A utility meter comprising: a detector for detecting interruption of a light path, the detector comprising: a plurality of light emitters; a plurality of light receivers; and a light guide comprising a plurality of portions (8, 8') each having an entry surface shaped to collimate light entering the light guide in an entry direction along an optical axis, a plurality of planar reflective surfaces (18, 20) arranged to reflect light inside the light guide along the optical axis to exit the light guide at an exit surface in an exit direction, the exit direction having a component in a direction opposite the entry direction, wherein each light guide portion (8, 8') comprises two limbs (10, 12) extending from either end of a back portion, free ends of the limbs (10, 12) being respectively shaped to provide the entry and exit surfaces, wherein the ends of the back portion are shaped to each provide a respective reflective portion, and wherein one of the limbs (10, 12) is shorter than the other one, thereby providing a space for accepting a moveable part capable of intersecting the optical axis, wherein each portion (8, 8') is disposed opposite a respective light emitter and receiver to define the space between the light guide and at least one of its respective light emitter and receiver, wherein a light path between each light emitter and receiver can be interrupted by the moveable part in the space, the moveable part being coupled to a consumption sensor such that movement of the moveable part is indicative of a quantity sensed by the consumption sensor. a detector for detecting interruption of a light path, the detector comprising: a plurality of light emitters; a plurality of light receivers; and a light guide comprising a plurality of portions (8, 8') each having an entry surface shaped to collimate light entering the light guide in an entry direction along an optical axis, a plurality of planar reflective surfaces (18, 20) arranged to reflect light inside the light guide along the optical axis to exit the light guide at an exit surface in an exit direction, the exit direction having a component in a direction opposite the entry direction, wherein each light guide portion (8, 8') comprises two limbs (10, 12) extending from either end of a back portion, free ends of the limbs (10, 12) being respectively shaped to provide the entry and exit surfaces, wherein the ends of the back portion are shaped to each provide a respective reflective portion, and wherein one of the limbs (10, 12) is shorter than the other one, thereby providing a space for accepting a moveable part capable of intersecting the optical axis, a plurality of light emitters; a plurality of light receivers; and a light guide comprising a plurality of portions (8, 8') each having an entry surface shaped to collimate light entering the light guide in an entry direction along an optical axis, a plurality of planar reflective surfaces (18, 20) arranged to reflect light inside the light guide along the optical axis to exit the light guide at an exit surface in an exit direction, the exit direction having a component in a direction opposite the entry direction, wherein each light guide portion (8, 8') comprises two limbs (10, 12) extending from either end of a back portion, free ends of the limbs (10, 12) being respectively shaped to provide the entry and exit surfaces, wherein the ends of the back portion are shaped to each provide a respective reflective portion, and wherein one of the limbs (10, 12) is shorter than the other one, thereby providing a space for accepting a moveable part capable of intersecting the optical axis, wherein each portion (8, 8') is disposed opposite a respective light emitter and receiver to define the space between the light guide and at least one of its respective light emitter and receiver, wherein a light path between each light emitter and receiver can be interrupted by the moveable part in the space, the moveable part being coupled to a consumption sensor such that movement of the moveable part is indicative of a quantity sensed by the consumption sensor.