\begin{align}L_o\left(\mathbf{x}, \omega_o\right) = L_e\left(\omega_o\right) + \int_{\Omega} {f_r \left(\omega_i\rightarrow \omega_o\right) \cdot L_i\left(\mathbf{x}\leftarrow\omega_i\right) \cdot \textbf{N}\cdot \omega_i \, d\omega_i}\end{align}