A method involving providing incident radiation of a first polarization state by an optical component into an interface of an object with an external environment, wherein a surface is provided adjacent the interface and separated by a gap from the interface, detecting, from incident radiation reflected from the interface and from the surface, radiation of a second different polarization state arising from the reflection of incident radiation of the first polarization at the interface as distinct from the radiation of the first polarization state in the reflected radiation, and producing a position signal representative of a relative position between the focus of the optical component and the object.