Perceived colour

The perceived colour of an object, surface or area within the field of vision is an attribute of visual perception. First and foremost, perceived colour refers to what an observer sees in any given situation and so is a subjective experience.

  • It is the human ability to perceive and distinguish between colours that provides an important basis for the way that we sense and make sense of the world.
  • A distinction can be made between the physical properties of things in the world around us and how they appear to a human observer.
    • When talking about perceived colour, a distinction can be made between:
    • The properties of light.
    • The properties of objects.
  • What an observer perceives as a result of the attributes of visual perception.
  • Perceived colour can be described by chromatic colour names such as pink, orange, brown, green, blue, purple, etc., or by achromatic colour names such as black, grey or white etc. Colour names can be qualified by adjectives such as dark, dim, light, bright etc.
  • Colour perception consists of any combination of chromatic and achromatic content.
  • Perceived colour depends on the spectral distribution of a colour stimulus and so the range and mixture of wavelengths and intensities of light that enter the eye.
  • Colour perception tends to provide visual information that is most important to an observer rather than information that is always objectively accurate.
  • Perceived colour depends on factors such as the size, shape and structure of all the objects in view, the composition and texture of their surfaces, their position and orientation in relation to one another, their location within the field of view of an observer and the direction of incident light.
  • Colour perception can be affected by the state of adaptation of an observer’s visual system. An example of this is when the photosensitive cells embedded in the retina become fatigued from long exposure to strong colour and then produce an afterimage when we look away.
  • Perceived colour is strongly influenced by factors such as an observer’s expectations, priorities, current activities, recollections and previous experience.
  • Perceived colour is defined in the International Lighting Vocabulary of the CIE (The International Commission on Illumination) as a characteristic of visual perception that can be described by attributes of hue, brightness (or lightness) and colourfulness (saturation or chroma).

Photometry

Definition

Photometry is the science concerned with measuring the human visual response to light.

Explanation

Measuring human visual response to light is not straightforward because the eye is a highly complex organ.

An internationally recognised system of measurements was first established in 1924 by an international commission called CIE (Commission Internationale de l’Eclairage).

The Commission established the typical spectral responsiveness of the human eye to wavelengths across the visible spectrum and compiled the data into the photopic curve.

The photopic curve shows that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes, red and violet.

A second set of measurements of the typical responsiveness of the human eye to wavelengths across the visible spectrum at low levels of light, where determining colour differences is difficult, resulted in data compiled into the scotopic curve.

Having defined the eye’s spectral response, CIE sought a standard light source to serve as a yardstick for luminous intensity. The first source was a specific type of candle, giving rise to the terms footcandle and candlepower. In an effort to improve repeatability, the standard was redefined in 1948 as the amount of light emitted from a given quantity of melting platinum.

https://en.wikipedia.org/wiki/Photometry_(optics)

Photon

Electromagnetic waves are carried by particles called photons.

Photon

A photon is a type of elementary particle, the quantum of the electromagnetic field.

  • As an electromagnetic field propagates through space it is configured as bundles of energy called photons.
  • Photons are the force carriers of radiant energy (electromagnetic radiation).
  • So a photon is a type of elementary particle and represents a quantum of light (eg. visible light). Another way of putting this is that a photon is the smallest quantity (quantum) into which light can be divided.
  • For everyday purposes, we might say that electromagnetic radiation travels through space as bundles of energy called photons and that streams of photons are configured as electromagnetic waves. So, the waves are the carriers of the energy (electromagnetic radiation).
  • It was Albert Einstein who first showed that while light travels in waves, it also is made of particles called photons.
  • The energy associated with a photon is determined by its wavelength. Photons with shorter wavelengths have more energy per photon than longer wavelength photons.
  • An electromagnetic field can be thought of as a single more complete object than its component electric and magnetic field.
  • A photon has zero mass when at rest.
  • A photon moves at the speed of light in a vacuum.

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

Photon energy

Photon energy is the energy carried by a single photon. The amount of energy is inversely proportional to the wavelength and directly proportional to the photon’s electromagnetic frequency.

