Geometry and parameters used to calculate interferometric visibility of an extended source star lensed by a single lens. In both panels the Einstein ring is shown as the black dotted circle, and both axes are in θ_E units. u₁ ≥ 0 is the position of the centre of the source along the y_E-axis, u₊ and u₋ are the intersections of the black line (polar angle θ) with the contour of the source, and () and () are their respective images given by the single-lens equation. Arc-like images are shown in the left panel (0 < η₁ < 1, where η₁ = p/u₁), while the case of a single ring-like image (η₁ > 1) is shown on the right.