LMS colour space

  • The LMS colour space was one of the first systematic demonstration of trichromatic colour theory.
  • The method used in the development of the LMS colour space produces a standardized representation of human colour perception.
  • The underlying principle is that any colour can be described in physiological terms by measuring the response of  L, M and S cone cells in the retina of the human eye to different wavelengths of light.
  • The response of cone cells is linear meaning that a single equation produces a set of interrelated values (tristimulus values) that correspond with the colour experience of a typical observer.
  • Tristimulus values have three components corresponding with the response of the L, M and S cone types. Each response is measured against a scale with values between 0 and 1.
  • Tristimulus colour values can be thought of as colour-matching functions. If you know a tristimulus value then you can predict the corresponding colour experience.
  • The LMS colour space is an extremely accurate physiological description but in practical terms has its limitations because some colours in the visible spectrum appear brighter than others. These differences are noticeable when converting a colour image to greyscale. The implications are that if the saturation of different colours is reduced incrementally some reach the point where they appear white before others.
  • The CIE (1931) XYZ colour space addresses the limitations of the LMS colour space by sacrificing physiologically accurate measurements of colour perceptions in favour of a solution that has enormous practical advantages.

Trivariance

Trivariance

The term trivariance is used to refer to this first stage of the trichromatic process. It refers to both the phototransductive response of the cone cells themselves and to the three separate channels used to convey their colour information forward to subsequent levels of neural processing.

Each channel conveys information about the response of one cone-type to both the wavelength of the incoming light it is tuned to and to its intensity. In both physiological and neurological terms this process is exclusively concerned with trivariance – three discernible differences in the overall composition of light entering the eye.

It is the separation of the signals produced on each channel that accounts for the ability of our eyes to respond to stimuli produced by additive mixtures of wavelengths corresponding with red, green and blue primary colours. But more of that later!

By way of summary, the rod and trivariant cone systems are composed of photoreceptors with connections to other cell types within the retina. Both specialize in different aspects of vision. The rod system is extremely sensitive to light but has a low spatial resolution. Conversely, the cone system is designed to function in stronger light. As a result, cones are relatively insensitive compared with rods but have a very high spatial resolution. It is this specialisation that results in the extraordinary detail, resolution and clarity of human vision.

Rod System Cone System
High sensitivity, specialized for night vision Lower sensitivity specialized for day vision
Saturate in daylight Saturate only in intense light
Achromatic Chromatic, mediate colour vision
Low acuity High acuity
Not present in the central fovea Concentrated in the central fovea
Present in larger number than cones Present in smaller number than rods

Caption

Trichromatic colour vision

Trichromatic colour vision (Trichromacy)

Photo-transduction by cone cell receptors is the physiological basis for trichromatic colour vision in humans. The fact that we see colour is, in the first instance, the result of interactions among the three types of cones, each of which responds with a bias towards its favoured wavelength within the visible spectrum. The result is that the L, M and S cone types respond best to light with long wavelengths (L =biased towards 560 nm), medium wavelengths (M =biased towards 530 nm), and short wavelengths (S = biased towards 420 nm) respectively.

Thermonuclear fusion

Thermonuclear fusion involves atoms fusing together. Thermonuclear fusion requires immense pressure and heat.

  • There are two forms of thermonuclear fusion:
    • Uncontrolled fusion, in which the resulting energy is released in an uncontrolled way, as it is in thermonuclear weapons (“hydrogen bombs”) and in most stars.
    • Controlled fusion, where the reaction takes place in an environment allowing some or all of the energy released to be harnessed for constructive purposes.

Thermonuclear fusion

Thermonuclear fusion involves atoms fusing together. Thermonuclear fusion requires immense pressure and heat.

  • There are two forms of thermonuclear fusion:
    • Uncontrolled fusion, in which the resulting energy is released in an uncontrolled way, as it is in thermonuclear weapons (“hydrogen bombs”) and in most stars.
    • Controlled fusion, where the reaction takes place in an environment allowing some or all of the energy released to be harnessed for constructive purposes.

Tangent

A tangent to a circle is a straight line that touches but does not intersect the circle and is at right angles to a radial line drawn from  the centre of the circle.

  • In geometry, a tangent (or tangent line) to a curve is a straight line that touches but does not intersect the curve. It can be defined as a line through a pair of infinitely close points on a curve.

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

Trough

A trough is the point on a wave with the maximum value of downward displacement within a wave-cycle. A crest is the opposite of a trough, so the maximum or highest point in a wave-cycle.

  • On a wave at sea, the trough is a point where the displacement of water reaches a minimum. A crest is the opposite of a trough, so a crest is a point where the displacement of the water is at a maximum.
  • In the case of an electromagnetic wave which has an electric and a magnetic axis,  a trough on either axis refers to minimum displacement in the negative direction whilst a crest refers to maximum displacement.
  • Wavelength refers to a complete wave-cycle from one crest to the next, or one trough to the next.
  • Frequency refers to the number of wave-cycles that pass a given point in a given amount of time.
  • The amplitude of a wave is a measurement of the distance from the centre line (or the still position) to the top of a crest or to the bottom of a corresponding trough.
  • Amplitude is related to the energy a wave carries. The energy a wave carries is related to frequency and amplitude. The higher the frequency, the more energy, and the higher the amplitude, the more energy.

Tristimulus colour values

LMS tristimulus colour values underpin the system of measurement and representation of colour perceptions that constitute the LMS colour space. The system works on the assumption that any colour can be described in physiological terms by measuring the response of  L, M and S cone cells in the retina of the human eye to different wavelengths of light.

XYZ tristimulus colour values are equivalent to LMS colour values. The CIE (1931) XYZ colour space uses XYZ tristimulus colour values as the foundation of the CIE colour system which has become the universal standard for communicating precise colour information around the world.

  • LMS tristimulus colour values have a real correspondence with the range of colours that fall within the observable visible spectrum of a typical human observer.
  • XYZ tristimulus colour values have a virtual correspondence with observable colours meaning that some colours are hypothetical, an anomaly resulting from adjusting colour values to account of variations in brightness. The adjustments allow for the fact that a fully saturated yellow, green or cyan is much lighter than red or blue for example.
  • Tristimulus values are the backbone of colour measurement whether in terms of the physiological response of the human eye to light or within the world of colour matching or colour management.
  • LMS tristimulus colour values have three components corresponding with the response of the L, M and S cone types. Each response is measured against a scale with values between 0 and 1.
  • XYZ tristimulus colour values also corresponding with the response of the L, M and S cone types
  • Tristimulus colour values are colour-matching functions in so far as, if you know a tristimulus value then you can predict the corresponding colour experience.
  • The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short (“S”, 420 nm – 440 nm), middle (“M”, 530 nm – 540 nm), and long (“L”, 560 nm – 580 nm) wavelengths.
  • These cone cells underlie human colour perception in conditions of medium and high brightness; in very dim light colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, denominated “rod cells”, become effective.
  • Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human colour sensation. Weighting a total light power spectrum by the individual spectral sensitivities of the three kinds of cone cells renders three effective values of stimulus; these three values compose a tristimulus specification of the objective colour of the light spectrum.
  • The three parameters denoted “S”, “M”, and “L”, are indicated using a 3-dimensional space denominated the “LMS colour space”, which is one of many colour spaces devised to quantify human colour vision.
  • Stimuli that account for colour perception: can be specified by a set of tristimulus values, defined as the “amounts of the 3 reference colour stimuli, in a given trichromatic system, required to match the colour of the stimulus considered”.
  • A human eye with normal vision has three kinds of cone cells and each senses light of different wavelengths. They are identified as follows:
    • L – Long (560 nm – 580 nm)
    • M = Medium ( 530 nm – 540 nm)
    • S = Short (420 nm – 440 nm)
  • Every human colour sensation can be explained in terms of the stimulus each cone type receives.
  • If the stimulus received by the three cone type is measured then any colour perception can be identified by the three components of a tristimulus colour value which is the basis for the LMS colour space.
  • The LMS colour space was the subject of intense scientific study during the 1920s because it established a direct link between the subjective human experience of colour and wavelengths of the visible spectrum.
  • There were technical problems interpreting the LMS colour space however that led to the development of the CIE 1931 colour space in which LMS tristimulus values are denoted by X, Y, and Z tristimulus values.
  • One of the most important innovations associated with the CIE 1931 colour space is the CIE xy chromaticity diagram.

Total internal reflection

Total internal reflection occurs when incoming light travelling through a medium strikes the boundary with a second medium at angles greater than a certain critical angle with the result that no light crosses the boundary and so all the light is reflected back into the medium.

Tone

Tone is sometimes used to refer to the perception of a colour’s brightness.

Tone can also be used to refer to the shades of grey between black and white.

  • A hue reduced in intensity is called a tone.
  • In the context of the HSB colour model, a darker tone of a hue is produced by reducing its brightness (intensity).
  • Tone is associated with the term value.
    • Value is the amount of light that is reflected from a surface or emitted by a computer screen.
    • As the amount of light associated with a hue increases the colour becomes brighter – a lighter tint of that hue.
    • As the amount of light associated with a hue decreases the colour becomes a darker –  a darker shade (tone) of that hue.
    • In terms of tone, white has a high value, black has a low value.
    • The value of an object is usually evaluated relative to the brightness of a similarly illuminated white in the same situation.

Temperature

All objects emit electromagnetic radiation, and the amount of radiation emitted at each wavelength depends on the temperature of the object.

  • Hot objects emit more of their light at short wavelengths.
  • Cold objects emit more of their light at long wavelengths.
  • The temperature of an object is related to the wavelength at which the object gives out the most light.

Tangent

A tangent to a circle is a straight line that touches but does not intersect the circle and is at right angles to a radial line drawn from  the centre of the circle.

In geometry, a tangent (or tangent line) to a curve is a straight line that touches but does not intersect the curve. It can be defined as a line through a pair of infinitely close points on a curve.

Transmission

Transmission takes place when light (electromagnetic radiation) passes through a medium.

  • Transmission refers to the passage of any form of electromagnetic radiation through a medium.
  • If no light is reflected or absorbed as light travels through a medium, then the light achieves 100% transmission.
  • The term transmittance of a material refers to its effectiveness in transmitting radiant energy. It is the fraction of incident electromagnetic power that is transmitted through the material.