An illumination apparatus is disclosed in the invention. The illumination apparatus includes a cavity with a diffusion surface, a light source, a light-spreading device, and at least one optically-conditioning surface with a wavelike array formed thereon. The light-spreading device and the optically-conditioning surface spread the light generated by the light source. The light-spreading device includes a wing-shaped protrusion part, a light incident surface, a recess located away from the light incident surface, and an optically-conditioning surface including a wavelike array, wherein the wavelike array has a wavefront direction.