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. So when talking about perceived colour, a distinction can be made between:
  • 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 after-image 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).

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Photometry

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

  • 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.

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Photon

A photon is the basic building block of light. A photon is a single indivisible bundle (particle or wave) of energy within an electromagnetic field.

  • In the field of optics, light is explained in terms of waves (wavelength, frequency and energy) but this description doesn’t always fit the evidence. It became clear during the 20th century that light sometimes exhibits wave-like behaviour, at others both waves and particles, or just particles.
  • Contemporary physics considers that electromagnetic fields propagate through space configured as bundles of energy. These are bundles of photons.
  • Photons are the force carriers of radiant energy (electromagnetic radiation).
  • 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, plural quanta) into which light can be divided.

Photon

A photon is the carrier of the electromagnetic force. It is the fundamental building block of light.

About the properties of photons
  • A photon is a single indivisible bundle (particle or wave) of energy within an electromagnetic field.
  • A photon is an elementary particle and represents a quantum (plural =quanta) of light – the smallest quantity into which light can be divided.
  • Whilst the field of optics often explains light in terms of waves, this description doesn’t always fit the evidence.
  • Light sometimes exhibits wave-like behaviour, at others, it behaves like both waves and particles or just as particles.

Other properties of photons include:

  • They have zero mass and rest energy. They only exist as moving particles.
  • They are elementary particles despite lacking rest mass.
  • They have no electric charge.
  • They are stable.
  • They carry energy and momentum which are dependent on the frequency.
  • They can have interactions with other particles such as electrons.
  • They can be destroyed or created by many natural processes, for instance when radiation is absorbed or emitted.
  • When in empty space, they travel at the speed of light.

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.

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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.

  • For everyday light levels, the photopic luminosity function best approximates the response of the human eye.
  • The photopic curve is the CIE standard curve used in the CIE 1931 color space.
  • For low light levels, the response of the human eye changes, and the scotopic curve applies.

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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.

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Pigment epithelium

Pigment epithelium

Pigment epithelium is a layer of cells at the boundary between the retina and the eyeball that nourish neurons within the retina. It is firmly attached to the underlying choroid is the connective tissue that forms the eyeball on one side but less firmly connected to retinal visual cells on the other.

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.

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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.

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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.

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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.

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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 rainbow

The most common atmospheric rainbow is a primary bow.

  •  Primary rainbows appear when sunlight is refracted as it enters raindrops, reflects once off the opposite interior surface, is refracted again as it escapes back into the air, and then travels towards an observer.
  • The colours in a primary rainbow are always arranged with red on the outside of the bow and violet on the inside.
  • The outside (red) edge of a primary rainbow forms an angle of approx. 42.40 from its centre, as seen from the point of view of the observer. The inside (violet) edge forms at an angle of approx. 40.70.
  • To get a sense of where the centre of a rainbow might be, imagine extending the curve of a rainbow to form a circle.
  • If your shadow is visible as you look at a rainbow its centre is aligned with your head.
  • A primary rainbow is only visible when the altitude of the sun is less than 42.4°.
  • Primary bows appear much brighter than secondary bows and so are easier to see.
  • The curtain of rain on which sunlight falls is not always large enough or in the right place to produce both primary and secondary bows.

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.

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Primary visual cortex

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. The point where the visual cortex receives sensory inputs is also the point where there is a vast expansion in 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.

 [Cerebral hemispheres, occipital lobes, primary visual cortex, optical radiations]

Prism

In the field of optics, a prism is an object made of glass or other transparent material and has 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.

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