Figure 4.

Admissible area on the polar angle plane $\left({\theta }_{\text{d}},{\theta }_{\text{a}}\right)$ (black rectangle; symmetric ${\theta }_{\text{d}}\leftrightarrow {\theta }_{\text{a}}$) has inequality 17 satisfied. To calculate this area, we use $r_{\text{d}}=0.275$, $r_{\text{a}}=0.292$, and $r_{\text{FRET}}=-0.026$. Because radiative dipole moments are projective vectors, it is enough to consider only a quarter of the symmetric admissible area with $0\le {\theta }_{\text{d}}\le \frac{\pi }{2}$ and $0\le {\theta }_{\text{a}}\le \frac{\pi }{2}$.