begin{equation}label{key}
frac{partial u}{partial t}=-vcirc nabla u-u left(v left(mu _a+mu _sright)right)+int _{4 pi }text{v$mu $}_sp_s left(Omega 'rightarrow Omega right)uleft(r,Omega ',tright)dOmega '+q(r,Omega ,t)
end{equation}

Where,\
$u(r,Omega,textit{E},t)$      &=  &   angular photon density\
$t$             &       =   &   time(s)\
$mathbit{Omega}$  &=  &   direction vector of solid angle in steradian\
$mathbit{r}$       &=  &position vector\
$textbf{v}$        &=  &   speed vector of photon in the medium\
$textit{E}$        &=  &   Energy of the photon\
$mu_a$             &=  &   absorption coefficients mm^{-1}\
$mu_s$             &=  &   scattering coefficients mm^{-1}\
$p_sleft(mathrm{Omega}^primerightarrowmathrm{Omega}right)$  = is the normalized probability of inscattering events of photons from mathrm{Omega}^prime to mathrm{Omega}\
$qleft(mathbit{r},mathbit{Omega},tright)$  &= &photon source term including emission events\