Geometry of lensing of a circumstellar disk. The dotted ring around the lens indicates the Einstein ring. The circumstellar disk is assumed to have an hole inside an inner radius and a cutoff outside an outer radius. Projected onto the sky plane, the disk appears to be an ellipse with the orientation angle of the major axis being φ₀. Any point on the disk is specified by its coordinates (r, φ) in a polar system centered at the star. The angles φ₀ and φ are with respect to the direction of the relative motion ﹩\boldsymbol{v}﹩. The position of the star (or the disk center) is specified by (R, Φ) with respect to the lens (Φ is relative to the direction of ﹩\boldsymbol{v}﹩). The impact parameter of the star is denoted as u₀.