Photometric cell of the type comprising two juxtaposed spherical half-mirrors M.sub.1 and M.sub.2 and a third spherical mirror M facing mirrors M.sub.1 and M.sub.2, the three mirrors having the same radius of curvature, while mirrors M.sub.1 and M.sub.2 have their centers of curvature C.sub.1 and C.sub.2 located on mirror M and slightly displaced with respect to one another and mirror M has its center of curvature C between the two mirrors M.sub.1 and M.sub.2, optical means being provided for introducing a measuring light beam into the cell through an entrance diaphragm E and for extracting it therefrom after multiple reflections on said mirrors through an exit diaphragm S, wherein the mirror M is truncated perpendicular to a line joining the centers C.sub.1 and C.sub.2, the truncated portion of the mirror being replaced by an optical system L operating by transmission, the entrance diaphragm E and exit diaphragm S being positioned level with said optical system L on a line perpendicular to lines C.sub.1 -C.sub.2, and wherein it also comprises a third half-mirror M.sub.3, the optical system L being such that the half-mirrors M.sub.1 and M.sub.2 are conjugate to one another, while the third half-mirror M.sub.3 has a center of curvature C.sub.3 located on the line joining the two diaphragms E and S conjugate to one another by the mirrors and the optical system L.