Absorption

When light strikes an opaque material the wavelengths that are not reflected are absorbed and their energy is converted to heat (notice that absorption has a ‘p’).

  • When sunlight strikes an opaque object or material, it can be reflected or absorbed.
  • If light is reflected it bounces off at the same wavelength.
  • If light is absorbed, the short wavelength energy is changed to longer wavelengths that produce heat near the infrared.
  • Of the light that reaches Earth’s surface from the sun:
    • 54% is already heat (infrared) before it reaches the earth’s atmosphere.
    • 45% is visible light.
    • 1% is shorter wavelengths (ultraviolet).
  • Absorption of a particular wavelength of light into a material takes place when the frequency of the wave matches the frequency at which electrons in the atoms of that material vibrate.
  • Because the electrons in different atoms and molecules have different frequencies they selectively absorb different frequencies of visible light.

Accommodation

Accommodation refers to the way the eye increases optical power by changing the shape of the lens. This is necessary to produce a clear, focused, image of an object when it is near to the eye.

  • The eye focuses on a given object by changing the shape of the eye lens through accommodation. This is controlled by the ciliary muscle, which surrounds the lens.
  • The distance between the retina (the detector) and the cornea (the refractor) is fixed in the human eyeball. The eye must be able to alter the focal length of the lens in order to focus images of both nearby and far away objects on the retinal surface. This is achieved by small muscles that alter the shape of the lens. The distance of objects of interest to an observer varies from infinity to next to nothing but the image distance remains constant.
  • The ability of the eye to adjust its focal length is known as accommodation. The eye accommodates by assuming a lens shape that has a shorter focal length for nearby objects in which case the ciliary muscles squeeze the lens into a more convex shape. For distant objects, the ciliary muscles relax, and the lens adopts a flatter form with a longer focal length.

https://simple.wikipedia.org/wiki/Accommodation_(eye)

Achromatic

Any colour that lacks strong chromatic content is said to be achromatic, unsaturated, or near neutral. Pure achromatic colours include black, white and all greys.

  • Achromatic, unsaturated colours include browns, tans, pastels and darker colours.
  • Achromatic, unsaturated colours can be of any hue or lightness.
  • Achromatic, unsaturated colours are obtained by mixing pure colours with white, black or grey, or by mixing two complementary colours.
  • In colour theory, neutral colours achromatic, unsaturated colours are easily modified by adjacent more saturated colours and appear to take on the hue complementary to the saturated colour.  Next to a bright red couch, a grey wall will appear distinctly greenish.

https://en.wikipedia.org/wiki/colour_scheme>

Additive colour

Additive colour involves mixing different wavelengths of light to produce other colours.

  • Whilst additive colour is the method used to mix wavelengths of light, subtractive colour is the method used to mix dyes, inks and paints.
  • An additive approach to colour is used in the case of emission of light from the (black) screens of mobile phones, computers and televisions.
  • An additive approach to colour is used in the case of the reflection of light off-white, neutral or black surfaces by digital projectors.
  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce all other colours.
  • RGB colour uses three light sources or beams. Each is called a component of the resulting colour.
  • Different colours are produced by varying the intensity of the component colours between fully off and fully on.
  • When fully saturated red, green and blue primary colours are combined, they produce white.
  • When any two fully saturated additive primaries are combined, they produce a secondary colour: yellow, cyan and magenta.
  • Some RGB colour models can produce over 16 million colours by varying the proportion and intensity of each of the three primary colours.
  • The additive RGB colour model cannot be used for mixing different colours of pigments, paints, inks, dyes or powder. To understand these colourants see subtractive colour.

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

Additive colour

Additive colour is a method of mixing different wavelengths of light to produce other colours.

