In the dyeing process of textiles, for example, they continuously detect the water colour, which allows conclusions to be drawn about the existing colour concentration. For this application, the CFO100 controller from Micro-Epsilon is used together with the CFS3-A30 transmission sensor. During the measurement, the water is pumped via a side arm through a glass tube, which is illuminated from the back with the transmission fibres of the CFS3 sensor. The colour turbidity is evaluated and classified in the controller. The colour value determined is then output directly to the process control. In this application, the measurement can be carried out both from the outside of the pipe and directly in the water.