Abstract: A method of monitoring for contaminants in highly elongate textile product such as yarn includes diffusely applying light to the yarn and monitoring light reflected by the yarn. The yarn is drawn past a background therefor and light is diffusely applied to both the yarn and said background. The background is arranged so that the total amount of monitored light reflected from the yarn and the background is substantially independent of the dimensions and density of the yarn and of the distribution of constituents in the yarn. In this way, a prescribed change in the reflected light intensity indicates a selected contaminant or range of contaminants in the yarn. The background is conveniently provided by an elongate channel so as to be close behind and to each side of the yarn.