2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/231112New ray-optical elements allow generalized refraction of light rays, but geometry imposes limitations on possible mappings between the positions of an object and its geometric image. Here I study the case of an infinite, planar, non-absorbing sheet that images the entire three-dimensional space. The most general case of such a sheet is equivalent to a thin lens with different object- and image-sided focal lengths. Special cases include ordinary thin lenses, confocal lenslet arrays, and negative refraction with n_2 = -n_1.10 pages, 1 figureOpticstext