A system comprising: a light-emitting device comprising: a curved optical element; a curved array of individually addressable light-emitting elements arranged to emit light towards the curved optical element, wherein a curvature of the curved array is substantially concentric to at least a portion of the circumference of the curved optical element, wherein the curved optical element is arranged to focus light emitted from each individually addressable light-emitting element to produce a substantially linear illumination pattern at a different corresponding scan angle within an angular range; and a control system operable to sequentially activate the individually addressable light-emitting elements such that the substantially linear illumination pattern sweeps out the angular range, characterized in that the system further comprises a light detector (420; 514), wherein the light detector is operable to detect light emitted from the light-emitting device over time while the substantially linear illumination pattern sweeps out the angular range, so as to generate a time-varying waveform of light intensity detected by the light detector; and a controller arranged to determine an angular position of the light detector relative to the light-emitting device based upon the time-varying waveform. a light-emitting device comprising: a curved optical element; a curved array of individually addressable light-emitting elements arranged to emit light towards the curved optical element, wherein a curvature of the curved array is substantially concentric to at least a portion of the circumference of the curved optical element, wherein the curved optical element is arranged to focus light emitted from each individually addressable light-emitting element to produce a substantially linear illumination pattern at a different corresponding scan angle within an angular range; and a control system operable to sequentially activate the individually addressable light-emitting elements such that the substantially linear illumination pattern sweeps out the angular range, a curved optical element; a curved array of individually addressable light-emitting elements arranged to emit light towards the curved optical element, wherein a curvature of the curved array is substantially concentric to at least a portion of the circumference of the curved optical element, wherein the curved optical element is arranged to focus light emitted from each individually addressable light-emitting element to produce a substantially linear illumination pattern at a different corresponding scan angle within an angular range; and a control system operable to sequentially activate the individually addressable light-emitting elements such that the substantially linear illumination pattern sweeps out the angular range, characterized in that the system further comprises a light detector (420; 514), wherein the light detector is operable to detect light emitted from the light-emitting device over time while the substantially linear illumination pattern sweeps out the angular range, so as to generate a time-varying waveform of light intensity detected by the light detector; and a controller arranged to determine an angular position of the light detector relative to the light-emitting device based upon the time-varying waveform.