  • The higher the photon’s frequency, the higher it’s energy. Equivalently, the longer the photon’s wavelength, the lower it’s energy.
  • Photon energy is solely a function of the photon’s wavelength and frequency.
  • Other factors, such as the intensity of the radiation, do not affect photon energy. In other words, two photons of light with the same colour and therefore, same frequency, will have the same photon energy, even if one was emitted from a wax candle and the other from the Sun.

Photon energy

Photon energy is the energy carried by a single photon. The amount of energy is inversely proportional to the wavelength and directly proportional to the photon’s electromagnetic frequency.

  • The higher the photon’s frequency, the higher it’s energy. Equivalently, the longer the photon’s wavelength, the lower it’s energy.
  • Photon energy is solely a function of the photon’s wavelength and frequency.
  • Other factors, such as the intensity of the radiation, do not affect photon energy. In other words, two photons of light with the same colour and therefore, same frequency, will have the same photon energy, even if one was emitted from a wax candle and the other from the Sun.
  • Units of energy commonly used to denote photon energy are the electronvolt (eV) and the joule.

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

Photopic curve

A photoptic curve is a diagram showing that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes of red and violet.

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

Pigment epithelium

Pigment epithelium is a layer of cells at the boundary between the retina and the choroid of the human eye that nourish neurons with the retina.

  • Pigment epithelium is firmly attached to the underlying choroid on one side but less firmly connected to retinal visual cells on the other. The choroid is the layer of connective tissue that supports the retina.

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

Pixel

A pixel is the smallest element of an image that can be uniquely processed, and is defined by its spatial coordinates and encoded with colour values.

  • In digital imaging, a pixel, dots, or picture element is a physical point in an image or the smallest addressable element in a display device; so it is the smallest controllable element of a picture represented on the screen.
  • In practical applications, each pixel represents a colour value for a specific point within an original image.
  • The intensity of each pixel is variable. In colour imaging systems, a colour is typically represented by three or four component intensities such as red, green, and blue, or cyan, magenta, yellow, and black.

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

Plank constant

The Plank constant enables an equation to be formulated for a conversion between hertz (Hz), a unit of frequency, and the joule (J) , a unit of energy.

  • Mathematical equations are constructed from expressions some of which are numerical constants (numbers) which do not change. Variables, on the other hand, are expressions, the value of which can change.
  • The equation, Energy = Planck Constant x Frequency, allows the quantity of energy associated with electromagnetic radiation to be calculated if the frequency is known.

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

Potential energy

Potential energy is energy in storage. When potential energy is released it becomes kinetic energy.

  • Potential energy comes in different forms such as:
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  • Electrical energy stored in a battery.
  • Chemical energy stored in coal.
  • Mechanical energy stored in a compressed spring.
  • Nuclear energy stored in the nucleus of an atom.

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

Power

In physics, power is the rate of doing work and so the amount of energy transferred per unit time.

  • Energy is measured in joules whilst power is measured in joules per second.
  • Another common and traditional measure is horsepower (comparing to the power of a horse).
  • The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. It was later expanded to include the output power of other types of piston engines, as well as turbines, electric motors and other machinery.

Primary colour

Primary colours are a set of colours from which others can be produced by mixing (pigments, dyes etc.) or overlapping (coloured lights).

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to the electromagnetic spectrum that results in the perception of colour.
  • A set of primary colours is a set of pigmented media or coloured lights that can be combined in varying amounts to produce a wide range of colour.
  • This process of combining colours to produce other colours is used in applications intended to cause a human observer to experience a particular range of colours when represented by electronic displays and colour printing.
  • Additive and subtractive models have been developed that predict how wavelengths of visible light, pigments and media interact.
  • RGB colour is a technology used to reproduce colour in ways that match human perception.
  • The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998) and sRGB are the result of an extensive investigation of the relationship between visible light and human colour vision.

Primary colour

Primary colours are a set of colours from which others can be produced by mixing (pigments, dyes etc.) or overlapping (coloured lights).

