An illumination system for machine vision inspection of agricultural products

An effect of PL filter on imaging that eliminates halation on object surface is well known. However, it has not been easy to use the filter in front of halogen, incandescent, or HID lamps which radiate a lot of heat, because PL film is melted with a temperature of about 60 degree centigrade. Although fluorescence lamps or LEDs can be used with PL filters, their intensity and color rendering are inferior to them. Diffusers or domes with reflection plates are often used when halogen lamps are used, but unavoidable halation and surroundings reflection occur on glossy surface. In this presentation, a direct lighting device (called DL) developed for fruit grading systems is introduced. DL mainly consisted of a PL filter, two heat absorption filters, a halogen lamp, and a fan. Since the absorption wavelength bands of the two filters were different, major heat did not reach the PL filter. In addition, cold air was introduced to the PL filter by its fan. Most parts of plants have cuticular layers on their surfaces to keep moisture within themselves. In this study, three categories are created for objects corresponding to degree of gloss on surfaces and surface materials: category 1 (apples, tomato, eggplants, bell peppers), category 2 (pear, kiwi, peach), and category 3 (potatoes, radish). A color CCD camera was set at the top of object with four DLs and images were acquired. To compare the PL filtering images, a dome and diffusers were also used for the image acquisition. From experiments through three types of illumination devices, it was observed that PL filtering images not only could eliminate halation on products of the category 1 but also could express true colors of the products. Microscopic images showed proportional relation between thickness of cuticular layers and halation on surfaces.