Geometry of major (red) and minor (blue) images when the images are arc-shaped (i.e. for 0 < η₁ < 1, where η₁ = ρ/u₁). The Einstein ring is shown as the black dotted circle, and both axes are in θ_E units. When the arcs are thin, the images thickness δ is not resolved by the interferometer and two sources 𝒮₁ and 𝒮₂ (in orange) of same ratio η₁ produce arc-shaped images of equal angular elongation, φ = 2 arcsin η₁. The projected extension of the arcs onto the x_E-axis is θ_a = η₁θ_E, is hence comparable to θ_E, and is thus resolved by the interferometer. When η₁ > 1, the image of the source is a ring (and φ = π).