Before cotton fiber can be spun, the raw cotton must be sorted to remove any foreign particles and fibers. While foreign particles can be clearly distinguished from the raw cotton by color, contrast and structure, foreign fibers such as polypropylene or polyethylene films are often light and transparent, making them difficult to detect using conventional foreign particle separators. To detect these particles and separate them safely, the raw cotton is illuminated underpolarized light, making the foreign plastic fibers appear colored. In this way, they can be distinguished from the raw cotton and separated.
Althoughcolor linescan cameras with trilinear sensors are often used in such machine-vision applications, trilinear sensors scan RGB images in different color planes. This can result in image artifacts appearing when such sensors are used in high-speed imaging applications. For such applications, a trisensor camera system with a color beamsplitter is the only option to ensure the highest possible color registration. Because of this, Trützschler (Mönchengladbach, Germany; www.truetzschler.de), a manufacturer of fiber preparation and processing systems for the textile and nonwovens industries, called on Linos (Goettingen, Germany; www.linos.com) to develop a custom prism camera for its SECUROPROP SP-FP foreign particle separator.
To correct for the aberrations of the triple-channel prism used in the camera, Linos developed a specialized lens design. Lateral chromatic aberration was minimized by aligning within less than 2 microns. Another central aspect is appropriatecolor splitting in the triple prism by dielectric color-splitting coatings. To produce high-fidelity color images, two coatings were used in the beamsplitter. While the first coating reflects blue and lets red and green pass, the second reflects red and lets green pass.
