A lithography method for use in fabricating integrated circuits on a substrate, the method comprising: supporting the substrate on a moveable stage; imaging spatially patterned radiation onto the substrate; adjusting the position of the stage; using a method for determining information about changes along a degree of freedom of an encoder scale to monitor the position of the stage, the method comprising: monitoring the position of a first component using a method for determining information about changes along a degree of freedom of the encoder scale, the method comprising: directing a first beam (112, 1854) and a second beam along different paths and combining the first and second beams to form an output beam, where the first and second beams are derived from a common source using a non-polarizing beam splitter (212, 240) such that the first and second beams contain both s- and p-polarized light, the first and second beams have different frequencies, where the first beam contacts the encoder scale at a non-Littrow angle and the first beam diffracts from the encoder scale at least once; detecting an interference signal based on the output beam, the interference signal comprising a heterodyne phase related to an optical path difference between the first beam (112, 1854) and the second beam; and determining information about a degree of freedom of the encoder scale based on the heterodyne phase, wherein the encoder scale is attached to the stage and the information corresponds to the position of the stage along an axis (x, y). supporting the substrate on a moveable stage; imaging spatially patterned radiation onto the substrate; adjusting the position of the stage; using a method for determining information about changes along a degree of freedom of an encoder scale to monitor the position of the stage, the method comprising: monitoring the position of a first component using a method for determining information about changes along a degree of freedom of the encoder scale, the method comprising: directing a first beam (112, 1854) and a second beam along different paths and combining the first and second beams to form an output beam, where the first and second beams are derived from a common source using a non-polarizing beam splitter (212, 240) such that the first and second beams contain both s- and p-polarized light, the first and second beams have different frequencies, where the first beam contacts the encoder scale at a non-Littrow angle and the first beam diffracts from the encoder scale at least once; detecting an interference signal based on the output beam, the interference signal comprising a heterodyne phase related to an optical path difference between the first beam (112, 1854) and the second beam; and determining information about a degree of freedom of the encoder scale based on the heterodyne phase, monitoring the position of a first component using a method for determining information about changes along a degree of freedom of the encoder scale, the method comprising: directing a first beam (112, 1854) and a second beam along different paths and combining the first and second beams to form an output beam, where the first and second beams are derived from a common source using a non-polarizing beam splitter (212, 240) such that the first and second beams contain both s- and p-polarized light, the first and second beams have different frequencies, where the first beam contacts the encoder scale at a non-Littrow angle and the first beam diffracts from the encoder scale at least once; detecting an interference signal based on the output beam, the interference signal comprising a heterodyne phase related to an optical path difference between the first beam (112, 1854) and the second beam; and determining information about a degree of freedom of the encoder scale based on the heterodyne phase, wherein the encoder scale is attached to the stage and the information corresponds to the position of the stage along an axis (x, y).