An optical assembly including a reflectance-based sensor emitting light into a detection plane and detecting reflections of the emitted light, reflected by an object located in the detection plane, a light redirector positioned away from the sensor redirecting light emitted by the sensor into one or more spatial planes parallel to the detection plane and, when the object is located in the one or more spatial planes, redirecting light reflected by the object into the detection plane, and a processor controlling light emitted by the sensor and receiving outputs from the sensor, and configured such that when an object passes through one or more of the spatial planes, the processor identifies both the spatial planes through which the object passes, and the location of the object within the spatial planes through which it passes, based on the received outputs and the position of the light redirector relative to the sensor.