${\Omega }_{e}$ is the equivalent angle which is calculated from both sides of the half-values of the angles, ${\vartheta }_{M}$ and ${\phi }_{M}$ of the two planes.
(1) ${\Omega }_{e}={\phi }_{M}\cdot {\vartheta }_{M}$
(2) $D=\frac{4\pi }{{\Omega }_{e}}$