An illumination device includes a raidation source; an optical integrator having an array of lenses disposed along a plane perpendicular to an optical axis of the device; a first optical system for amplitude-dividing a coherent beam from the radiation source and producing plural beams which are substantially incoherent with each other, the first optical system also being effective to project the beams to the optical integrator in different directions and to superpose the beams upon one another on the optical integrator; and a second optical system for directing beams from the lenses of the optical integrator to a surface to be illuminated and for superposing the beams upon one another on the surface to be illuminated.