Fiber surface characterization by laser back scattering

Fiber surface structure is important because it affects the behavior of fibers, yarns and fabrics during their processing as well as during their use in service. For example, the geometric roughness of a fiber's surface greatly affects frictional characteristics and luster of the fiber or yarns and fabrics made from the fiber.

Since the advent of lasers, scattering phenomena have received considerable attention both theoretically and experimentally. According to the theory of Fourier optics, the far-field diffraction pattern is the Fourier transform of the diffracting object. This can be postulated to back scattering also. Therefore, one ought to be able to obtain information about the surface structure of materials from their laser back scattering patterns. The application of laser back scattering to fiber surface roughness characterization was studied in this thesis from both theoretical and experimental approaches. This study reveals relationships between back scattering patterns and fiber surface roughness.

In order to quantify experimentally measured roughness, a parameter called the "roughness coefficient" was defined as the ratio of scattered intensities at non zero spatial frequencies to the intensity at zero spatial frequency. A computer simulation study was employed to investigate the theoretical influence of rootmean-square fiber surface roughness, fiber cross-sectional shapes, fiber diameters and surface asperity shapes on back scattering patterns.

Two instruments were employed to measure fiber surface roughness based on optical Fourier transforms. The first one used a vidicon-camera to capture the scattering pattern and provide input to a micro-computer for data analysis. The second simpler instrument consisting of a few of photo-detectors were used to reduce cost. Several applications of laser back scattering have been examined, such as measuring the effect of "delustering" fibers with T_iO₂ and detecting surface crystallization. The optical transform method discussed in this study seems to have potential as a simple means of providing nonintrusive, real- time measurements of fiber surface structure during processing or after use in service.