An integrating optical system having a chamber, the chamber having an aperture and at least one portion having a diffuse reflective material; a light source; and a diffuse transmissive baffle. The baffle is located in relation to the chamber such that it is also located in an optical path between the light source and a treatable target. A light-ray originating from the light source is diffusely transmitted from the diffuse transmissive baffle and impinges on an interior surface of the chamber before impinging on the treatable target.