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.

Etymology

• 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

• 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