The invention relates to an optical assembly with a laterally graded reflective multilayer whose reflecting surface reflects incident X-rays under low incidence angles to produce a two-dimensional optical effect. The reflecting surface comprises a single surface conformed along two curvatures corresponding to two different directions. The invention also relates to a manufacturing method of such an optical assembly. The method includes coating a substrate already having a curvature. The invention also relates to a device for generating and conditioning X-rays for applications for angle-dispersive X-ray reflectometry. The device includes the optical assembly connected to an X-ray source so that X-rays emitted by the source are conditioned along two dimensions so as to adapt the beam emitted by the source to the sample, with the X-rays having different angles of incidence on the sample under consideration.