Laser imaging method for fast detecting white foreign fibers in cotton

Abstract: Foreign fibers, such as polypropylene baler twine, packaging materials, ropes, small pieces of cloths, nylon, and other different synthetic fibers, are almost invariably present in cotton during the cotton picking, storing, drying, and transporting. Their presence in textile processing may cause a disastrous quality problem in the textile industry because their physical and chemical properties are different from cotton. In recent years, machine vision systems have been used for sorting out the foreign fibers from cotton, however, all of the sorting machines cannot efficiently identify white foreign fibers in cotton, because their color is the same or very similar under the illumination of visible light. The laser imaging method previously suggested by our team can distinguish most of the white foreign fibers from cotton based on the difference of the microstructure of the sample surface, but the successful detection rate was not high and the speed of the detection was slow. Because the method was based on the calculation of the target size and distribution, or in the image acquisition, the laser light must project from some special angles of the sample surface for the camera can obtain the reflected light from the white foreign fibers. In this study, the image contrast of the white foreign fibers and the cotton background was first quantized by the calculation of the average pixel value in a frame of the laser image. Then, under the illumination of a line laser at a fixed power and wavelength, the images of 12 kinds of typical white foreign fibers on the cotton surface were obtained by a camera with a fixed aperture with different lengths of exposure time. Finally, the relationship between the lengths of different imaging exposure time and the image contrast was analyzed. It was found that there was a shared wave crest in all of the relationship curves and imaging in the exposure time of the wave crest in the same frame image. The gray level of the white foreign fibers had reached a saturated status, but the gray level of cotton was still unsaturated. Thus, using an optimized exposure time, the difference of the gray level of the laser images can be used for the detection of the white foreign fibers in cotton, including white paper, white plastic film, white semitransparent plastic mulch, white cloth, white density foam plastic, white nylon cord, white cotton string, white plastic cord, white plastic cardboard, white semitransparent polypropylene bags, white polyethylene foamed sheets, and white feathers. The experimental results indicated that, by a simple binary segmentation algorithm, using the images obtained under the illumination of the line laser at the wavelength of 650 nm, a power of 0.8 W, and by the camera aperture of 8C with an exposure time of 1.6 ms, the 12 kinds of white foreign fibers can be easily distinguished from cotton. In Matlab, the processing time of a frame image was less than 3 ms, and the successful detecting rate was up to 95.8 percent. In conclusion, a fast imaging method based on line laser and optimized exposing time was presented in the study, which can be used for on-line detection of white foreign fibers in cotton.