  • An additive approach to colour is used in the case of emission of light from the screens of mobile phones, computers and televisions.
  • An additive approach to colour is used in the case of the reflection of light off-white, neutral or black surfaces by digital projectors.
  • RGB colour is an additive colour model that combines wavelengths of light corresponding with red, green and blue primary colours to produce other colours.
  • Red, green and blue are called additive primary colours in an RGB colour model because they can be added together to produce all other colours.
  • RGB colour uses three light sources or beams. Each is called a component of the resulting colour,
  • Different colours are produced by varying the intensity of the component colours between fully off and fully on.
  • When fully saturated red, green and blue primary colours are combined they produce white.
  • When any two fully saturated additive primaries are combined they produce a secondary colour: yellow, cyan and magenta.
  • Some RGB colour models can produce over 16 million colours by varying the proportion and intensity of each of the three primary colours.

Adobe RGB colour space

The Adobe RGB (1998) colour space is an RGB colour space developed by Adobe Systems, Inc.

  • Adobe RGB (1998) colour space was designed to encompass most of the colours achievable on CMYK colour printers.
  • By using RGB primary colours on a device such as a display device, the Adobe RGB (1998) colour space encompasses roughly 50% of the visible colours specified by the CIELAB colour space.
  • The Adobe RGB (1998) colour space improves on the gamut of the sRGB colour space, primarily in cyan-green hues.

https://en.wikipedia.org/wiki/Adobe_RGB_color_space>

Amacrine cell

Amacrine cells are interneurons in the human retina that interact with retinal ganglion cells and/or bipolar cells.

  • Amacrine cells interact with bipolar cells and/or ganglion cells. They are another type of interneuron which in this case monitor and augment the stream of data through bipolar cells and then control and refines the response of ganglion cell subtypes.
  • Amacrine cells are located in a central but inaccessible region of the retinal circuitry. Most are without tale-like axons. Whilst they clearly have multiple connections to other neurons around them their precise inputs and outputs are difficult to find. They are driven by and feedback to the bipolar cells but also synapse on ganglion cells, and with each other.
  • Amacrine cells are known to serve narrowly task-specific visual functions including:
    • Efficient transmission of high fidelity visual information with a good signal-to-noise ratio.
    • Maintaining the circadian rhythm which keeps our lives tuned to the cycles of day and night and so helps to govern our lives throughout the year.
    • Measuring the difference between the response of specific photoreceptors compared with surrounding cells (centre-surround antagonism) which enables edge detection and contrast enhancement.
    • Object motion detection which provides an ability to distinguish between the true motion of an object across the field of view and our own eye movements.
  • Centre-surround antagonism refers to the way retinal neurons organize their receptive fields.  The centre component is primed to measure the sum-total of signals received from a small number of cones directly connected to a bipolar cell. The surround component is primed to measure the sum of signals received from a much larger number of cones around the centre point. The two signals are then compared to find the degree to which they disagree.

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

Amplitude

The amplitude of a wave is a measurement of the distance from the top of a crest through the centre line  (the still position, zero-point, mid-point) to the bottom of a trough.

In the case of an electromagnetic wave, amplitude corresponds with the intensity of light and the brightness of colour perceived by an observer.

  • When the amplitude of an electromagnetic wave of a specific wavelength increases so does the overall distance between the peak and the trough of the wave.
  • The measurement taken between the topmost point on a wave (the peak) and the centre line of the wave is called peak amplitude.
  • Amplitude is measured in metres (m).
  • The greater the amplitude of a wave, the more energy it carries.
  • When amplitude increases so does the perceived vividness of a colour.
  • Whilst the perceived brightness of a colour depends on the amplitude of a light wave, brightness is also somewhat influenced by wavelength. So for example yellows tend to look brighter than reds or blues.

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

Analogous colours

Analogous colours are groups of colours that are adjacent to each other on a colour wheel, with one being the dominant colour.

  • The dominant colour amongst a group of analogous colours is usually a primary or secondary colour.
  • The immediately adjacent colours to secondary colours are tertiary colours.
  • An analogous colour scheme creates a rich, monochromatic look but is less vibrant than complementary schemes.
  • Red, reddish-orange, orange, yellow-orange is one example of a set of analogous colours.

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

Angle of incidence

The angle of incidence measures the angle at which incoming light strikes a surface.

  • The angle of incidence is measured between a ray of incoming light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), the boundary between two media.
  • If the boundary between the media is curved, then the normal is drawn at a tangent to the boundary.

