A reflective optical component is configured to reflect EUV radiation. The reflective optical component has a reflective layer with a bimetal cap layer of differing first and second metals selected to ensure that the outer surface of the cap layer is substantially unreactive or non-adsorptive to sulfur. The bimetal cap layer may be an alloy of the two metals or may consist of a base layer of the first metal deposited on the reflective layer and a surface layer of the second metal on the base layer. The interaction of the two metals may lead to modification of the bonding energy to the outer face of the cap layer of sulfur-containing molecules such as SO2 so that sulfur adsorption, which leads to loss of reflectivity, is reduced or eliminated.