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to the electromagnetic spectrum that results in the perception of colour.
  • A set of primary colours is a set of pigmented media or coloured lights that can be combined in varying amounts to produce a wide range of colour.
  • This process of combining colours to produce other colours is used in applications intended to cause a human observer to experience a particular range of colours when represented by electronic displays and colour printing.
  • Additive and subtractive models have been developed that predict how wavelengths of visible light, pigments and media interact.
  • RGB colour is a technology used to reproduce colour in ways that match human perception.
  • The primary colours used in colour-spaces such as CIELAB, NCS, Adobe RGB (1998) and sRGB are the result of an extensive investigation of the relationship between visible light and human colour vision.

Primary visual cortex

The visual cortex of the brain is part of the cerebral cortex and processes visual information. It is in the occipital lobe at the back of the head.

  • Visual information coming from the eyes goes through the lateral geniculate nucleus within the thalamus and then continues towards the point where it enters the brain. At the point where the visual cortex receives sensory inputs is also a point where there is a vast expansion of the number of neurons.
  • Both cerebral hemispheres contain a visual cortex. The visual cortex in the left hemisphere receives signals from the right visual field, and the visual cortex in the right hemisphere receives signals from the left visual field.

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

Prism

In the field of optics, a prism is made of glass or other transparent material with flat, polished surfaces.

  • Prisms are often used for experimental purposes to study the refraction and dispersion of light.
  • A triangular prism consists of two triangular ends and three rectangular faces.
  • If white light is to be refracted or dispersed by a prism into its component colours a narrow beam is pointed towards one of the rectangular faces.
  • Dispersive prisms are used to break up light into its constituent spectral colours.
  • Reflective prisms are used to reflect light, in order to flip or invert a light beam.
  • Triangular reflective prisms are a common component of cameras, binoculars and microscopes.

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

Propagate

Wave propagation refers to any of the ways in which waves travel.

Pure colour

  • Each wavelength of light at full saturation and brightness is perceived as a pure colour.
  • A monochromatic colour is a colour produced by a single wavelength of light.
  • Colours produced by a narrow band of adjacent wavelengths often appear to be pure colours.
  • Spectral colours are the pure colours associated with a natural rainbow.
  • Natural rainbow colours include red, orange, yellow, green, blue and violet but the human eye can distinguish many thousands of other pure colours as well as each of these.
  • In a continuous spectrum of sufficiently close wavelengths, separate colours are indistinguishable.

Qualitative

A qualitative measure is a measurement of the quality of something rather than its quantity.

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

Quantitative

Definition

A quantitative measure is a measurement of the quantity of something rather than its quality.

Explanation

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

Radiant energy

Radiant energy is the energy of electromagnetic radiation, the energy carried by light.

  • Electromagnetic (EM) radiation can be thought of as a stream of photons, in which case radiant energy can be viewed as photon energy – the energy carried by these photons.
  • Alternatively, EM radiation can be viewed as an electromagnetic wave, carrying energy in its oscillating electric and magnetic fields. These two views are completely equivalent and are reconciled to one another in quantum field theory (see wave-particle duality).
  • Radiant energy includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.
  • The quantity of radiant energy is measured in terms of radiant flux over time.
  • Radiant energy also applies to gravitational radiation. For example, the first gravitational waves ever observed were produced by a black hole collision that emitted about 5.3×1047 joules of gravitational-wave energy.

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

Radiometry

Radiometry is the science of measurement of radiant energy in terms of absolute power.

  • Radiant energy is the electromagnetic energy transported by electromagnetic waves.
  • Radiant energy can also be described in terms of elementary particles called photons.
  • Radiometric techniques characterize the distribution of the radiation’s power (transfer of energy per unit of time) in space.
  • The symbol Qe is often used to denote radiant energy (“e” for “energetic”, to avoid confusion with photometric quantities).
  • The SI unit of radiant energy is the joule (J).
  • Whilst radiometry deals with electromagnetic radiation, photometry deals with the interaction of light with the human eye.
  • Outside of the field of radiometry, electromagnetic energy is referred to using E or W. The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.

