Foreground-light contamination in mock observable lensed LRDs. The histogram shows the distribution of the flux ratio f/F, measured within a circular aperture of diameter 2 × PSF_FWHM, where f is the flux of a lensed image and F is the local flux contributed by the foreground lens. The solid histogram corresponds to the selected image, defined as the second-brightest image in doubles and the third-brightest image in quads, while the dashed histogram corresponds to the brightest lensed image. The image stamps show representative systems. In each pair, the left panel presents the mock observed image, including foreground-lens light and noise, whereas the right panel shows the corresponding noise-free, PSF-convolved lensed-source image with the foreground deflector omitted. The magnitude and effective radius of each deflector are listed in the lower-right corner of the corresponding right-hand panel. Blue crosses mark the centers of the foreground deflectors. Orange and white crosses indicate the brightest and selected lensed images, respectively, while the remaining lensed images are marked by green crosses. The definitions of f₁ and f₂ are given in Section 4.1.