Spectral power distribution

The spectral power distribution (spectral distribution) of a light or colour stimulus refers to the range, combination and intensity of wavelengths of light that it emits.

  • The precise measurement of the wavelengths, frequencies, and intensities of light emitted by a source or reflected from a surface can provide comprehensive information about the spectral power distribution of that stimulus.
  • Spectral power distribution is usually measured with a spectroscope.
  • The spectral power distribution of a stimulus is not apparent to the human eye:
    • The human eye has three types of colour receptors, also known as cones.
    • These cones respond differently to various wavelengths of light, which allows us to perceive a wide range of colours.
    • Tristimulus values can be used to represent the response of these cones to different wavelengths of light.
    • By measuring tristimulus values, we can accurately describe the perceived colour of an object.
    • While tristimulus values are commonly used to quantify colour perception, they do not provide a complete characterization of the spectral power distribution of light.
  • Different stimuli can produce the same colour sensation for an observer. Visually indistinguishable colour stimuli that appear the same but have different spectral compositions are often described as being metameric.
  • Metamerism occurs because each type of cone responds to the distribution of energy across the entire spectral power distribution of a light source, and not just to the intensity of individual wavelengths.