Following from: Discrete ordinates and finite volume methods
Many practical problems involving combined heat transfer modes are transient, e.g., the boundary conditions may be time dependent. However, the timescales characteristic of convective and diffusive transport of mass, momentum, and energy are typically much larger than the timescale characteristic of radiation propagation. Photons travel at the speed of light, which implies that the characteristic timescale of radiative transport, defined as the ratio of a characteristic length to the speed of light, is very small. As a consequence, the transient term of the radiative transfer equation (RTE) may be neglected in most cases.
Recently, however, new technologies involving short-pulsed lasers have received significant development in a variety of applications such as optical tomography, biomedical diag ...