Dictionary tag: R

Rest mass

Rest mass, (also known as invariant mass, intrinsic mass, proper mass, or simply mass) in the case of bound systems, is a fundamental property of a particle or a system of particles that remains constant regardless of the observer’s frame of reference.

  • For a single particle, its invariant mass equals its energy at rest divided by the square of the speed of light.
  • Photons, which have zero rest mass, still possess an invariant mass calculated using their energy and momentum, related by the equation E=pc, where E is energy, p is momentum, and c is the speed of light.
  • Invariant mass is a crucial concept in particle physics because it is conserved in all interactions, including those involving the conversion of mass into energy and vice versa.
  • The conservation of invariant mass stems from the conservation of energy and momentum, fundamental principles in physics.

The fact that photons have zero rest mass has several important implications in physics, especially regarding their behaviour, speed, and role in electromagnetic interactions:

  • Speed of light: Photons always travel at the speed of light (approximately 299,792,458 meters per second) in a vacuum. Since they have zero rest mass, they don’t need any energy to accelerate to this speed. This is a fundamental feature of special relativity, where massless particles must always travel at the speed of light.
  • Energy and momentum: Even though photons have no rest mass, they still carry energy and momentum. According to Einstein’s equations, the energy of a photon is proportional to its momentum, with no contribution from rest mass. This allows photons to transfer energy to other particles, as seen in phenomena like the photoelectric effect or Compton scattering.
  • Interaction with gravity: Despite having zero mass, photons are still affected by gravity. This is because gravity influences spacetime itself, curving the path of photons. For example, light bends around massive objects like stars or black holes, a phenomenon known as gravitational lensing, predicted by general relativity.
  • Infinite range of electromagnetic force: The fact that photons are massless ensures that the electromagnetic force, which they mediate, has an infinite range. If photons had mass, the electromagnetic force would diminish over distance, limiting its range.
  • Time perception for photons: Since photons travel at the speed of light, from their perspective, time doesn’t pass. This means that for a photon, the moment it is emitted is the same as the moment it is absorbed, making time essentially non-existent in its frame of reference.
Related diagrams

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

References
    • Rest mass, also known as invariant mass, intrinsic mass, proper mass (or simply mass in the case of bound systems,) is a fundamental property of a particle or a system of particles that remains constant regardless of the observer’s frame of reference.
    • For a single particle, its invariant mass equals its energy at rest divided by the square of the speed of light.
    • Photons, which have zero rest mass, still possess an invariant mass calculated using their energy and momentum, related by the equation E=pc, where E is energy, p is momentum, and c is the speed of light.
    • Invariant mass is a crucial concept in particle physics because it’s conserved in all interactions, including those involving the conversion of mass into energy and vice versa.
    • The conservation of invariant mass stems from the conservation of energy and momentum, fundamental principles in physics.

Retina

THE RETINA

Human beings see the world in colour because of the way their visual system processes light. The retina contains light-sensitive receptors, rod and cone cells, that respond to light stimuli. It is the variety of wavelengths and intensities of light entering the eyes that produces the impression of colour.

The retina is the innermost, light-sensitive layer of tissue inside our eyes. It forms a sheet of tissue barely 200 micrometres (μm) thick, but its neural networks carry out almost unimaginably complicated feats of image processing.

The physiology of the eye results in a tiny, focused, two-dimensional image of the visual world being projected onto the retina’s surface. Because of the optics of lenses, it appears upside down and the wrong way around. But no worry, sorting that out is child’s play for the human brain! The real challenge is that the photosensitive receptors in the retina must produce precise chemical responses to light and translate every minute detail of the image into electrical impulse ready to be sent to the brain where they produce visual impressions of the world. In a very limited sense, the retina serves a similar function to a photosensitive chip in a camera.

As research continues to reveal ever-increasing amounts of detail about these signalling processes across and beyond the retina, it required new thinking, not only of the retina’s function but also of the mechanisms within the brain that shape these signals into behaviourally useful perceptions.

