The radiative transfer equation (RTE) well describes the photon propagation but its high computational cost is prohibitive for tomographic imaging. The diffusion approximation (DA) to RTE is the most popular but it only works well in weakly absorbing and highly scattering media, and breaks down near sources and across boundaries. In 2007, we derived the phase approximation (PA) model from RTE based on the generalized Delta-Eddington phase function. The generalized phase function is a linear combination of isotropic scattering and strongly peaked forward scattering with anisotropy weight as a free coefficient. PA is highly accurate over a broad range of coefficients with a computational complexity comparable to that of DA.