Radiopacity is preferentially used to describe opacity of
contrast media, which can be passed through the bloodstream, the gastrointestinal tract, or into the cerebral spinal fluid and utilized to highlight CT scan or X-ray images. Radiopacity is one of the key considerations in the design of various devices such as guidewires or stents that are used during radiological
intervention. The radiopacity of a given endovascular device is important since it allows the device to be tracked during the interventional procedure.
The words "opacity" and "opaque" are often used as colloquial terms for objects or media with the properties described above. However, there is also a specific, quantitative definition of "opacity", used in astronomy, plasma physics, and other fields, given here.
In this use, "opacity" is another term for the
here
) at a particular frequency of electromagnetic radiation.
More specifically, if a beam of light with frequency travels through a medium with opacity and mass density , both constant, then the intensity will be reduced with distance x according to the formula
where
x is the distance the light has traveled through the medium
is the intensity of light remaining at distance x
is the initial intensity of light, at
For a given medium at a given frequency, the opacity has a numerical value that may range between 0 and infinity, with units of length2/mass.
Opacity in air pollution work refers to the percentage of light blocked instead of the attenuation coefficient (aka extinction coefficient) and varies from 0% light blocked to 100% light blocked:
Planck and Rosseland opacities
It is customary to define the average opacity, calculated using a certain weighting scheme. Planck opacity (also known as Planck-Mean-Absorption-Coefficient[1]) uses the normalized Planck black-body radiation energy density distribution, , as the weighting function, and averages directly:
where is the
Stefan–Boltzmann constant
.
Rosseland opacity (after
Planck distribution
, , as the weighting function, and averages ,
The photon mean free path is . The Rosseland opacity is derived in the diffusion approximation to the radiative transport equation. It is valid whenever the radiation field is isotropic over distances comparable to or less than a radiation mean free path, such as in local thermal equilibrium. In practice, the mean opacity for Thomson electron scattering is: