Photon diffusion is a situation where photons travel through a material with a high optical depth and very short mean free path. Their behavior is then dominated by scattering and the path of any given photon is effectively a random walk. A large ensemble of such photons can be said to exhibit diffusion in the material, described accurately 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:
where σ 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, the diffusion of photons can be used to create images of the body (mainly brain and breast) and has been contributing a lot to the advance of certain fields of research, such as neuroscience. This technique is known as diffuse optical imaging.
See also
- Global dimming
- Radiative transfer equation and diffusion theory for photon transport in biological tissue
- Diffusion damping
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