Rainbow

rainbow is an optical phenomenon produced by illuminated droplets of water. Rainbows are caused by reflectionrefraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

  • Rainbows can be produced by meteorological phenomena, waterfalls, lawn sprinklers and other things that create a fine mist of water.
  • A rainbow is formed from millions of individual raindrops each of which reflects and refracts a tiny image of the sun towards the observer.
  • It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
  • If the sun is behind an observer then the rainbow will appear in front of them.

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

Rainbow

rainbow is an optical phenomenon produced by illuminated droplets of water. Rainbows are caused by reflectionrefraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

  • Rainbows can be produced by meteorological phenomena, waterfalls, lawn sprinklers and other things that create a fine mist of water.
  • A rainbow is formed from millions of individual raindrops each of which reflects and refracts a tiny image of the sun towards the observer.
  • It is the dispersion of light as refraction takes place that produces the rainbow colours seen by an observer.
  • If the sun is behind an observer then the rainbow will appear in front of them.
  • When a rainbow is produced by sunlight, the angles between the sun, each droplet and the observer determine which ones will form part of a rainbow and which colour each will produce.
  • Rainbows always form arcs around a centre point because each colour is at a different angle to an observer.
  • Seen from the air a rainbow can appear as a complete circle. It is only because the ground around the observer gets in the way that a rainbow produced by sunlight is reduced from a circle to a semi-circle or an arc.
  • The sky inside a rainbow is bright because raindrops direct light there too.
  • Rainbows caused by sunlight always appear in the section of sky directly opposite the sun.
  • and centred on a line from the sun to the observer’s eye.
  • When a single rainbow is observed then red will appear on the outside, followed by orange, yellow, green, blue, with violet on the inside.
  • When a double rainbow is observed then the second rainbow will be outside the first, it will be less intense, and the colours will be in the reverse order with violet on the outside.

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

Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

  • The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
  • Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
  • Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
  • Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.

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

Rainbow colours

Rainbow colours are the bands of colour seen in rainbows and in other situations where visible light separates into its component wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.

  • The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. It is the sensitivity of the eye to this small part of the electromagnetic spectrum that results in the perception of colour.
  • Defining rainbow colours is a question more closely related to the relationship between perception and language than to anything to do with physics or scientific accuracy.
  • Even the commonplace colours associated with the rainbow defy easy definition. They are concepts we generally agree on, but are not strictly defined by anything in the nature of light itself.
  • Whilst the visible spectrum and spectral colour are both determined by wavelength and frequency it is our eyes and brains that interpret these and create our perceptions after a lot of processing.
  • Modern portrayals of rainbows have reduced the number of colours to six ROYGBV. One reason for this is because it is easier to portray using RGB colour.
  • RGB colour is a technology principally used to reproduce colour using digital and electronic equipment. RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.

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

Ray

In a diagram, a light ray is a way of tracing the motion of light, including its direction of travel, and what happens when absorption, dispersion, polarization, reflection, refraction, scattering or transmission take place.

  • A light ray is a component of a geometric model (a ray diagram) of the properties of light.
  • Whilst the world in general terms literally appear before our eyes, many of the properties of the electromagnetic spectrumare invisible to us.
  • Simplified models are used to explore and explain some of the invisible optical phenomena. Ray diagrams represent light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.
  • So a light ray is a diagrammatic representation of a narrow beam of light travelling through a vacuum or medium.
  • Light waves are part of a similar model (wave diagram). Wave diagrams imagine light to be a wave, with peaks and troughs and allow other properties to be shown that a ray does not make evident.
  • Light can also be described in terms of photons which allows additional properties to be modelled.
  • One of the conventions used to represent photons is a  Feynman diagram.

Ray diagram

A ray diagram (ray tracing) uses a set of drawing conventions and labels to visualise the path that a single ray of light takes in order to visualise what happens as it encounters different media, materials or objects.

Geometric optics treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces.