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

Angle of reflection

The angle of reflection measures the angle at which reflected light bounces off a surface.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Angle of reflection

The angle of reflection measures the angle at which reflected light bounces off a surface.

  • The angle of reflection is measured between a ray of light which has been reflected off a surface and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
  • Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction when referring to light passing across the boundary between two different transparent media, such as water, glass, or air.
  • In optics, the law is used in ray diagrams to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a medium.

Angle of refraction

The angle of refraction measures the angle to which light bends as it passes across the boundary between different media.

  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is a line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.
  • Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction when referring to light passing across the boundary between two different transparent media, such as water, glass, or air.
  • In optics, the law is used in ray diagrams to compute the angles of incidence or refraction, and in experimental optics to find the refractive index of a medium.

Atom

An atom is the smallest particle that can be described as a chemical.

  • At the centre of an atom is a nucleus containing protons which are positively charged sub-atomic particles. The number of protons determines what chemical the atom is. A hydrogen atom has a single proton.
  • The nucleus also contains a number of neutrons, sub-atomic particles with no charge.
  • Surrounding the nucleus are very small negatively charged particles called electrons, held in place by their attraction to the positively charged nucleus.
  • Quantum mechanics is used to describe the orbital motion of electrons around a nucleus.
  • In an atom, the number of electrons always matches the number of protons.
  • If an electron is removed from an atom the result is a charged particle called an ion.
  • For a single atom, the number of protons and neutrons combined determines its atomic mass.
  • Atomic mass is a measure of the total mass of protons and neutrons in an atom and is used to order elements in the periodic table.

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

Attributes of visual perception

Attributes of visual perception are the innate abilities and the skills we develop over the course of a lifetime that enable us to make sense of what we see. They are evident in the diverse properties of the world we see around us.

  • Innate attributes of visual perception associated with the response of the human eye and brain to light include:
  • Colour perception: The ability to see colour in the presence of light including all the greys between black and white.
  • Visual attention: The ability to focus on important visual information and filter out unimportant background information.
  • Sensory processing: Accurate registration, interpretation and response through the coordination of visual information with other forms of sensory stimulation.
  • Visual discrimination: The ability to recognise differences or similarities in objects based on size, colour, shape, etc.
  • Spatial relationships: The ability to understand the relationships of objects, particularly their distance, direction of movement and position relative to an observer.
  • Figure-ground: The ability to locate something against a busy background.
  • Form constancy: The ability to know that a form or shape is the same, even if it becomes larger or smaller, or its orientation changes.
  • Visual closure: The ability to recognise a form or object when part of it is hidden or missing.
  • Visual memory: The ability to recall visual traits of a form or object.
  • Visual sequential memory: The ability to recall a sequence of objects in the correct order.

Bands of colour

An observer perceives bands of colour because:

  • The human eye is able to distinguish between some ranges of wavelengths of visible light better than others.
  • Some ranges of wavelengths appear more intense to a human observer than others.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • It is the human brain that draws lines between different bands of colour when an observer looks at a rainbow for example.
  • A human observer can distinguish between colours corresponding with many thousands of single wavelengths of light in the visible spectrum. These colours are called spectral colours.
  • Combinations of wavelengths from different areas of the visible spectrum produce other colours when perceived by a human observer which are called non-spectral colours.
  • There is no property belonging to electromagnetic radiation that causes bands of colour to appear to an observer. The fact that we do see distinct bands is often described as an artefact of human colour vision.
  • The visible spectrum is formed of a smooth and continuous range of wavelengths that can be demonstrated to produce a smooth and continuous range of colours.
  • Cone cells in our eyes are particularly sensitive to red, green and blue wavelengths.
  • Our brains process information received from the eye to produce all the colours of the visible spectrum.

