Photon diffusion

This article needs additional citations for verification. Please help by adding citations to reliable sources. Unsourced material may be challenged and removed. Find sources:  –  ·  ·  · scholar · JSTOR (December 2009) (Learn how and when to remove this template message)

Photon diffusion is a situation where photons 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. A large ensemble of such photons can be said to exhibit diffusion in the material, and can be described with a diffusion equation.

Astrophysics[]

In astrophysics, photon diffusion occurs inside a stellar atmosphere. To describe this phenomenon, one should develop the transfer equation in moments and use the Eddington approximation to radiative transfer (i.e. the diffusion approximation). In 3D the results are two equations for the photon energy flux:

${\displaystyle {\vec {F}}=-{\frac {c}{12\pi \sigma }}{\vec {\nabla }}U,}$

${\displaystyle {\vec {\nabla }}\cdot {\vec {F}}=0,}$

where ${\displaystyle \sigma }$ is the opacity. By substituting the first equation into the second, one obtains the diffusion equation for the photon energy density:

${\displaystyle \nabla ^{2}U-{\frac {1}{\sigma }}{\vec {\nabla }}U\cdot {\vec {\nabla }}\sigma =0.}$

Medical science[]

In medicine, 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. The first model on photon diffusion in biological media has been developed by Bonner et al.^[1] For more recent studies see Refs.^[2][3]

See also[]

• Diffuse reflection
• Diffusion damping
• Global dimming
• Medical optical imaging
• Optical tomography
• Radiative transfer equation and diffusion theory for photon transport in biological tissue

References[]

This physics-related article is a stub. You can help Wikipedia by .

• v
• t