Abstract: The diffuser is made from N=2.sup.n light ducts divided into two sets of 1/2N ducts, with every other duct belonging to a different one of the sets. The two sets of ducts are helically wound in opposite directions but at the same helical pitch over a cylinder (1). Each duct in either set crosses over the 1/2N ducts in the other set, and couplers are provided at those cross-overs between any given duct and the 1/2N ducts of the other set having cross-over numbers 2.sup.q, where q=0, 1, 2, . . . , (n-1). The diffuser has stages each comprising 1/2N couplers, with each stage being disposed in a ring around the cylinder.