Fiber-optic light paves way for more accurate inspection

Current line-scan cameras used in inspection lack ideal light sources to image three dimensional objects properly, and this is the problem scientists at Princeton Lightwave and OFS Labs sought to solve. "The new fiber-based light source combines all the ideal features necessary for accurate and efficient scanning -- uniform, intense illumination over a rectangular region, a directional beam that avoids wasting unused light by only illuminating the rectangle, and a 'cool' source that does not heat up the objects to be imaged,"​ the scientists say in a paper to be presented next week. Current inspection machines lack at least one of these features, they say. Such machines are used in the industry to automate quality and safety inspection of foods. The scientists studied a more accurate source -- a fiber-optics-based solution, which they describe in new research to be presented 29 March at the Optical Fiber Communication Conference in the US. "An efficient lighting arrangement should distribute significant power density over a rectangular region that is conjugate with the row of pixels in the line scan camera,"​ they wrote. "This requires a highly directional, distributed source." ​ In their design, a bright light source such as a laser sends light through an optical fiber. Along the length of the fiber is an ultraviolet-light-treated region called a "fiber grating"​. The grating deflects the light so that it exits perpendicularly to the length of the fiber as a long, expanding rectangle of light. This optical rectangle is then collimated by a cylindrical lens. The technique allows the rectangle to illuminate objects at various distances from the source, such as potatoes or other produce moving along a conveyor belt. The bright rectangle allows line scan cameras to sort products at higher speeds with improved accuracy, the scientists claimed in their paper. The fiber-based device can be customised for a specific inspection application within four to six weeks, and then manufactured for that application in 16 to 20 weeks, according to the researchers. The scientists say the light source could also improve the inspection of paper, currency, recyclables and other products. "We expect this approach will provide key improvments in the length, efficiency, brightness, thermal management, and lifetime of lighting solutions for line scan sensing applications spanning numerous economic sectors,"​ they stated. Current inspection machines used for foodstuffs often use line-scan cameras. These cameras sense the presence of moving objects by reading one row of pixels at uniform temporal increments using a light source, such as tungsten halogen lamps, arrays of light-emitting diodes, laser line generators, arrays of fiber tips, patterned lightguides, and side emitting fibers. "These approaches do not combine sufficiently intense uniform power density in a directional beam without thermal loading,"​ the scientists stated. The information about the object obtained from the light source is then processed by rapid electronic computers. The software detects whether there are any problems with the items and instruct mechanical actuators, such as air jets, to separate out the good from the bad. In addition to foodstuffs, many other objects are currently inspected in this manner paper, currency, and recyclables. OFS Labs is a US-based division of Furukawa Electric. Source: Paper: "FBG Based Distributed Lighting for Sensing Applications" by G.E. Carver, K.S. Feder, P.S. Westbrook.