Abstract: The density of successive increments of a cigarette rod is measured by an apparatus which employs one or more photocells operating with ultraviolet, infrared or visible light. The radiation source of each photocell emits a beam of ultraviolet, infrared or visible light which penetrates through the wrapper and the filler of the rod and thereupon impinges upon one or more transducers which transmit signals denoting the density of the monitored portion of the rod to an evaluating circuit. The beams which are emitted by two or more discrete radiation sources are or can be angularly offset with reference to each other, and the evaluating circuit processes the signals from the photocells to generate a single signal which is indicative of the density of the monitored increment of the rod and is used to adjust the trimming device and/or the ejector for defective cigarettes.