Figure 3.

Geometry of the FRET pair. The donor fluorophore is located at distance R from the acceptor fluorophore. Donor and acceptor dipole orientations are described by the unit vectors $\widehat{d}$ and $\widehat{a}$, but in reality they are unit projective vectors (opposite directions are equivalent), which is reflected in all equations describing observable physical quantities. The donor plane is formed by the donor vector $\widehat{d}$ and the unit vector ${\widehat{r}}_{\text{da}}$ in the direction from the donor to acceptor, the angle ${\theta }_{\text{d}}$ is the angle between these two vectors. The acceptor plane and the angle ${\theta }_{\text{a}}$ are defined similarly. The angle ${\varphi }_{\text{da}}$ is the angle between the donor and acceptor planes. The angle ${\beta }_{\text{da}}$ is the angle between $\widehat{d}$ and $\widehat{a}$.