Feasibility of using Polarizing Filters to reduce Halation Effects during Image Acquisition in the Field

摘要

Machine vision is widely applied in automated inspection and monitoring systems. One of the most important components of machine vision systems is illumination. To remove and avoid halation on images, indirect lighting devices such as diffusers or domes with several lamps have been studied for post-harvest operations. However, it is difficult to control natural illumination in the field, because sunlight constitutes a point source, which causes shiny and shady parts on objects, and its intensity fluctuates with time. In addition, most parts of plants have a cuticular layer, causing halation effects, which reduce image quality. In this study, a tractor-mounted imaging system was developed to acquire high quality crop images. The system consisted of aframe box (1m X 1m X 1m), covered by a polarizing film (1m X 1m), 2 identical cameras in the box, and a PC with two image capture boards. All 4 sides of the frame box were covered with black curtains so that only polarized sunlight could enter. Polarizing filters were also fitted to the lens of one camera to compare 2 images from the cameras at a time. In experiments, images of coffee plants were acquired under varying sunlight conditions. From the results, it was observed that the polarizing filteringtechnique was capable not only reduce by 97% of halation on the surfaces of leaves but also of achieving high accuracy color representation of the leaves.