Geometry of a flat accretion disk ring. An element of the flat equatorial disk, defined by the unit radius-vector, $\widehat{\mathit{r}\mathbf{,}}$ at azimuthal angle, φ, moves along the direction, $\widehat{\mathit{v}}$. The coordinate system is based on the normal of the disk, $\widehat{\mathit{n}},$ and the observer vector, $\widehat{\mathit{o}}$. A photon is emitted from the disk element along ${\widehat{\mathit{k}}}_0$. The important angles used in this work are also shown.