The retina consists of 60-plus distinct neuron types, each of which plays a specialized role in turning variations in the patterns of wavelengths and intensities of light into visual information. Neurons are electrically excitable nerve cells that collect, process and transmit vast amounts of this information through both chemical and electrical signals. Retinal neurons work together to convert the signals produced by a hundred and twenty million rods and cones and send them along around one million fibres within the optic nerve of each eye to connections with higher brain functions. In this process rods and cones are first responders whilst ganglion cells are the final port of call before information leaves the retina.

There are three principal forms of processing that take place within the retina itself. The first organises the outputs of the rod and cone photoreceptors and begins to compose them into around 12 parallel information streams as they travel through bipolar cells. The second connects these streams to specific types of retinal ganglion cells. The third modulates the information using feedback from horizontal and amacrine cells to create the diverse encodings of the visual world that the retina transmits towards the brain.

As mentioned above, the image of the outside world focused on the retina is upside down and the wrong way around. But the human retina is also inverted in the sense that the light-sensitive rod and cone cells are not located on the surface where the image forms, but instead are embedded inside, where the retina attaches to the fabric of the eyeball. As a result, light striking the retina, passes through layers of other neurons (ganglion, bipolar cells etc.) and blood-carrying capillaries, before reaching the photoreceptors.

The overlying neural fibres do not significantly degrade vision in the inverted retina. The neurons are relatively transparent and accompanying Müller cells act as fibre-optic channels to transport photons directly to photoreceptors. However, some estimates suggest that overall, around 15% of all the light entering the eye is lost en-route to the retina. To counter this, the fovea centralis, at the centre of our field of vision, is free of rods and there are no blood vessels running through it, so optimising the level of detail where we need it most.

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

Retinal image

Retinal image

It is the cornea-lens system that determines where light falls on the surface of the retina which results in discernible images.

The images are inverted and obviously very small compared with the world outside that they resolve. The inversion poses no problem. Our brains are very flexible and even when tricked by prisms will always turn the world right-side-up given time. The reduction in size is part of the process by which the fit of the image on the retina determines our field of view.

The images are real in the sense that they are formed by the actual convergence of light rays onto the curved plane of the retina. Only real images of this kind provide the necessary stimulation of rod and cone cells necessary for human perception.

Retinal input

Retinal input

Visual input is initially encoded in the retina as a two-dimensional distribution of light intensity, expressed as a function of position, wavelength and time. This retinal image is transferred to the visual cortex where primary sensory cues and, later, inferred attributes, are eventually computed in the process of actualising our perceptions. Parallel processing strategies are employed from the outset to overcome the constraints of the individual ganglion cell’s limited bandwidth and anatomical bottlenecks as data approaches the optic nerves that connect each eye to the visual cortex.

References: DeYoe and Van Essen (1988): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2771435/

RGB & the trichromatic colour model

About RGB & the trichromatic colour model

To make sense of the physiological basis of the RGB colour model we can relate it to how the trichromatic colour model explains colour vision. Let’s look at the Trichromatic colour model first:

  • The trichromatic colour theory, which is also known as the Young-Helmholtz theory, established that there are three types of cone cells in the human eye that carry out the initial stage of colour processing that ultimately produces the world of colours we see around us.
  • Cone cells are daylight photoreceptors which means they are able to convert light into electrical charges through a process called photo-transduction.
  • The sensitivity of cone cells was established using spectroscopy which measures which wavelengths are absorbed and which are reflected.
  • The three types of cone cells were identified along with the range of wavelength they absorbed:
    • L = Long (500–700 nm)
    • M = Medium (440 – 670 nm)
    • S = Short (380 – 540 nm)
  • The trichromatic colour theory also established the visual effect of exposing a human observer to mixtures of light produced by three monochromatic light sources, one in the red, one in the green, and one in the blue part of the spectrum.
  • It proved that by incrementally adjusting the intensity of the light produced by each source an observer can be induced to see any colour within the visible spectrum.
  • The outcome was that a match was produced between how the L, M and S cone cells responded to light of different wavelengths and calibrated mixtures of wavelengths of light corresponding with R, G and B. This is the basis of the RGB colour model.
  • The fact that mixtures of red, green and blue light at different levels of intensity can be used to stimulate the L, M and S cones types to produce any human observable colour underpins almost every form of colour management in practice today.

