Photon diffusion is a situation where
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they a ...
s travel through a material without being absorbed, but rather undergoing repeated
scattering events which change the direction of their path. The path of any given photon is then effectively a
random walk
In mathematics, a random walk is a random process that describes a path that consists of a succession of random steps on some mathematical space.
An elementary example of a random walk is the random walk on the integer number line \mathbb Z ...
. A large
ensemble
Ensemble may refer to:
Art
* Architectural ensemble
* Ensemble (album), ''Ensemble'' (album), Kendji Girac 2015 album
* Ensemble (band), a project of Olivier Alary
* Ensemble cast (drama, comedy)
* Ensemble (musical theatre), also known as the ...
of such photons can be said to exhibit
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...
in the material, and can be described with a
diffusion equation.
Astrophysics
In
astrophysics, photon diffusion occurs inside a
stellar atmosphere
The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.
Overview
The stellar atmosphere is divided into several regions of distinct character:
* The photosphere, whi ...
. To describe this phenomenon, one should develop the transfer equation in moments and use the Eddington approximation to
radiative transfer
Radiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission, and scattering processes. The equation of radiative trans ...
(i.e. the diffusion approximation). In 3D the results are two equations for the photon energy flux:
:
:
where
is the opacity. By substituting the first equation into the second, one obtains the diffusion equation for the photon energy density:
:
Medical science
In
medicine
Medicine is the science and practice of caring for a patient, managing the diagnosis, prognosis, prevention, treatment, palliation of their injury or disease, and promoting their health. Medicine encompasses a variety of health care pr ...
, the diffusion of photons can be used to create images of the body (mainly brain and breast) and has contributed much to the advance of certain fields of research, such as neuroscience. This technique is known as
diffuse optical imaging.
See also
*
Diffuse reflection
Diffuse reflection is the reflection of light or other waves or particles from a surface such that a ray incident on the surface is scattered at many angles rather than at just one angle as in the case of specular reflection. An ''ideal'' di ...
*
Diffusion damping
*
Global dimming
Global dimming is the reduction in the amount of global direct irradiance at the Earth's surface that has been observed since systematic measurements began in the 1950s. The effect varies by location, but worldwide it has been estimated to be of ...
*
Medical optical imaging
*
Optical tomography
Optical tomography is a form of computed tomography that creates a digital data, digital volumetric model of an object by reconstructing images made from light transmitted and scattered through an object. Optical tomography is used mostly in medi ...
*
References
Diffusion
Photonics
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