Dictionary tag: T

Tristimulus colour values

Human eyes perceive colour through the response of three types of cone cells: L (long wavelength), M (medium wavelength), and S (short wavelength). Tristimulus colour values are a way to quantify colour based on this response. They represent the stimulation levels of these three cone cell types for a particular colour. These values are crucial for various colour spaces and applications in colour science, colour matching, and colour management.

Tristimulus colour values
  • 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
    • LMS tristimulus colour values form the foundation for measuring and representing colour perceptions within the LMS colour space. The system is based on the premise that any colour can be described physiologically by measuring the response of L, M, and S cone cells in the human eye’s retina to different wavelengths of light.
    • LMS tristimulus colour values have a genuine association with the range of colours that fall within the observable visible spectrum of a typical human observer.
    • LMS tristimulus colour values have three components corresponding to the response of the L, M, and S cone types. Each response is measured on a scale with values between 0 and 1.
XYZ tristimulus colour values
  • XYZ tristimulus colour values are equivalent to LMS colour values. The CIE (1931) XYZ colour space utilizes XYZ tristimulus colour values as the basis of the CIE colour system, which has become the global standard for conveying accurate colour information worldwide.
  • XYZ tristimulus colour values have a virtual correspondence with observable colours, meaning that some colours are hypothetical and require adjustments to account for variations in brightness. For instance, fully saturated yellow, green, or cyan may appear much lighter than red or blue.
  • XYZ tristimulus colour values correspond with the response of the L, M and S cone types.
  • Tristimulus colour values are colour-matching functions insofar as they allow you to predict the corresponding colour experience when you know a tristimulus value.
LMS tristimulus colour values & the human eye
  • The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in:
    • Short wavelengths: S = 420 nm – 440 nm.
    • Middle wavelengths: M = 530 nm – 540 nm.
    • Long wavelengths: L = 560 nm – 580 nm.
  • Every human colour sensation can be explained in terms of the stimulus each cone type receives.
  • The LMS cone cells underlie human colour perception in conditions of medium and high brightness.
  • However, in very dim light, colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, known as “rod cells,” become effective.
  • The three parameters denoted as “S”, “M”, and “L” are represented in a 3-dimensional space known as the “LMS colour space,” which is one of many colour spaces designed to quantify human colour vision.
  • 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, which led to the development of the CIE 1931 colour space. In the CIE 1931 colour space, 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.
Related diagrams

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

References
  • Human eyes perceive colour through the response of three types of cone cells: L (long wavelength), M (medium wavelength), and S (short wavelength). Tristimulus colour values are a way to quantify colour based on this response. They represent the stimulation levels of these three cone cell types for a particular colour. These values are crucial for various colour spaces and applications in colour science, colour matching, and colour management.
  • 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.
  • See this page for more information: Tristimulus colour values

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

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 the lowest point in the wave cycle, where the water displacement is furthest down from its rest position. A crest, on the other hand, is the highest point where the displacement is furthest up.
  • For electromagnetic waves, which have electric and magnetic fields, a trough on either axis represents the point where the field reaches its minimum value in the downward direction. A crest represents the point of maximum value in the upward direction.
  • 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.
Related diagrams

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

References
  • 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 the lowest point in the wave cycle, where the water displacement is furthest down from its rest position. A crest, on the other hand, is the highest point where the displacement is furthest up.
  • For electromagnetic waves, which have electric and magnetic fields, a trough on either axis represents the point where the field reaches its minimum value in the downward direction. A crest represents the point of maximum value in the upward direction.