Schematics of the three reference single-lens models detailed in the text. In all cases, the source centre lies on the y_E-axis, and the source surface is uniformly bright. Both axes are in θ_E units. The microlens is the black dot at the centre of the frame, and the Einstein ring is indicated by the dotted circle of radius unity. In the point-source approximation, the source star 𝒮₁ (in orange) is point-like and has two point-like images: the major image (in red) is located outside of the Einstein ring, while the minor image (in blue) is inside. In general, a source 𝒮₂ has a spatial extension, and its major and minor images and are elongated along the Einstein ring. When both the source radius and the lens-source separation are smaller than unity, as for source 𝒮₃, the two images take the form of two thin arcs, and . When the microlens lies inside the source, the two images merge into a single ring-like image (not shown here).