Abstract: For the detection of contaminants, a yarn or a thread is led through a zone of measurement, which is formed as a flat slit in an optically transparent body. This transparent body has outer surfaces for diffusely or non-diffusely reflecting light back into the body. Light is led into the body from several sources of light, such that the yarn is diffusely illuminated from all sides. The sources of light are arranged and adjusted such that the brightness of the yarn detected by a sensor is equal to the brightness of the yarn's background field. In this way, the measurement becomes independent of the yarn's thickness. Due to the use of a flat slit for receiving the yarn, the yarn can be inserted into the zone of measurement by the same procedures as they are used in conventional yarn cleaners.