Abstract: The present invention relates to a method of cleaning and after treatment of optical surfaces in an irradiation unit, said irradiation unit comprising a radiation source (1, 31) emitting EUV-radiation and/or soft X-rays, a first volume (40) following said radiation source (1, 31) and containing first optical components (3, 33) with said optical surfaces, and a second volume (41) following said first volume (40) and containing second optical components (38). The method comprises at least one cleaning step in which a first gas or gas mixture is brought into contact with said optical surfaces, thereby forming volatile compounds with contaminations deposited on said optical surfaces, wherein said compounds are pumped out of the first volume (40) together with the first gas or gas mixture.