The aim of a ray diagram is to demonstrate optical phenomena such as absorption, dispersion, polarization, reflection, refractionscattering and transmission.

https://en.wikipedia.org/wiki/Ray_tracing_(physics)

Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.

If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.

Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of objects that the light has already encountered become visible to an observer.

Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in all directions and so no image is produced.

Reflection

Reflection takes place when incoming light strikes the surface of a medium, obstructing some wavelengths which bounce back into the medium from which they originated.

  • The laws of reflection are as follows:
    • The incident ray, the reflected ray and the normal to the surface all lie in the same plane.
    • The angle the incident ray makes with the normal is equal to the angle which the reflected ray makes with the same normal.
    • The reflected ray and the incident ray are on the opposite sides of the normal.
  • Reflection takes place when light is neither absorbed by an opaque medium nor transmitted through a transparent medium.
  • As stated above, an important feature of reflection is that when light reflects off a surface, the angle of incidence of an incoming ray as it approaches the surface is equal to the angle of reflection.
  • If the reflecting surface is very smooth, the reflected light is called specular or regular reflection.
  • Specular reflection occurs when light waves reflect off a smooth surface such as a mirror. The arrangement of the waves remains the same and an image of the surfaces of objects that the light has already encountered become visible to an observer.
  • Diffuse reflection takes place when light reflects off a rough surface. In this case, scattering takes place and waves are reflected randomly in many different directions and so no image is produced.
  • Reflection off a surface can take place regardless of the optical density of the medium through which the incident light is propagating or of the medium it bounces off.

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

  • As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
  • As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
  • In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
  • A fast (optically rare) medium is one that obstructs light less than a slow medium.
  • A slow (optically dense) medium is one that obstructs light more than a fast medium.
  • The speed at which light travels through a given medium is expressed by its index of refraction.
  • If we want to know in which direction light will bend at the boundary between transparent media we need to know:
  • Which is the faster, less optically dense (rare) medium with a smaller refractive index?
  • Which is the slower, more optically dense medium with the higher refractive index?
  • The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
  • Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

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

Refraction

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

  • As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
  • As light passes from a slow medium such as diamond to a faster medium such as glass it bends away from the normal and speeds up.
  • In a diagram illustrating optical phenomena like refraction or reflection, the normal is a line drawn at right angles to the boundary between two media.
  • A fast (optically rare) medium is one that obstructs light less than a slow medium.
  • A slow (optically dense) medium is one that obstructs light more than a fast medium.
  • The speed at which light travels through a given medium is expressed by its index of refraction.
  • If we want to know in which direction light will bend at the boundary between transparent media we need to know:
  • Which is the faster, less optically dense (rare) medium with a smaller refractive index?
  • Which is the slower, more optically dense medium with the higher refractive index?
  • The amount that refraction causes light to change direction, and its path to bend, is dealt with by Snell’s law.
  • Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.

Refractive index

The refractive index of a medium is the amount by which the speed (and wavelength) of electromagnetic radiation (light) is reduced compared with the speed of light in a vacuum.

  • Refractive index (or, index of refraction) is a measure of how much slower light travels through any given medium than through a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index of a medium is a numerical value and is represented by the symbol n.
  • Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
  • If the speed of light in a vacuum = 1. Then the ratio is 1:1.
  • The refractive index of water is 1.333, meaning that light travels 1.333 times slower in water than in a vacuum. The ratio is therefore 1:1.333.
  • As light undergoes refraction its wavelength changes as its speed changes.
  • As light undergoes refraction its frequency remains the same.
  • The energy transported by light is not affected by refraction or the refractive index of a medium.
  • The colour of refracted light perceived by a human observer does not change during refraction because the frequency of light and the amount of energy transported remain the same.

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

Refractive index

The refractive index of a medium is the amount by which the speed (and wavelength) of electromagnetic radiation (light) is reduced compared with the speed of light in a vacuum.