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

Bands of colour

An observer perceives bands of colour because:

  • The human eye is able to distinguish between some ranges of wavelengths of visible light better than others.
  • Some ranges of wavelengths appear more intense to a human observer than others.
  • Colour is not a property of electromagnetic radiation, but a feature of visual perception.
  • It is the human brain that draws lines between different bands of colour when an observer looks at a rainbow for example.
  • A human observer can distinguish between colours corresponding with many thousands of single wavelengths of light in the visible spectrum. These colours are called spectral colours.
  • There is no property belonging to electromagnetic radiation that causes bands of colour to appear to an observer. The fact that we do see distinct bands is often described as an artefact of human colour vision.
  • The visible spectrum is formed of a smooth and continuous range of wavelengths that can be demonstrated to produce a smooth and continuous range of colours.
  • Cone cells in our eyes are particularly sensitive to red, green and blue wavelengths.
  • Our brains process information received from the eye to produce all the colours of the visible spectrum.

Bipolar cells

Bipolar cells are a type of neuron found in the retina of the human eye. They are located between photoreceptors (rod cells and cone cells) and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.

  • Bipolar cells are connected to rod and cone cells by synapses. These cells are located within the retina between these photoreceptors and ganglion cells.
  • There are around 12 types of bipolar cells that function as integrating centres. Each type acts directly or indirectly, as a conduit from a photoreceptor to ganglion cells and each carries a different parsing of its output. So, each type of bipolar cell that contacts a given rod or cone transmits a different analysis and interpretation of information extracted from its output.
  • The output of bipolar cells onto ganglion cells includes both the direct response of the bipolar cell to signals derived from phototransduction but also responses to those signals received indirectly from information and actions provided by amacrine cells.
  • We might imagine a type of bipolar cell that connects directly from a cone to a ganglion cell and simply compares signals on the basis of what is known of their wavelength. The ganglion cell uses 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 enabling them to carry packets of information to an array of different locations and cell types.

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

Black body

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

https://simple.wikipedia.org/wiki/Blackbody_radiation

Brightness

The brightness (luminance, brilliance) of an object refers to the quality, apart from hue and saturation that an observer uses to determine the comparative brightness of another object.

In terms of tonal differences, pure white has the maximum brightness, and pure black the minimum brightness.

In terms of colour, a pure yellow may appear to be much brighter than a pure blue.

  • Brightness is a colour coordinate in the HSB colour model.
  • Brightness is the perception elicited by the luminance of a visual target.
  • Brightness should not be confused with lightness.
  • In general terms, brightness is an attribute of visual perception used to refer to whether one object appears to be radiating or reflecting more or less light than another.
  • The field of photometry, the science of the measurement of light, recognises that the human eye is not equally sensitive to all wavelengths of visible light.

Chemical bond

A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds.

  • A chemical compound consists of two or more atoms from different elements chemically bonded together.
  • There are two types of chemical bond: covalent bonds and ionic bonds:
    • A covalent bond forms when two atoms share a pair of electrons.
    • Atoms can lose or gain electrons in chemical reactions. When they do this they form charged particles called ions.
  • Chemical bonds occur because opposite charges attract via the electromagnetic force.
  • Negatively charged electrons orbiting the nucleus of an atom and the positively charged protons in the nucleus attract each other.
  • An electron positioned between two nuclei will be attracted to both of them, and the nuclei will be attracted toward electrons in this position. This attraction constitutes the chemical bond.
  • Due to the matter-wave nature of electrons and their smaller mass, they must occupy a much larger amount of volume compared with the nuclei, and this volume occupied by the electrons keeps the atomic nuclei in a bond relatively far apart, as compared with the size of the nuclei themselves.
  • The physical world is held together by chemical bonds, which dictate the structure and the bulk properties of matter.

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

Colour

Things appear coloured because colour corresponds with a property of light that is visible to the human eye. The visual experience of colour is associated with words such as red, blue, yellow, etc.

The colour an observer sees depends on:

Light is electromagnetic radiation (radiant energy), which, detached from its source, is transported by electromagnetic waves (or their quanta, photons) and propagates through space. Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.

The experience of colour is a feature of human vision that depends first of all on the construction of our eyes and the wavelength, frequency and amplitude of visible light that strikes the retina at the back of each eye.

Because colour is a visual experience that is specific to each and every one of us at any given moment, we can try and share our experiences of colour using language but colour cannot be defined without examples.

