A lithographic apparatus includes an illumination system configured to transmit a beam of radiation, the beam of radiation comprising desired radiation having a predetermined wavelength or a predetermined wavelength range, and undesired radiation having another wavelength or another wavelength range; a support structure configured to support a patterning structure, the patterning structure being configured to impart the beam of radiation with a pattern in its cross-section; a substrate table configured to hold a substrate; and a projection system configured to project the patterned beam of radiation onto a target portion of the substrate; wherein at least part of the lithographic apparatus, in use, includes a gas substantially transmissive for at least part of the desired radiation and substantially less transmissive for at least part of the undesired radiation.