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).

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

Electromagnetic waves are carried by particles called photons.

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

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.

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.

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)