The name given to light that contains all wavelength of the visible spectrum is white light.

When white light strikes a neutral coloured object, and all wavelengths are reflected, then it appears white to an observer.

The term white light doesn’t mean light is white as it travels through the air.

As light travels through the air it is invisible to our eyes.

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.

The colour an observer sees depends on the wavelengths of visible light emitted by a light source and on which of those wavelengths are reflected off an object.

Although a human observer can distinguish between many thousands of wavelengths of light in the visible spectrum our brains often produce the impression of bands of colour.

As light travels from one medium to another, such as from air to glass, the wavelength changes but the frequency remains the same so the colour seen by an observer remains the same.

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

Colour

Things appear coloured because colour corresponds with a property of light that is visible to the human eye. The visual experience of colour is associated with words such as red, blue, yellow, etc.

  • The experience of colour is a feature of human vision that depends first of all on the construction of our eyes and the wavelength, frequency and amplitude of visible light that strikes the retina at the back of each eye.
  • 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.
  • The colour an observer sees depends on the wavelengths of visible light emitted by a light source and on which of those wavelengths are reflected off an object.
  • Although a human observer can distinguish between many thousands of wavelengths of light in the visible spectrum our brains often produce the impression of bands of colour.

Colour constancy

Colour constancy refers to the ability of the human eye and brain to automatically compensate when objects change colour because of changes in illumination.

  • Colour vision relies on colour constancy to enable an observer to perceive the colour of an object as almost unchanged as levels of illumination change and the spectral distributions of light changes.
  • A human observer will often not notice when the colour of object changes as the source of illumination changes e.g. sunlight to artificial light.
  • Colour vision allows us to distinguish different objects by their colour. In order to do so, colour constancy can keep the perceived colour of an object relatively unchanged when the illumination changes among various broad (whitish) spectral distributions of light.
  • Colour constancy is achieved by chromatic adaptation. The International Commission on Illumination defines white (adapted) as “a colour stimulus that an observer who is [chromatically] adapted to the viewing environment would judge to be perfectly achromatic and to have a luminance factor of unity. The colour stimulus that is considered to be the adapted white may be different at different locations within a scene.
  • The effect of changes in colour balance is very noticeable when comparing photographs of the same subject taken in different lighting conditions. Cameras use white balance to compensate for changes in illumination.

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

Colour notation

The most common forms of colour notation used with computer software and by digital equipment are the triplets of the  RGB and HSB colour models and the quadruplets used by CMYK model.

  • RGB notation can be represented in decimal or hexadecimal formats.
    • Decimal RGB notation for orange is: R=255, G=128, B=0.
    • Hexadecimal RGB notation  for orange is: #FF8000.
  • HSB notation is represented in a decimal format.
    • The HSB notation for orange is: H=30.12, S=100, B=100.
  • CMYK notation is represented in a decimal format.
    • The CMYK notation for orange is: C=0, M=61.48, Y=100, K=0.

Colour vision

Colour vision is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a range of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
  • Colours can be measured and quantified in various ways. But an observer’s perception of colour is a subjective process whereby their brain responds to stimuli that are produced when incoming light reacts with light-sensitive cells at the back of their eye. As a result, different people may see the same illuminated object or light source in different ways.

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

Colour vision

Colour vision is the ability of an organism or machine to distinguish objects based on the wavelengths (or frequencies) of the light they emit, reflect or transmit. The human eye and brain together translate light into colour.

  • The human eye, and so human perception, is tuned to the visible spectrum and so to spectral colours between red and violet. Light, however, is rarely of a single wavelength, so an observer will usually be exposed to a range of different wavelengths of light or a mixture of wavelengths from different areas of the spectrum.
  • Colours can be measured and quantified in various ways; indeed, a person’s perception of colour is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with several types of cone cells in the eye. In essence, different people see the same illuminated object or light source in different ways.

Colour wheel

A colour wheel is a diagram based on a circle divided into segments. The minimum number of segments is three with a primary colour in each. Segments added between the primaries can then be used to explore the result of mixing adjacent pairs of primary colours together. Additional segments can then be added between all the existing segments to explore the result of mixing further pairs of adjacent colours.

