Criterion used to classify lensed images of extended sources. Six examples are shown. First column: caustics (red curves) and source position (blue circle). The radius of the circle corresponds to the radius at which the surface brightness is equal to 2σ the sky background rms fluctuation for a single pixel. In other words, the blue circle delimits the area of the source that can be detected. Second column: mock image of the lensed source, with added noise. Third column: footprint of the source. The purple footprint is obtained with a 2σ detection criterion. The coral region is the largest footprint with the highest number of detected images.