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.



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.


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”.


(CIE, 2011, 17-1345).