  • 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 rainbow colours.
  • Colour wheels are often used in technologies which reproduce colour in ways that match the light sensitivity of the three different types of cone cells and the rod cells in the human eye.
  • Colour wheels exploring additive colour models and subtractive colour models use different sets of primary colours.
  • An RGB colour wheel, used to explore additive mixing of light, starts with red, green and blue primary colours.
  • The colours produced in between the primary colours in a colour wheel are called secondary colours.
  • The colours produced in between the secondary colours in a colour wheel are called tertiary colours.
  • A CMY colour wheel, used to explore subtractive mixing of pigments and inks (used in digital printing) starts with cyan, magenta and yellow primary colours.
  • An RYB colour wheel used to explore the subtractive mixing of art pigments and paints starts with red, yellow and blue primaries.
  • The colour wheels described above all depend on trichromatic colour vision which involves three receptor types (cone cells) processing colour stimuli.

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

Colour wheel

A colour wheel is a diagram based on a circle divided into segments. The minimum number of segments is three with a primary colour in each. Segments added between the primaries can then be used to explore the result of mixing adjacent pairs of primary colours together. Additional segments can then be added between all the existing segments to explore the result of mixing further pairs of adjacent colours.

  • 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 rainbow colours.
  • Colour wheels are often used in technologies which reproduce colour in ways that match the light sensitivity of the three different types of cone cells and the rod cells in the human eye.
  • Colour wheels exploring additive colour models and subtractive colour models use different sets of primary colours.
  • An RGB colour wheel, used to explore additive mixing of light, starts with red, green and blue primary colours.
  • The colours produced in between the primary colours in a colour wheel are called secondary colours.
  • The colours produced in between the secondary colours in a colour wheel are called tertiary colours.
  • A CMY colour wheel, used to explore subtractive mixing of pigments and inks (used in digital printing) starts with cyan, magenta and yellow primary colours.
  • An RYB colour wheel used to explore the subtractive mixing of art pigments and paints starts with red, yellow and blue primaries.
  • The colour wheels described above all depend on trichromatic colour vision which involves three receptor types (cone cells) processing colour stimuli.

Complementary

In the context of a discussion of light (as opposed to pigments) complementary colours are two colours that, when mixed together, produce white light.

  • When working with the RGB colour model, combining the wavelengths corresponding with red, green and blue primary colours produces white for a human observer.
  • The complementary colour of each primary colour when working with the RGB colour model is the secondary colour produced by combining the other two primaries.
  • The complementary primary–secondary combinations of light are red–cyan, green–magenta, and blue–yellow.
  • Combinations of complementary primary–secondary colours at full intensity make white light because together they contain wavelengths corresponding with all three primaries.
  • A complementary colour plus a primary colour combine to produce white because each complementary colour is the product of combing two primaries.
  • So a complementary colour produced by combining green and blue primaries makes white when combined with red.

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

Compound

A compound is a substance made from the combination of two or more elements and held together by chemical bonds that are difficult to break. The bonds form as a result of sharing or exchanging electrons among atoms. The smallest unbreakable unit of a compound is a molecule.

  • A compound is formed when different elements react with each other, forming bonds between atoms that produce molecules.  When a compound is exposed to a new element further reactions can take place which produce new compounds.
  • A compound differs from a mixture because the atoms in a mixture are not bonded together. In this case, different elements mix together but no chemical reaction takes place, so each element remains separate and distinct.

Cone cell

Cone cells, or cones, are one of three types of photoreceptor cells (neurons) in the retina of the human eye. They are responsible for colour vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.

  • The three types of photoreceptor cells (neurons) in the retina of the human eye are cone, rod and ganglion cells.
  • Cone cells are cone-shaped whilst rod cells are rod-shaped.
  • Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
  • There are believed to be around six million cone cells and 90 million rod cells in the human retina.

