A device and corresponding module are for a signaling light and/or lighting an exterior or interior ambient of a motor-vehicle passenger compartment including a type of light source. The device comprises an optical assembly of a first reflector including a mirror and a second reflector including a semi-transparent mirror. Light emitted by an LED is diffused in a curtain diffuser of a transparent material. Planes or mid-planes of the mirror, the semi-transparent mirror, and the diffuser are mutually parallel. Light transmitted or reflected by the assembly is transmitted and visible to an observer from a surface of the second reflector that is opposite to that facing other elements of the assembly. The diffuser includes a lighting sub-assembly arranged in space between the mirror and semi-transparent mirror and having a transparent optical material and a light source arranged for emission of light rays propagated within a thickness of the optical material.