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Article: Opacity (optics)

Opacity (optics)

Opacity Definition and Characteristics

  • Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light.
  • It describes the absorption and scattering of radiation in a medium.
  • An opaque object is neither transparent nor translucent.
  • Light striking an interface between substances can be reflected, absorbed, scattered, or transmitted.
  • Opacity depends on the frequency of the light being considered.


  • The word 'opacity' originated from the Late Middle English term 'opake.'
  • The current spelling was influenced by the French form.
  • The spelling became more common in the 19th century.


  • Radiopacity is used to describe the opacity of X-rays.
  • Radiodense substances do not allow X-rays or similar radiation to pass through.
  • Radiodense contrast media revolutionized radiographic imaging.
  • Radiopacity is a key consideration in the design of medical devices used during radiological intervention.
  • It allows the tracking of devices during procedures.

Quantitative Definition

  • Opacity has a specific, quantitative definition used in astronomy, plasma physics, and other fields.
  • It is synonymous with the mass attenuation coefficient or mass absorption coefficient.
  • Opacity is measured at a particular frequency of electromagnetic radiation.
  • In air pollution work, opacity refers to the percentage of light blocked.
  • Opacity can vary from 0% light blocked to 100% light blocked.

Planck and Rosseland Opacities

  • Planck opacity uses the normalized Planck black-body radiation energy density distribution as the weighting function.
  • Rosseland opacity uses the temperature derivative of the Planck distribution as the weighting function.
  • Photon mean free path is the inverse of the Rosseland opacity.
  • The Rosseland opacity is derived in the diffusion approximation to the radiative transport equation.
  • The mean opacity for Thomson electron scattering and nonrelativistic thermal bremsstrahlung can be calculated.

Opacity (optics) Data Sources

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