An apparatus for monitoring soiling of an optical element of a processing head of a machine for laser processing of a workpiece includes a first transparent optical element, positioned in a path of a laser beam, a second transparent optical element, positioned in the path of a laser beam and separated from the first transparent optical element by a gap, a first detector for detecting laser light scattered from the first transparent optical element, and a second detector for detecting laser light scattered within the gap between the first transparent optical element and the second transparent optical element.