  • Refractive index (or, index of refraction) is a measure of how much slower light travels through any given medium than through a vacuum.
  • The concept of refractive index applies to the full electromagnetic spectrum, from gamma-rays to radio waves.
  • The refractive index of a medium is a numerical value and is represented by the symbol n.
  • Because it is a ratio of the speed of light in a vacuum to the speed of light in a medium there is no unit for refractive index.
  • If the speed of light in a vacuum = 1. Then the ratio is 1:1.
  • The refractive index of water is 1.333, meaning that light travels 1.333 times slower in water than in a vacuum. The ratio is therefore 1:1.333.
  • As light undergoes refraction its wavelength changes as its speed changes.
  • As light undergoes refraction its frequency remains the same.
  • The energy transported by light is not affected by refraction or the refractive index of a medium.
  • The colour of refracted light perceived by a human observer does not change during refraction because the frequency of light and the amount of energy transported remain the same.

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

RGB colour

  • To be clear about RGB colour it is useful to remember first that:
  • RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.
  • Except for the three primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
  • RGB colour provides the basis for a wide range of technologies used to reproduce digital colour.
  • RGB colour provides the basis for reproducing colour in ways that are well aligned with human perception.
  • When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue coloured lights they are able to create a match for a very extensive range of colours.
  • When looking at any modern display device such as a computer screen, mobile phone or projector we are looking at RGB colour.
  • Magenta is an RGB colour for which there is no equivalent spectral colour.

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

RGB colour

RGB colour is an additive colour model in which red, green and blue light is combined in various proportions to reproduce a wide range of other colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. However, because of the way the eye works, we can see many other colours which are produced by mixing colours from different areas of the spectrum. A particularly useful range of colours is produced by mixing red, green and blue light.
  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • A spectral colour is a colour corresponding with a single wavelength of visible light, or with a narrow band of adjacent wavelengths.
  • Except for the three RGB primary colours, RGB colours are not spectral colours because they are produced by combining colours from different areas of the visible spectrum.
  • Magenta is an example of an RGB colour for which there is no equivalent spectral colour.
  • RGB colour is a technology used to reproduce colour in ways that are well aligned with human perception. When an observer has separate controls allowing them to adjust the intensity of overlapping red, green and blue RGB primary coloured lights they are able to create a match for an extremely wide range of colours.
  • When looking at any modern display device such as a computer screen, mobile phone or video projector we are looking at RGB colour.
  • RGB colours are produced:
    • On a computer or mobile phone screen:  By Juxtaposing tiny dots of light corresponding with the three primary colours, red, green and blue.
    • On a digital projector: By projecting three carefully aligned but separate images, one red, one green and one blue onto a screen. Each image is made up of pixels of different intensity. Where the pixels from each image overlap they produce RGB colour by varying their relative intensity.
    • In computer software and apps: By selecting RGB colours using swatches or by selecting RGB colour values.

 

  • An extremely wide range of colours can be produced using RGB colour simply by varying the brightness of the three primary colours by different amounts.
  • The RGB colour model itself does not define what is meant by red, green and blue, and so the results of mixing them are relative to the choice of the particular red, green and blue lights that are used. When the exact chromaticity of the red, green, and blue primaries are defined, the colour model then becomes an absolute colour space, such as sRGB or Adobe RGB.
  • Look at a screen with large pixels (such as a TV) using a magnifying glass to see the three RGB primary colours. Then step back to see how they produce different colours when all the pixels merge together.
  • When working with a computer graphics application such as Adobe Illustrator, tints and shades can be produced by adjusting opacity in conjunction with white and black backgrounds.
  • Where an RGB colour model is used to control the output of a display device (computer screen, mobile phone or projector), tints are produced by increased the colour value of each RGB component proportionally, so increasing the intensity of the output, whilst shades are produced by proportionally decreasing the colour value of each RGB component, so decreasing the intensity of the output and dimming the output of the device.
  • RGB works by asking three questions of any colour: how red it is (R), how green it is (G), and how blue it is (B).
  • The RGB model is popular because it can easily be used to produce a comprehensive palette of 1530 spectral colours.
  • This RGB model is particularly useful where the output is to a web page or is to be presented on an RGB display device because of the system of notation that allows exact colours to be specified.