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

Cone cell

Cone cells, or cones, are one of three types of photoreceptor cells (neurons) in the retina of the human eye. They are responsible for colour vision and function best in relatively bright light, as opposed to rod cells, which work better in dim light.

  • Cone cells are cone-shaped whilst rod cells are rod-shaped.
  • Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
  • There are believed to be around six million cone cells in the human retina.

Crest

The crest is the point on a wave with the maximum value of upward displacement within a wave-cycle. A trough is the opposite of a crest, so the minimum or lowest point in a wave-cycle.

  • On a wave at sea, the crest is a point where the displacement of water is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the medium is at a minimum.
  • In the case of an electromagnetic wave which has an electric and a magnetic axis then a crest on either axis refers to maximum displacement in the positive direction whilst a trough refers to minimum displacement.

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

Crest and trough

The crest is the point on a wave with the maximum value of upward displacement within a wave-cycle. A trough is the opposite of a crest, so the minimum or lowest point in a wave-cycle.

  • On a surface wave, the crest is a point where the displacement of the medium (water for example) is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the medium is at a minimum.
  • In the case of an electromagnetic wave which has an electric and a magnetic axis then a crest on either axis refers to maximum displacement in the positive direction whilst a trough refers to minimum displacement.

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

Critical angle

The critical angle for light approaching the boundary between two different media is the angle of incidence above which it undergoes total internal reflection. The critical angle is measured with respect to the normal at the boundary between two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • Internal reflection takes place when light travelling through a medium strikes the boundary of another medium with a lower refractive index at an angle greater than the critical angle.
  • For example, internal reflection takes place when light reaches air from glass at an angle greater than the critical angle, but not when light reaches glass from air.
  • In general, light will be partially refracted and partially reflected because of irregularities in the surface at the boundary.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.

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

Critical angle

The critical angle for light approaching the boundary between two different media is the angle of incidence above which it undergoes total internal reflection. The critical angle is measured with respect to the normal at the boundary between two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • Internal reflection takes place when light travelling through a medium strikes the boundary of another medium with a lower refractive index at an angle greater than the critical angle.
  • For example, internal reflection takes place when light reaches air from glass at an angle greater than the critical angle, but not when light reaches glass from air.
  • In general, light will be partially refracted and partially reflected because of irregularities in the surface at the boundary.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.

Crown glass

Crown glass is a type of optical glass. It is made without lead or iron and is used in the manufacture of lenses and other tools and equipment concerned with the visible part of the electromagnetic spectrum.

  • Crown glass is produced from alkali-lime silicates containing approximately 10% potassium oxide and is one of the earliest low dispersion glasses.
  • As well as the specific material called crown glass, there are other optical glasses with similar properties that are also called crown glasses.

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

Digital printing

Digital printing usually involves mapping the colours in a digital file (JPEG, PNG, SVG) to the CMYK colour model and printing the data onto paper using cyan, magenta, yellow and black inks.

  • Digital printers typically use highly reflective white paper overlaid with cyan, magenta, yellow and black pigments in the form of ink or toner.
  • Printing has a smaller gamut than display devices (screens, monitors, projectors) which rely on light emission, rather than reflection
  • Display devices produce comparatively brighter, more intense colours than printers because the amplitude of each wavelength is larger on an emissive source.
  • Digital printers produce dull and less intense colours than display devices because the amplitude of each wavelength is smaller when light is reflected off paper through inks or pigments.
  • A display device, such as a computer screen, starts in a black state and is illuminated with red, green and blue light to produce colour.
  • Printers typically use a highly reflective white paper and add CMYK (cyan, magenta, yellow and black) ink or toner to produce colour.
  • CMYK pigments are the standard for colour printing because they have a larger gamut than RGB pigments.
  • Highlights are produced by reducing the amount of coloured ink and printing without black to allow the maximum amount of light possible to shine through and reflect off the paper.
  • Mid tones rely on the brilliance and transparency of the pigments and the reflectivity of the paper to produce fully saturated colours.
  • Shadows are produced by adding black to both saturated or desaturated hues.

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

Dispersion

Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.

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

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

Dispersion

Dispersion (or chromatic dispersion) refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.

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