RGB colour & colour perception

About RGB colour and colour perception
  • The human eye, and so visual perception, is tuned to the visible spectrum and so to spectral colours between red and violet.
  • RGB colour is a model used to reproduce colour in a way that matches 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 neutral-coloured surface.
    • The additional colours produced by mixing adjacent pairs of colours such as adjacent primary, secondary, tertiary colours etc.
  • Every imaginable colour can be produced by the RGB colour model.
  • Remember that the RGB is an additive colour model used when mixing light of different wavelengths.
  • The CMYK subtractive colour model is often used when mixing paints, dyes and pigments.

RGB colour model

RGB colour is an additive colour model in which red, green and blue light is combined to reproduce a wide range of other colours.

RGB colour model in practice
  • The RGB colour model works in practice by asking three questions of any colour: how red is it (R), how green is it (G), and how blue is it (B).
  • The RGB model is popular because it can easily produce a comprehensive palette of 1530 vivid hues simply by adjusting the combination and amount of each of the three primaries it contains.
  • When the saturation or brightness of a hue needs to be adjusted it is sometimes easier to switch to the HSB colour model.
Hardware applications of the RGB colour model

The RGB colour model is deeply embedded in many contemporary forms of hardware and is the industrial standard for capturing colour on cameras or scanners and reproducing colour on TVs, phones, computers and projectors.

  • RGB sensors are used in cameras, scanners, phones, optical mouse devices, medical imaging equipment, and night-vision equipment such as thermal imaging devices, radar, sonar, and others.
  • Both analogue and digital image sensors can detect, capture and convey the colour information needed to produce images.
  • When viewing images using contemporary displays such as computer screens, mobile phones and video projectors we are looking at digital information conveyed by the RGB colour model.
  • RGB colour is produced on computer or mobile phone screens by juxtaposing tiny dots of light corresponding with the three primary colours, red, green and blue.
  • RGB colour is produced on digital projectors by projecting three carefully aligned but separate images, one red, one green and one blue onto a screen.
  • In the RGB colour model, each primary colour is typically represented by 8 bits (256 levels of intensity), allowing for a total of over 16 million possible colours.
  • The RGB colour model forms the basis for creating images in digital formats such as JPEG, PNG, and GIF.
  • RGB technologies used to create displays in use today include:
    • Liquid crystal display (LCD) and thin-film transistor (TFT) LCD.
    • Light-emitting diode (LED), Quantum dot (QLED), OLED, AMOLED and Super AMOLED display
Software applications of the RGB colour model

In the implementation of the RGB colour model used by Illustrator in Adobe CC:

  • The RGB colour model is the default setting in the Colour Panel. If the colour panel is not visible then look for Colour in the Windows menu.
  • Use the hamburger menu in the top right of the panel to change from the default RGB to other colour models.
  • The amount of red, green and blue in a colour can be adjusted using the corresponding sliders or by inputting a value between 0 and 255 for each colour.
  • However, there are easier ways to select a colour in the RGB colour model:
    • Select a colour by clicking anywhere on the RGB Spectrum right below the sliders and then make fine adjustments using the sliders.
    • Use a colour picker app to find the colour you want and paste its hexadecimal value into the Colour Panel.
    • Build your own library of swatches and save them into the swatches panel.
    • Open an existing document that contains all the colour swatches you need, Save as using a different file name and then delete the previous content. This way you start with a blank document but all the settings and colours used last time are retained.
    • Switch to the HSB colour model using the hamburger menu in the Colour Panel and adjust the hue, saturation and brightness of a colour.
RGB colour notation
  • The RGB colour model uses both decimal and hexadecimal triplets for colour notation. So RGB decimal and hexadecimal triplets look like this:
    • R = 255, G = 128, B = 0 is the decimal notation for orange.
    • #FF8000 is the hexadecimal notation for orange.
About the human eye, light and RGB colour
  • The human eye, and so human perception, is tuned to the range of wavelengths of light that make up the visible spectrum and so to the corresponding spectral colours between red and violet.
  • The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
  • To be exact, spectral colour is a colour corresponding to a single wavelength of visible light, but in everyday terms, spectral colours are usually composed of a narrow band of adjacent wavelengths.
  • Because of the way the eye works, we can see all the colours of the visible spectrum when red, green and blue lights are combined at different intensities.
  • The RGB colour model is designed to provide the exact stimuli to the light-sensitive cone cells in the retina to illicit perception of any predetermined colour.
  • Mixing wavelengths of light corresponding with the RGB primaries enables the human eye to see almost any imaginable colour including colours such as magenta that are not part of the visible spectrum.
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About RGB and digital devices
  • RGB colour is deeply embedded in many contemporary technologies.
  • 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.
    • 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.
Related diagrams

