An inspection device for a continuous cigarette rod comprises a plurality of infra-red emitter-detector units circumferentially spaced around the path of a rod. The light from each unit is focussed onto and collected from a specific area of the rod. Two, or more, axially displaced arrays of units are each arranged to inspect areas of the rod which are staggered in relation to the areas inspected by the other array or arrays.Signals from the units are multiplexed and transmitted to processing circuitry. Instantaneous signals are compared with predetermined values derived for each optical unit as a proportion of the running average of the instantaneous signal. If the instantaneous signal falls below this predetermined value a warning device is operated and the cigarette is ejected.Known devices use expensive optical fibres requiring a close fit to the cigarette rod.