Luminosity is about light given off by a source such as the Sun or a light-bulb.

  • If one light bulb produces one unit of light, then two produces twice the luminosity and so on.
  • Because luminosity is about light being given off in all directions it is not something that an observer can see directly.
  • A light source’s luminosity depends on how much power it consumes. In the case of a light bulb, for example, it relates to the amount of electrical energy the bulb is burning and how much of that energy is being turned into visible light.
  • Luminosity is something that can be measured and so is an objective term.
  • If you have read about trichromatic vision, you will know that it is possible to match all the colours in the visible spectrum seen by an observer by appropriate mixing of wavelengths and intensities (luminosity) of light corresponding with three primary colours and this can be achieved without any loss of information so far as an observer is concerned.
  • Now imagine three light sources with wavelengths corresponding with red, green and blue connected to sliders that allows the luminosity of each component to be adjusted between a minimum of 0% (off) and a maximum of 100% (fully on).
  • Zero luminosity for each component means no light, so an observer in a windowless room would be in darkness. If each component is set to full luminosity our observer will see white. The exact quality of white depends on the type of lights, how accurately their wavelength is controlled and the surface the light falls upon.
  • If the sliders that control each light are set to the same value (and so to the same luminosity) between 1% and 99% then the result is a shade of grey, which appears darker as the intensity decreases or brighter as the intensity increases.
  • When the luminosity of each slider is set to different values, the result is the perception of a colour.
  • When one of the components has the highest luminosity, the colour will be a hue near that primary colour and so appear more reddish, greenish or bluish. When two components have the same high luminosity, then the observer sees the hue of a secondary colour (a shade of cyan, magenta or yellow).
  • Maximum luminosity of a spectral colour corresponds with its lightest tint.
  • Maximum luminosity of a display device corresponds with the brightest white it can reproduce and is called the white point.
  • The black point corresponds with the minimum luminosity of a device, so corresponds with the device being turned off.
  • The contrast ratio of maximum and minimum luminosity of a television or computer screen is typically more than 280:1