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

References
  • RGB colour is an additive colour model in which red, green and blue light is combined to reproduce a wide range of other colours.
  • The primary colours in the RGB colour model are red, green and blue.
  • In the RGB model, different combinations and intensities of red, green, and blue light are mixed to create various colours. When these three colours are combined at full intensity, they produce white light.
  • Additive colour models are concerned with mixing light, not dyes, inks or pigments (these rely on subtractive colour models such as the RYB colour model and the CMY colour model).
  • The RGB colour model works in practice by asking three questions of any colour: how red is it (R), how green is it (G), and how blue is it (B).
  • The RGB model is popular because it can easily produce a comprehensive palette of 1530 vivid hues simply by adjusting the combination and amount of each of the three primaries it contains.

RGB colour model

RGB colour is an additive colour model in which red, green and blue light is combined to reproduce a wide range of other colours.

  • The primary colours in the RGB colour model are red, green and blue.
  • In the RGB model, different combinations and intensities of red, green, and blue light are mixed to create various colours. When these three colours are combined at full intensity, they produce white light.
  • Additive colour models are concerned with mixing light, not dyes, inks or pigments (these rely on subtractive colour models such as the RYB colour model and the CMY colour model).
  • The RGB colour model works in practice by asking three questions of any colour: how red is it (R), how green is it (G), and how blue is it (B).
  • The RGB model is popular because it can easily produce a comprehensive palette of 1530 vivid hues simply by adjusting the combination and amount of each of the three primaries it contains.

RGB colour model in practice

About the RGB colour model in practice
  • RGB colour model works in practice 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 vivid hues simply by adjusting the intensity of the three primaries.
  • When the saturation or brightness of a hue needs to be adjusted it is sometimes easier to switch to the HSB colour model.

RGB colour notation

RGB colour notation refers to the method used by the RGB colour model to identify and store colour values in a format recognizable to both computers and humans.

  • RGB stands for red, green, and blue and is a commonly used colour model in digital imaging and computer graphics.
  • The RGB colour model is an additive colour model, meaning that colours are created by adding different amounts of red, green, and blue light together.
  • RGB colour notation is widely used in web design, graphic design, and other digital media.

RGB colour values are expressed as either:

    • Decimal triplets. For example, the colour yellow is represented as 255, 255, 0.
    • Hexadecimal triplets. For example, the colour green is represented as #00FF00.
  • Computer software is usually programmed to recognize both decimal and hexadecimal forms of notation.
  • Both decimal and hexadecimal notations represent the intensity of red, green, and blue used to create a specific colour.
RGB decimal notation
  • An RGB decimal triplet is made up of three decimal numbers representing the intensities of the red, green, and blue components of a colour. Each component has a value between 0 and 255 and is separated by a comma.
  • These are some familiar RGB decimal triplet:
    • Red = 255, 0, 0
    • Yellow = 255, 255, 0
    • Green = 0, 255, 0
    • Cyan = 0, 255, 255
    • Blue = 0, 0, 255
    • Magenta = 255, 0, 255
RGB hexadecimal notation
  • RGB hexadecimal notation starts with a # sign.
  •  An RGB hexadecimal triplet is made up of three pairs of hexadecimal digits representing the intensities of the red, green, and blue components of a colour. Each pair has a value of 00 to FF and is written without spaces between. So red = 00 to FF, green = 00 to FF, blue = 00 to FF.
  • Hexadecimal numbers count in base 16, and each digit represents a value from 0 to 15, where A-F represent 10 to 15.
  • 00 to FF represents a sequence of 255 values.
    • 0 to 15 appear as: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F
    • 16 to 31 appear as: 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E and 1F
    • 32 to 47 appear as: 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 2A, 2B, 2C, 2D, 2E and 2F
    • 48 to 63 appear as: 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 3A, 3B, 3C, 3D, 3E and 3F
    • etc.
    • 240 to 255 appear as: F0, F1, F2, F3, F4, F5, F6, F7, F8, F9, FA, FB, FC, FD, FE and FF
  • Look carefully at the following examples that show matching hex and decimal values:
    • Red = #FF0000 = 255, 00, 00
    • Yellow = #FFFF00 = 255, 255, 0
    • Green = #00FF00 = 0, 255, 0
    • Cyan = #00FFFF = 0, 255, 255
    • Blue = #0000FF = 0, 0, 255
    • Magenta = #FF00FF = 255, 0, 255
Related diagrams

