Abstract: A sensor for non-intrusively measuring the volumetric flow rate of materials, such as cotton, in a confined air stream is disclosed. The sensor is mounted on a conduit through which an air entrained material such as cotton flows. The sensor has an emitter unit having a number of light sources such as an LED which emit light beams normal to the longitudinal axis of the conduit. The sensor also has a detector unit which detects the light beams from the light sources. The light beams are detected by light-to-frequency circuits which generate a signal indicative of the sensed light. The light beams are attenuated depending on the amount of material passing through the conduit. The generated signal is sent to a counter circuit. The counter outputs are coupled to a processor circuit which calculates the quantity of material flowing through the light beam as a function of the frequency signal and a baseline frequency signal indicative of the level of light detected when no material is flowing through the light beam.