Spectral purity of a radiation beam of a first wavelength may be improved by providing an optical element that includes a structure having at least first layer including a first material, which structure is configured to be substantially reflective for a radiation of the first wavelength and substantially transparent or absorptive for a radiation of a second wavelength, a second layer including a second material, the second layer being configured to be substantially reflective, absorptive or scattering for the radiation of the second wavelength, and vacuum between the first layer and the second layer, wherein the first layer is located upstream in the optical path of incoming radiation with respect to the second layer.