Photon diffusion
Encyclopedia
Photon diffusion is a situation where photon
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
. A large ensemble of such photons can be said to exhibit diffusion
in the material, and can be described with a diffusion equation.
, 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
is the opacity. By substituting the first equation into the second, one obtains the diffusion equation for the photon energy density:
, 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.
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s travel through a material without being absorbed, but rather undergoing repeated scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
events which change the direction of their path. The path of any given photon is then effectively a random walk
Random walk
A random walk, sometimes denoted RW, is a mathematical formalisation of a trajectory that consists of taking successive random steps. For example, the path traced by a molecule as it travels in a liquid or a gas, the search path of a foraging animal, the price of a fluctuating stock and the...
. A large ensemble of such photons can be said to exhibit diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
in the material, and can be described with a diffusion equation.
Astrophysics
In astrophysicsAstrophysics
Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...
, photon diffusion occurs inside a stellar atmosphere
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. It is divided into several regions of distinct character:...
. To describe this phenomenon, one should develop the transfer equation in moments and use the Eddington approximation to radiative transfer
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 transfer describes these interactions mathematically...
(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 medicineMedicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....
, 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
- Global dimmingGlobal dimmingGlobal dimming is the gradual reduction in the amount of global direct irradiance at the Earth's surface that was observed for several decades after the start of systematic measurements in the 1950s. The effect varies by location, but worldwide it has been estimated to be of the order of a 4%...
- Radiative transfer equation and diffusion theory for photon transport in biological tissueRadiative transfer equation and diffusion theory for photon transport in biological tissuePhoton transport in biological tissue can be equivalently modeled numerically with Monte Carlo simulations or analytically by the radiative transfer equation . However, the RTE is difficult to solve without introducing approximations. A common approximation summarized here is the diffusion...
- Diffusion dampingDiffusion dampingIn modern cosmological theory, diffusion damping, also called photon diffusion damping, is a physical process which reduced density inequalities in the early universe, making the universe itself and the cosmic microwave background radiation more uniform...
- Diffuse reflectionDiffuse reflectionDiffuse reflection is the reflection of light from a surface such that an incident ray is reflected at many angles rather than at just one angle as in the case of specular reflection...