Apparatus for tracking an object (12A, 12B, 12C) including radiator modules (16A, 16B, 16C), disposed in an array of known locations adjoining a region (13) in which the object moves; each module includes at least one emitter (44, 46, 48) which emits a respective color selected from among a first plurality of colors. The apparatus includes a controller (32), which drives the at least one emitter to emit during a respective time slot, selected from among a second plurality of time slots during which the modules may emit. The apparatus also includes a location unit (22), fixed to the object and including at least one camera (72), which captures a sequence of electronic images containing some of the locations of the modules. The apparatus further includes a processing unit (26), which processes the electronic images to determine, responsively to the colors emitted by the modules and the time slots in which the colors are emitted, a location of the object.