A sensor for an immersion system is disclosed. The sensor comprises: a sensing device, a transparent layer and an opaque patterning layer. The sensing device is configured to sense a property of a beam of radiation. The transparent layer is configured to allow the passage of a beam of radiation therethrough. The transparent layer covers the sensing device. The opaque patterning layer is configured to impart a pattern to the beam of radiation. In the patterning layer is an opening in which is located an infilling material. The infilling material is transparent to the beam of radiation and has a similar refractive index to that of the transparent layer.