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

References
  • https://en.wikipedia.org/wiki/Comparison_of_color_models_in_computer_graphics
  • https://en.wikipedia.org/wiki/Decimal
  • https://en.wikipedia.org/wiki/Hexadecimal
  • RGB colour notation refers to the method used by the RGB colour model to identify and store colour values in a format recognizable to both computers and humans.
    • RGB stands for red, green, blue and is a commonly used colour model in digital imaging and computer graphics.
    • The RGB colour model is an additive colour model, meaning that colours are created by adding different amounts of red, green, and blue light together.
    • RGB colour notation is widely used in web design, graphic design, and other digital media.
    • RGB colour values are expressed as either:
      • Decimal triplets. For example, the colour yellow is represented as 255, 255, 0.
      • Hexadecimal triplets. For example, the colour green is represented as #00FF00.

RGB colour notation

RGB colour notation refers to the method used by the RGB colour model to identify and store colour values in a format recognizable to both computers and humans.

  • RGB stands for red, green, and blue and is a commonly used colour model in digital imaging and computer graphics.
  • The RGB colour model is an additive colour model, meaning that colours are created by adding different amounts of red, green, and blue light together.
  • RGB colour notation is widely used in web design, graphic design, and other digital media.
  • RGB colour values are expressed as either:
    • Decimal triplets. For example, the colour yellow is represented as 255, 255, 0.
    • Hexadecimal triplets. For example, the colour green is represented as #00FF00.

RGB colour notation

About RGB colour notation

RGB colour values are expressed as decimal triplets (yellow = 255, 255, 0) or hexadecimal triplets (green = #00FF00). Computer software is programmed to recognise RGB colour values.

In both cases, the triplets determine the amount of red, green and blue used to produce a specific colour.
A decimal triplet is made up of three numbers between 0 and 255 divided by commas.
A hexadecimal triplet starts with a # sign followed by three two-digit numbers with values between  00 and FF written without spaces between.

RGB colour values are based on decimal notation (triplets with a base 10) or hexadecimal notation (triplets with a base 16).

  • Decimal notation uses 10 digits from 0 to 9 as follows, 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9.
  • The hexadecimal notation uses 16 digits from 0 to F as follows, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E and F.
  • Hexadecimal notation for values between 16 and 31 are as follows 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E and 1F.

RGB colour space

About the RGB colour space
  • The Adobe RGB (1998) colour space is designed to encompass the colours that can be output by CMYK colour printers.
  • When the RGB colour model is used on a modern computer screen, the Adobe RGB (1998) colour space aims to reproduce roughly 50% of the range of colours that an observer is capable of seeing in ideal conditions.
  • The purpose of the  Adobe RGB (1998) colour space was to improve on the gamut of colours that could be produced by the earlier sRGB colour space, primarily in the reproduction of cyan-green hues.

RGB colour values

About RGB colour values
  • RGB colour values are represented as decimal triplets (e.g., yellow = 255, 255, 0) or hexadecimal triplets (e.g., green = #00FF00). These values are used by software applications to choose particular colours.
  • In both instances, the values determine the levels of red, green, and blue used to create a specific colour.
    • A decimal triplet is made up of three numbers between 0 and 255 divided by commas.
    • A hexadecimal triplet starts with a # sign followed by three two-digit numbers with values between  00 and FF written without spaces between.