 

  • In the implementation of the RGB colour model used in Adobe Illustrator CC:
    • Colours can be selected using swatches which by default are identified by their decimal RGB values (right click a swatch to open Swatch Options).
    • Once a swatch has been selected right click on the icon for stoke/fill (top right of Swatches panel) and use the Color Picker to find a colour or adjust RGB (decimal and hex), HSB and CMYK values.

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

RGB colour values

RGB colour values are expressed as decimal triplets or hexadecimal triplets and are used in software applications to select specific colours colour.

  • RGB colour values (codes) are represented by decimal triplets (base 10) or hexadecimal triplets (base 16).
  • In decimal notation, an RGB triplet is used to represent the values of red, then green, then blue. A range of decimal numbers from 0 to 255 can be selected for each value.
    • Red = 255, 00, 00
    • Yellow = 255, 255, 0
    • Green = 00, 255, 00
    • Cyan = 00, 255, 255
    • Blue = 00, 00, 255
    • Magenta = 255, 00, 255
  • In hexadecimal notation an RGB triplet is used to represent the value of red, then green, then blue. A range of  hexadecimal numbers from 00 to FF can be selected for each value.
  • The hash symbol (#) is used to indicate hex notation.
    • Red = #FF0000
    • Yellow = #FFFF00
    • Green = #00FF00
    • Cyan = #00FFFF
    • Blue = #0000FF
    • Magenta = #FF00FF
  • The sequence of hexadecimal values between 1 and 16 = 0,1,2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.
  • The sequence of hexadecimal values between 17 and 32 = 10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E and 1F.

RGB colour values

RGB colour values are expressed as decimal triplets or hexadecimal triplets and are used in software applications to select specific colours colour.

  • RGB colour values (codes) are represented by decimal triplets (base 10) or hexadecimal triplets (base 16).
  • In decimal notation, an RGB triplet is used to represent the values of red, then green, then blue. A range of decimal numbers from 0 to 255 can be selected for each value.
    • Red = 255, 00, 00
    • Yellow = 255, 255, 0
    • Green = 00, 255, 00
    • Cyan = 00, 255, 255
    • Blue = 00, 00, 255
    • Magenta = 255, 00, 255
  • In hexadecimal notation an RGB triplet is used to represent the value of red, then green, then blue. A range of  hexadecimal numbers from 00 to FF can be selected for each value.
  • The hash symbol (#) is used to indicate hex notation.
    • Red = #FF0000
    • Yellow = #FFFF00
    • Green = #00FF00
    • Cyan = #00FFFF
    • Blue = #0000FF
    • Magenta = #FF00FF
  • The sequence of hexadecimal values between 1 and 16 = 0,1,2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.
  • The sequence of hexadecimal values between 17 and 32 = 10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E and 1F.

RGB colour wheel

The main purpose of an RGB colour wheel is to understand the representation and display of colour used with RGB display devices such as televisions, computers, mobile phones, cameras and the software applications used with them.

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet.
  • RGB colour is a technology used to reproduce colour in a way that matches human perception.
  • An RGB colour wheel helps to simulate:
    • The effect of projecting lights with wavelengths corresponding to the three primary colours, red, green and blue onto a dark surface.
    • The additional colours produced by mixing adjacent pairs of colours i.e. adjacent primary, secondary, tertiary colours etc.

 

  • An RGB colour wheel demonstrates the gradation of colours as the number of intermediate colours between primary colours increases.
  • RGB colour wheels are particularly useful when trying to visually identify a specific RGB colour, the relationship between different RGB colours or find an RGB colour value (code).
  • The primary colours selected for RGB colours are intended to closely match those seen by an observer when white light undergoes dispersion and produces rainbow colours.
  • The bands of wavelengths corresponding with the observation of red, green and blue in a rainbow are typically:
  • Red = between 620 – 750 nanometres.
  • Green = between 495 – 570 nanometres
  • Blue = between 450 – 495 nanometres

 

  • An LED light source is often used when demonstrating the effect of projecting primary coloured lights onto a dark surface because they emit light in very narrow bands of wavelengths. The peak wavelength for the selected lights might typically be red = 625 nanometres, green = 500 nm, blue = 440 nm.
  • Typical peak wavelength of red, green and blue LED lights: red = 625, green = 500, blue =440
  • RGB colour wheels, therefore, have a minimum of three segments. These are filled with the red, green and blue additive primary colours.

