an optical vend sensing system for use in vending machines employs a row of light emitters on one side of a region through which a vend product passes during delivery and a row of light detectors on the opposite side . each detector is substantially aligned with a corresponding one of the emitters , and is capable of receiving light from any of more than one of the emitters when that emitter is activated . the emitters are individually activated in sequential , round - robin fashion and multiple detectors are monitored for each emitter when activated . interruption of detectible beams between an activated emitter and one of the multiple emitters monitored for the activated emitter . power applied to an activated emitter is calibrated in steps to a minimum level sufficient to ensure detection by all monitored detectors of light emitted by an activated emitter , plus a safety margin .