A rotation detection apparatus comprising: a beam source for emitting a beam of polarised light; a detector (46A, 46B) for outputting a signal in dependence on the intensity of light which is incident thereupon, the detector (46A, 46B) being arranged so that the beam of polarised light is incident upon it; a polariser for location in the path of the beam of polarised light such that rotation of the polariser relative to the beam of polarised light results in a variation of the intensity of light incident upon the detector (46A, 46B) from the beam source and a corresponding variation in the signal output from the detector (46A, 46B); and a processor (48A, 48B, 48C) for detecting the relative rotation based on the variation of the signal output from the detector (46A, 46B); wherein the beam of polarised light comprises first and second beam portions having first and second different respective polarisation states; wherein the apparatus comprises an arrangement for providing, for each of the first and second beam portions, first and second different relative orientations between a polarisation axis of the polariser and a polarisation direction of the polarised light of that beam portion, thereby correspondingly providing first to fourth different output signals (A1, A2, B1, B2) from the detector (46A, 46B) relating respectively to: (a) the first relative orientation for the first beam portion; (b) the first relative orientation for the second beam portion; (c) the second relative orientation for the first beam portion; and (d) the second relative orientation for the second beam portion; wherein the processor (48A, 48B, 48C) is configured for detecting the relative rotation based on the first to fourth output signals (A1, A2, B1, B2) from the detector (46A, 46B); and wherein the processor (48A, 48B, 48C) is configured for deriving first and second values from the first to fourth output signals (A1, A2, B1, B2), and for detecting the relative rotation based on a difference between the first and second values, wherein the first and second values are based respectively on (i) a difference (A1 - A2) between the first and second output signals (A1, A2) and a difference (B1 - B2) between the third and fourth output signals (B1, B2); or (ii) a difference (A1 - B1) between the first and third output signals (A1, B1) and a difference (A2 - B2) between the second and fourth output signals (A2, B2). a beam source for emitting a beam of polarised light; a detector (46A, 46B) for outputting a signal in dependence on the intensity of light which is incident thereupon, the detector (46A, 46B) being arranged so that the beam of polarised light is incident upon it; a polariser for location in the path of the beam of polarised light such that rotation of the polariser relative to the beam of polarised light results in a variation of the intensity of light incident upon the detector (46A, 46B) from the beam source and a corresponding variation in the signal output from the detector (46A, 46B); and a processor (48A, 48B, 48C) for detecting the relative rotation based on the variation of the signal output from the detector (46A, 46B); wherein the beam of polarised light comprises first and second beam portions having first and second different respective polarisation states; wherein the apparatus comprises an arrangement for providing, for each of the first and second beam portions, first and second different relative orientations between a polarisation axis of the polariser and a polarisation direction of the polarised light of that beam portion, thereby correspondingly providing first to fourth different output signals (A1, A2, B1, B2) from the detector (46A, 46B) relating respectively to: (a) the first relative orientation for the first beam portion; (b) the first relative orientation for the second beam portion; (c) the second relative orientation for the first beam portion; and (d) the second relative orientation for the second beam portion; wherein the processor (48A, 48B, 48C) is configured for detecting the relative rotation based on the first to fourth output signals (A1, A2, B1, B2) from the detector (46A, 46B); and wherein the processor (48A, 48B, 48C) is configured for deriving first and second values from the first to fourth output signals (A1, A2, B1, B2), and for detecting the relative rotation based on a difference between the first and second values, wherein the first and second values are based respectively on (i) a difference (A1 - A2) between the first and second output signals (A1, A2) and a difference (B1 - B2) between the third and fourth output signals (B1, B2); or (ii) a difference (A1 - B1) between the first and third output signals (A1, B1) and a difference (A2 - B2) between the second and fourth output signals (A2, B2).