Methods and devices for reducing the dimensions of an incident light beam of large dimensions are disclosed. The method includes the dispatching of a first light beam toward a partially reflecting plate of dimensions suitable for the dimensions of the light beam of large dimensions, the dispatching onto a convergent reflective element of a second light beam arising from the transmission through the partially reflecting plate of the first light beam, the dispatching of a third light beam arising from the reflection on the convergent reflective element of the second light beam, toward said partially reflecting plate, and the reflection of the third beam on the partially reflecting plate so as to form a fourth light beam.