Dichromatic Fabrics – A Challenge in Colourimetry

When a textile is woven, different colour threads may be used for the warp (vertical) and the weft (horizontal). The primary colour is seen on viewing the textile orthogonally (at right angles) to a flat surface. But a secondary colour may be seen if the textile is tilted or rotated (dichromatism). The colours of the main threads in six dichromatic textiles were measured by microscopy using a rotating stage. Colours did not change with rotation, although to an observer to one side of the microscope the whole sample appeared to change colour on rotation. Several methods of illumination were used to show that a wide angle of illumination and recording was required to produce dichromatism. Relationships between microcolourimetry and macrocolorimetry then became detectable. Chromaticity x in a fibre was correlated (r = 0.85, P < 0.025) with chromaticity x of primary macroscopic reflectance as well as with the chromaticity x of secondary macroscopic reflectance (r = 0.86, P< 0.025).