RGB colour values

RGB colour values refer to the numeric codes used in RGB colour notation to define specific colours in a digital image or on a digital display.

About RGB colour values
  • Colour values are numbers and/or characters used by colour models to represent and store colour information in a format recognizable to both computers and humans.
  • Each colour within a colour model is assigned a unique colour value.
  • RGB colour values (codes) are represented by decimal triplets (base 10) or hexadecimal triplets (base 16).
RGB decimal values
  • 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
RGB hexadecimal values
  • In decimal notation, an RGB triplet represents the values of red, green, and blue, in that order. Each value can range from 0 to 255.
  • The hash symbol (#) is used to indicate hexadecimal 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.
Related diagrams

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

References
    • RGB colour values refer to the numeric codes used to define a specific colour in a digital image or on a digital display.
    • Colour values are numbers and characters used by colour models to represent and store colour information in a format recognizable to both computers and humans.
    • Each colour within a colour model is assigned a unique colour value.
    • The notation for RGB colour values (codes) are decimal triplets (base 10) or hexadecimal triplets (base 16).
    • RGB decimal and hexadecimal triplets look like this:
      • R = 255, G = 128, B = 0 (255,128,00) is the decimal notation for orange.
      • #FF8000 is the hexadecimal notation for orange.

RGB colour values

RGB colour values refer to the numeric codes used in RGB colour notation to define specific colours in a digital image or on a digital display.

  • Colour values are numbers and characters used by colour models to represent and store colour information in a format recognizable to both computers and humans.
  • Each colour within a colour model is assigned a unique colour value.
  • The notation for RGB colour values (codes) are decimal triplets (base 10) or hexadecimal triplets (base 16).
  • RGB decimal and hexadecimal triplets look like this:
    • R = 255, G = 128, B = 0 (255,128,00) is the decimal notation for orange.
    • #FF8000 is the hexadecimal notation for orange.

RGB colour wheel

A colour wheel is a diagram based on a circle divided into segments and can be used to explore the effect of mixing adjacent colours.

An RGB colour wheel provides a graphic representation of the RGB colour model.

  • RGB colour wheels have a minimum of three segments or spokes. These are filled with the additive primary colours red, green and blue.
  • Starting with the three primary colours, an RGB colour wheel can demonstrate the effect of mixing adjacent segments to produce progressively more subtle gradations of intermediate hues.
  • An RGB colour wheel is particularly useful when trying to visually identify and specify:
    •  A particular RGB colour
    • The relationship between different RGB colours
    • Find the colour value (code) for an RGB colour.
  • LED light sources producing very narrow bands of wavelengths can be used when demonstrating RGB colour wheels by projecting red, green and blue LEDs onto a neutrally toned surface.
  • The peak wavelength for selected colours might typically be red = 625 nanometres, green = 500 nm and blue = 440 nm.

RGB colour wheel

A colour wheel is a diagram based on a circle divided into segments and can be used to explore the effect of mixing adjacent colours.

An RGB colour wheel provides a graphic representation of the RGB colour model.

