| abstract
(eng) |
The flop index is a widely used metric in the coatings industry that quantifies directional lightness variation between specular and off-specular viewing geometries within a fixed in-plane configuration. It describes the perceptual transition between the face color observed near the specular direction and the flop color seen at a glancing angle, characterizing the “color travel” typical of metallic and pearlescent coatings. We extend this concept to the domain of fabrics, introducing a quantitative method for evaluating their directional optical behavior. Directional effects in shiny polyester textiles arise from the combined influence of yarn geometry, reflective properties, and weave structure, which generate local shadowing, masking, and inter-fiber reflections. Unlike coatings, which are largely isotropic under in-plane rotation, woven materials exhibit strong azimuthal anisotropy, causing their appearance to vary significantly with orientation relative to illumination and view. In current practice, apparel designers assess directionality through visual rotation tests under controlled lighting, short drape-and-motion videos, or by using gloss meters and multi-angle color measurements sampled over azimuth. However, these methods yield fragmented indicators rather than a unified quantitative descriptor. We compute the classic flop index across a series of in-plane orientations using bidirectional reflectance distribution function (BRDF) data acquired with a goniometric setup. The resulting directional flop profiles reveal dominant anisotropy directions and relative strengths, capturing subtle variations in sheen and highlight travel caused by weave geometry. The same approach can be implemented using simplified multi-angle instruments outside laboratory conditions. This method establishes a direct link between fabric structure and perceived visual dynamics, offering an objective tool for comparing textiles and guiding digital appearance modeling and apparel design, where controlled directional highlights enhance visual expressiveness, paralleling the coatings industry’s use of the flop index to optimize color travel. |