Abstract: The present invention relates to a method and apparatus for measuring the content of suspended substances in a flowing medium by illuminating the medium with light from a light source. Reflected light from the medium is detected by at least two detectors, at least one of which measures directly reflected light while the other detectors measure multiply reflected light. The detector for detecting directly reflected light is located such that a linear relationship is established between its output signal and the concentration of the flowing medium. The other detectors are located such that their output signals will be essentially constant in relation to the concentration of particles in the flowing medium within a large range. The measuring scheme provides for obtaining the quotient of the signal provided by direct reflection divided by the signal provided by multiple reflection.