 

  • When exploring RGB colour wheels the next thing to establish is what happens when pairs of primary colours of equal intensities overlap.
  • Where red and green light sources overlap they produce yellow.
  • Where green and blue light sources overlap they produce cyan.
  • Where blue and red light sources overlap they produce magenta.
  • Yellow, cyan and magenta are called secondary colours.
  • Mixtures of equal intensities of pairs of secondary colours are called tertiary colours.

 

  • Additional colours on an RGB colour wheel are produced by continuing to overlap equal intensities of adjacent pairs of colours.
  • The range of colours that can be produced by an RGB colour wheel is limited only by the system of notation and the resolution of the device they are displayed on.

ROYGBV

ROYGBV is an acronym for the sequence of hues (colours) commonly described as making up a rainbow: red, orange, yellow, green, blue, and violet.

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

ROYGBV

ROYGBV is an acronym for the sequence of hues (colours) commonly described as making up a rainbow: red, orange, yellow, green, blue, and violet.

Saturation

Saturation refers to the perceived vividness of a colour.

  • When colours are more saturated, our eyes interpret it as a colour’s luminance and chroma. This makes us believe that the colours are actually brighter.
  • As saturation decreases, colours appear dull and washed out until all colour disappears leaving only a grey tone.
  • On many colour wheels, saturation increases from the centre to the edge.

https://en.wikipedia.org/wiki/Colorfulness#Saturation

Scattering

Light scattering takes place when light waves are reflected in random directions at the boundary between two media, or by particles and other irregularities within the medium through which they propagate.

  • Light scattering can be caused by an uneven surface at the boundary between two media or by particles and other irregularities within the medium through which they propagate.

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

Scattering

Light scattering takes place when light waves are reflected in random directions at the boundary between two media, or by particles and other irregularities within the medium through which they propagate.

  • Light scattering can be caused by an uneven surface at the boundary between two media or by particles and other irregularities within the medium through which they propagate.

Scotopic curve

A scotopic curve is a diagram showing that, at low levels of light, where determining colour differences is difficult, the strongest response of the human eye moves towards blue and violet end of the visible spectrum with less sensitivity towards the red when compared with a photopic curve.

  • Whilst a  scotopic curve describes the response of the human eye to low levels of light a photopic curve is a diagram showing that, in bright light, the strongest response of the human eye is to the colour green with less sensitivity towards the spectral extremes of red and violet.

Secondary colour

secondary colour is a colour made by mixing two primary colours in a given colour space. The colour space may be produced by an additive colour model that involves mixing different wavelengths of light or by a subtractive colour model that involves mixing pigments or dyes.

Secondary colour

secondary colour is a colour made by mixing two primary colours in a given colour space. The colour space may be produced by an additive colour model that involves mixing different wavelengths of light or by a subtractive colour model that involves mixing pigments or dyes.

  • Secondary colours produced by an additive colour model are quite different from the spectral colours seen in a rainbow.
  • A spectral colour is produced by a single wavelength, or a narrow band of wavelengths, within the visible spectrum.
  • A secondary colour produced by an additive colour model results from superimposing wavelengths of light from different areas of the visible spectrum.
  • For the human eye, the best additive primary colours of light are red, green, and blue.
  • RGB colour can be used to produce an extremely wide range of colour.
  • Because RGB colour involves adding different wavelengths of light together (thus the term “additive colour”), the resulting combinations always appear lighter to an observer.
  • When all three primaries (or for that matter all three secondaries) are combined in equal amounts, the result is white.
  • The RGB secondary colours produced by the addition of light turn out to be the best primary colours for pigments, the mixing of which subtracts light.