Improvement of Luster of Cotton

Abstract

The flattened, twisted-ribbon shape of the natural cotton fiber is unique among textile fibers, and leads to characteristic differences in the reflection of light from cotton, in comparison with that from more rounded fibers. With natural cotton, there are a large number of small planes set at every possible angle, so that light is not merely scattered, but also is in part reflected in a regular mirror-like way at every angle. However, small planes which are set at the more glancing angles reflect a larger proportion of the light which falls on them, in a manner described quantitatively by Fresnel's law. This explains the shift of the peak of reflection to angles higher than the mirror angle for the general surface, a shift which is greater for cotton than for most other fibers. If corrections based on Fresnel's law are applied, the peak of corrected reflectance is found to be at the mirror angle for cotton. The relations between luster and the balance between regular reflection and scattering are also shown in the higher luster of sheer fabrics and the decreased luster caused by increased scattering from deeper layers. Another demonstration of the favorable effect of decreased scattering on luster is shown by the increased luster of dark colors, where absorption prevents scattering and leaves the field dominated by surface reflection. In fabrics, there is scattering by the crossways system of yarns. If the Fresnel corrections are applied, the effect of the crossways yarns can be largely removed, and the effect of weaving crimp made clearer. Weaving crimp produces secondary or shifted peaks of reflection close to and on each side of the main peak, which correspond to the yarn rising up, going over the cross-yarn, and dipping down again.