Tangent

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

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

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

Tone

Tone refers to the perception of a colour’s brightness (intensity).

Tone can also be used to refer to 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.

Total internal reflection

Total internal reflection occurs when incoming light traveling 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.

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

Tristimulus values

The tristimulus values associated with a trichromatic colour space determine the amounts of each of the three primary colours are to be used to select a particular colour.

  • 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”.

https://en.wikipedia.org/wiki/CIE_1931_color_space#Tristimulus_values

(CIE, 2011, 17-1345).