In an apparatus at a spinning preparation machine, for example a cleaner, opener, carding machine or the like, for detecting waste which is separated out from fibre material, for example cotton, and consists of foreign matter and good fibres and which is collected in a collecting device, there is provided an optical measuring device having a brightness sensor, which measuring device examines the waste.In order to make it possible, by simple means, for the content of good fibres in the waste to be detected and to allow optimum adjustment of the composition of the waste, especially with a high content of trash and low content of good fibres, the waste material is moved past at least one sensor arrangement responding to good fibres, and the sensor arrangement comprises a light source, the light reflected by the moving good fibres being detected by the brightness sensor and being converted into electrical signals, from which the good fibre content can be determined.