**Definition:**A set of simplifying physical assumptions that enable one to derive the Diffusion Equation from the Radiative Transport Equation. The diffusion approximation makes several assumptions about photon transport in tissue: (1) the scattering coefficient is much higher than the absorption coefficient; (2) the fluence rate is significantly larger than the photon flux; (3) the light source is isotropic; (4) the scattering angle of a scattering event in tissue does not depend on the incident angle, and; (5) the rate of change of the photon flux is very slow, which requires that the time scale of source modulation is much longer than the time between photon scattering events. As a result of the diffusion approximation, the modeling of light transport in highly scattering media is drastically simplified, which allows the derivation of analytical solutions to the problem and, therefore, a valuable and powerful tool for analyzing light distribution in tissues. Most tissues satisfy these assumptions, although even in these cases some of the assumptions may fail either too close to boundaries or too near sources. Definition: A set of simplifying physical assumptions that enable one to derive the Diffusion Equation from the Radiative Transport Equation. The diffusion approximation makes several assumptions about photon transport in tissue: (1) the scattering coefficient is much higher than the absorption coefficient; (2) the fluence rate is significantly larger than the photon flux; (3) the light source is isotropic; (4) the scattering angle of a scattering event in tissue does not depend on the incident angle, and; (5) the rate of change of the photon flux is very slow, which requires that the time scale of source modulation is much longer than the time between photon scattering events. As a result of the diffusion approximation, the modeling of light transport in highly scattering media is drastically simplified, which allows the derivation of analytical solutions to the problem and, therefore, a valuable and powerful tool for analyzing light distribution in tissues. Most tissues satisfy these assumptions, although even in these cases some of the assumptions may fail either too close to boundaries or too near sources.

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**Related terms:**Diffusion Equation, Radiative Transport Equation, photon fluence, photon flux