  • RGB colour wheels have a minimum of three segments or spokes. These are filled with the additive primary colours red, green and blue.
  • Starting with the three primary colours, an RGB colour wheel can demonstrate the effect of mixing adjacent segments to produce progressively more subtle gradations of intermediate hues.
  • An RGB colour wheel is particularly useful when trying to visually identify and specify:
    •  A particular RGB colour
    • The relationship between different RGB colours
    • Find the colour value (code) for an RGB colour.
  • LED light sources producing very narrow bands of wavelengths can be used when demonstrating RGB colour wheels by projecting red, green and blue LEDs onto a neutrally toned surface.
  • The peak wavelength for selected colours might typically be red = 625 nanometres, green = 500 nm and blue = 440 nm.
About RGB colour and colour perception
  • The human eye, and so visual perception, is tuned to the visible spectrum and so to spectral colours between red and violet.
  • RGB colour is a model used to reproduce colour in a way that matches 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 neutral-coloured surface.
    • The additional colours produced by mixing adjacent pairs of colours such as adjacent primary, secondary, tertiary colours etc.
  • Every imaginable colour can be produced by the RGB colour model.
  • Remember that the RGB is an additive colour model used when mixing light of different wavelengths.
  • The CMYK subtractive colour model is often used when mixing paints, dyes and pigments.
About RGB secondary colours
  • RGB secondary colours are the hues formed by combining two primary colours of light in equal proportions.
  • The three RGB secondary colours are cyan, magenta, and yellow:
    • When green and blue light sources overlap, they produce cyan.
    • When blue and red light sources overlap, they produce magenta.
    • When red and green light sources overlap, they produce yellow.
  • Mixing adjacent RGB primary and secondary colours of equal intensity results in tertiary colours:
    • Mixing red (primary) and yellow (secondary) creates orange.
    • Mixing yellow (secondary) and green (primary) creates lime green.
    • Mixing green (primary) and cyan (secondary) creates spring green.
    • Mixing cyan (secondary) and blue (primary) creates turquoise.
    • Mixing blue (primary) and magenta (secondary) creates purple.
    • Mixing magenta (secondary) and red (primary) produces fuchsia.
About RGB colour wheels & intermediate colours
  • Intermediate colours on an RGB colour wheel are produced by mixing equal intensities of adjacent pairs of colours.
  • Secondary colours are created by mixing two primary colours in equal amounts. The RGB secondary colours are yellow (red + green), cyan (green + blue) and magenta (red + blue).
  • Tertiary colours are created by mixing a primary colour with a secondary colour. For example, mixing equal amounts of red and yellow creates the tertiary colour orange.
  • 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.
  • The best way to find the correct code for an intermediate colour on a colour wheel is to work from a table (calculate a colour) or swatches (visually match a colour). You can find examples of tables here.
Related diagrams

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

References

RGB colour wheels & intermediate colours

About RGB colour wheels & intermediate colours
  • Intermediate colours on an RGB colour wheel are produced by mixing equal intensities of adjacent pairs of colours.
  • Secondary colours are created by mixing two primary colours in equal amounts. The RGB secondary colours are yellow (red + green), cyan (green + blue) and magenta (red + blue).
  • Tertiary colours are created by mixing a primary colour with a secondary colour. For example, mixing equal amounts of red and yellow creates the tertiary colour orange.
  • 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.
  • The best way to find the correct code for an intermediate colour on a colour wheel is to work from a table (calculate a colour) or swatches (visually match a colour). You can find examples of tables here.

Rods and cones

Rods and cones

Both the photosensitive rods and cones form a regularly spaced mosaic of cells across the entirety of the retina – bar the absence of rods in the fovea centralis. Because there are 100 million rod receptors and 20 million cone receptors in each eye, rods are packed more densely per unit area. The synaptic connections of both rods and cones vary in function in different locations across the retina, reflecting the specialisations of different regions. This, for example, allows the eyes to deal with daylight and darkness and with what we see at the centre and periphery of our field of view.

Rods and cones are easily distinguished by their shape, from which they derive their names, the type of photo pigment they contain and by the distinct patterns of synaptic connections with the other neurons around them.

Neurons (nerve cells) are present throughout the human central and peripheral nervous systems and fall into three main categories: sensory, motor and interneurons. Rods and cones are both sensory neurons. Rods don’t produce as sharp an image as cone cells because they share more connections with other types of neurons. But a rod cell is believed to be sensitive enough to respond to a single photon of light whilst cone cells require tens to hundreds of photons to be activated.

The principal task of rod and cone cells alike is photo-transduction. This refers to the type of sensory transduction that takes place in the visual system. It is the process of photo-transduction that enables pigmented chemicals in the rods and cones to sense light and convert it into electrical signals. Many other types of sensory transduction occur elsewhere within the body enabling touch and hearing for example.

References: Functional Specialization of the Rod and Cone Systems: https://www.ncbi.nlm.nih.gov/books/NBK10850/