A method of cleaning an ion source. The method includes: at a first reflective surface of a mirror, reflecting light that has a wavelength in a first wavelength band onto a surface of the ion source so that contaminant material is desorbed from the surface of the ion source; at a second reflective surface of the mirror, reflecting light that has a wavelength in a second wavelength band and that comes from the surface of the ion source towards an imaging apparatus for producing an image of the surface of the ion source, wherein the light that has a wavelength in the second wavelength band passes through the first reflective surface of the mirror before being reflected at the second reflective surface of the mirror.