A direct-light generator includes an optical unit positioned downstream of an output surface of a collimated light source and upstream from a first emitting surface with respect to a direct light direction. The optical unit includes a first planar light mixing element characterized by a first response function having a first angular profile with a peak having a first divergence angle, and a second planar light mixing element characterized by a second response function having a second angular profile with a peak having a second divergence angle. The first planar light mixing element at least partially intercepts collimated light exiting the output surface of the collimated light source and defines a unit input surface. The second planar light mixing element is positioned downstream of the unit input surface to at least partially intercept light crossing the unit input surface and to define a unit emitting surface.