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This paper describes various illumination and image processing techniques for yarn characterization. Darkfield and back-lit illuminations are compared in terms of depth of field tolerance and image quality. Experiments show that back-lit illumination is superior in terms of depth of field tolerance and contrast. Three different back-lit illumination configurations are studied: one simply employing a light source placed behind the yarn, the other incorporating a field lens to increase the light intensity passing through the aperture, and the third using a mirror placed at 45° to the optical axis to enable imaging of two orthogonal views of the yarn core. Problems in defining the hair–core boundaries in high resolution yarn pictures are addressed and a filtering process is introduced for back-lit images. A comparison of the diameter and diameter coefficient of variation percentage measurements for different illumination and image …