviolet).
Fire
Candles		Thermal radiationLight emission due to the thermal excitation of atoms and molecules during the combustion of a fuel source.Burning of a fuel source, releasing heat and light.
Artificial light source
Fluorescent lights Highlighters
Safety vests		Chemiluminescence Light emission from chemical reactions.Fluorescence (absorption and re-emission of light).
Glow sticks
Emergency signs		ChemiluminescenceLight emission due to phosphorescence - a type of chemiluminescence.A type of chemiluminescence where light emission is delayed after the initial excitation.
Glow sticks
Light sticks		Chemiluminescence Chemiluminescence Light emission from a chemical reaction that does not involve combustion.
Tungsten light bulbs
Toasters		Thermal radiationHeated filament radiates light and heat.Light emission from a hot filament.
Fluorescent lamps
LED lights		ElectroluminescenceExcitation of atoms by electric current.Light emission when electric current excites atoms in a material.
Neon signsElectrical DischargeDischarge of electricity through gas.Light emission when electricity flows through a gas.
Sugar crystals cracking
Pressure-sensitive adhesives		TriboluminescenceLight emission from friction or pressure.Light emission due to mechanical forces.
Fluorescent paint Highlighters
Safety vests		PhotoluminescenceAbsorption and subsequent re-emission of light at a lower energy.Absorption and re-emission of light.

Light Sources: Mechanism, examples, and everyday applications

Footnote: Cerenkov radiation and Synchrotron radiation are not included in the table because they are not conventionally classified as light sources.

Related diagrams

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

References
  • Bioluminescence, meaning “living light,” is the production and emission of light by living organisms. It occurs in a wide variety of creatures, from bacteria and fungi to fish, insects, and even some deep-sea animals. Unlike artificial light sources, bioluminescence doesn’t involve heat generation, making it a truly cold light.
    • The chemistry behind bioluminescence lies in a special molecule called luciferin. When luciferin reacts with another molecule, usually an enzyme called luciferase, in the presence of oxygen, energy is released. This energy takes the form of visible light, creating the characteristic glow. Different luciferins and luciferases determine the emitted light’s colour and intensity.
    • Whilst the light is what we see, bioluminescence is a complex biological process.
    • The materials involved in bioluminescence include:
      • Luciferin: The “fuel” molecule, often a small organic molecule with a specific chemical structure depending on the organism.
      • Luciferase: The “spark,” typically an enzyme that acts as a catalyst, accelerating the reaction between luciferin and oxygen.
      • Oxygen: Essential for most bioluminescent reactions, acting as the final electron acceptor.
    • The reactions involved in bioluminescence include:
      • Activation: Luciferase activates luciferin through various mechanisms, depending on the specific type.
      • Oxidation: Oxygen reacts with the activated luciferin, transferring energy to an excited state.
      • Light emission: As the excited molecule returns to its ground state, energy is released as visible light with a specific wavelength determined by the energy change.

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

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.

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.
  • 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

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

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 solely on its temperature.
  • The radiation emitted by a black body is known as blackbody radiation.
  • 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.

Bohr model

The Bohr model of the atom, proposed by Danish physicist Niels Bohr in 1913, represented a significant development in the understanding of atomic structure. It revolutionized the view in classical physics of the atom by introducing the concept of quantized energy levels for electrons.

  • The Bohr model of the atom includes:
    • Central Nucleus: Bohr’s model retained the idea of a central nucleus consisting of positively charged protons and neutral neutrons, around which electrons orbit.
    • Quantized Energy Levels: Unlike the classical model, where electrons could orbit the nucleus at any distance, Bohr proposed that electrons can only occupy certain specific energy levels. These energy levels are quantized, meaning they are discrete and not continuous. Electrons can move between these levels by absorbing or emitting energy in discrete amounts (quanta).
    • Stationary Orbits: Bohr suggested that electrons orbit the nucleus in fixed, circular paths or orbits. Each orbit corresponds to a specific energy level, with the lowest energy level being closest to the nucleus. These orbits are sometimes referred to as “stationary states.”
    • Energy Absorption and Emission: When an electron absorbs energy, it can jump to a higher energy level (further away from the nucleus). Conversely, when it emits energy, it falls back to a lower energy level (closer to the nucleus). The energy emitted or absorbed corresponds to the difference in energy between the initial and final states of the electron.
    • Stability of Orbits: According to the Bohr model, electrons in stable orbits do not emit energy and hence do not spiral into the nucleus. They remain in their orbits until a change in energy causes them to transition to a different orbit.
  • While the Bohr model provided valuable insights into atomic structure and spectral lines, it had limitations, especially when applied to larger atoms. It was eventually superseded by quantum mechanics, which provided a more comprehensive understanding of the behaviour of electrons in atoms.
Related diagrams

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

References

Bohr model

The Bohr model of the atom, proposed by Danish physicist Niels Bohr in 1913, represented a significant development in the understanding of atomic structure. It revolutionized the view in classical physics of the atom by introducing the concept of quantized energy levels for electrons.

  • While the Bohr model provided valuable insights into atomic structure and spectral lines, it had limitations, especially when applied to larger atoms. It was eventually superseded by quantum mechanics, which provided a more comprehensive understanding of the behaviour of electrons in atoms.

Brightness

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

  • 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.

Brightness

In this resource, the terms brightness and colour brightness have distinct meanings. The first refers to a property of light, and 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
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Related diagrams

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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: HSB colour model

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

  • 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.
  • 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.
  • 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.
    • Saturation refers to the vividness of a colour compared to an unsaturated colour.
    • 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.

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.

About amplitude, brightness, colour brightness and intensity

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

Amplitude
Brightness
  • 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:
  • 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 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.
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.
Related diagrams

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

References
  • 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.
  • Follow this link for a discussion of colour brightness in relation to the HSB colour model, where H represents hue, S represents saturation, and B represents brightness.