A light emitter/sensor includes a lamp, a light sensor, a light guide, and a light controller. The light guide has a pair of opposite ends and is optically-coupled to the lamp and the light sensor at one of the ends. The light guide is configured to convey light emitted from the lamp to the other end of the light guide, and from that other end to the light sensor. The light controller is coupled to the lamp and the light sensor and is configured to flash the lamp and to determine the intensity of light at that other end of the light guide, via the light guide and the light sensor, during an interval between the flashes.