Patent ID: 8675210

Claim:
A level sensor for measuring a position of a surface of a substrate comprising: a projection system; and a detector; wherein the projection system comprises: an emitter for emitting a radiation beam towards the substrate; a projection grating positioned in between the emitter and the substrate, said projection grating comprising a measurement grating and an aperture, such that the radiation beam from the emitter incident on the projection grating is divided by the projection grating into a measurement radiation beam associated with the measurement grating and a capture radiation beam associated with the aperture; wherein the detector comprises: a first and second measurement detector; a first and second capture detector; a detection grating; and an optical system with a first optical unit and a second optical unit; wherein the detection grating comprises a ruled grating with multiple rules, wherein each rule of the multiple rules of the ruled grating comprises a first measurement facet associated with the first measurement detector and a second measurement facet associated with the second measurement detector; wherein the first measurement facet is configured to refract radiation of the measurement radiation beam incident on said first measurement facet towards the first optical unit, wherein the first optical unit is configured to divert radiation refracted by the first measurement facet to the first measurement detector; wherein the second measurement facet is configured to refract radiation of the measurement radiation beam incident on said second measurement facet towards the second optical unit; wherein the second optical unit is configured to divert radiation refracted by the second measurement facet to the second measurement detector; wherein the detection grating comprises a first capture element having a first capture facet associated with the first capture detector and a second capture facet associated with the second capture detector, said second capture facet being adjacent to the first capture facet; wherein the first capture facet is configured to refract radiation of the capture radiation beam incident on said first capture facet towards the first optical unit, wherein the first optical unit is configured to divert radiation refracted by the first capture facet to the first capture detector; wherein the second capture facet is configured to refract radiation of the capture radiation beam incident on said second capture facet towards the second optical unit; and wherein the second optical unit is configured to divert radiation refracted by the second capture facet to the second capture detector.