Abstract: Optical measuring devices for textile yarns require a homogeneous measuring field in order not to interpret changes in position of the yarn as additional changes in the cross-section or diameter. To form a homogeneous measuring field, it is proposed to insert in each case one light guide between the light source and the measuring field or between the measuring field and the light receiver, the purpose of the said light guide being to achieve a homogeneous light distribution in a measuring field intercepted by the yarn by means of internal reflection of the light passing therethrough. Such light guides can be made of transparent plastic and have the advantageous property that they can be curved, folded or twisted without the light guided by them escaping through the surface. By polishing or mirror-coating the surface of the light guides, the dispersion can be further reduced.