The density of a tobacco stream which is advanced by a foraminous conveyor toward the surplus removing station of a cigarette rod making machine is measured by one or more detectors each having one or more sources of infrared light and one or more photoelectric transducers. In order to avoid the generation of distorted signals in response to impingement of high-intensity radiation upon the transducer or transducers, each transducer is out of line with the path of direct propagation of radiation from the respective source or sources toward successive increments of the tobacco stream. The transducer or transducers receive infrared light which is scattered in and/or reflected by tobacco particles in the stream. Signals from the transducer or transducers are used to regulate the quantity of surplus which is removed from the stream.