Design and implementation of an illumination device for optical inspection of defects in glass substrates

A compact and cost-effective illumination platform was developed for a versatile optical inspection system to improve the detection accuracy of defects in glass substrates. The illumination device was developed in two phases, initially to demonstrate its feasibility for surface defect inspection in glass based on dark field images, and subsequently to optimize the design so it can provide multi-directional lighting and increase light scattering from defects on the substrate. Three LED arrays were installed above the substrate carrier and projected at an angle onto the glass substrate for the phase-I illumination device. Surface defects on the glass substrate were successfully reconstructed from images acquired by a line scanned CCD camera, but non-uniformity of defects intensity distribution on images was revealed. To optimize the illumination, two sets of tightly arrayed 3-watt LEDs were symmetrically installed at the entrance slit of the lens-camera module for phase-II illumination device. The inspection data were able to show clearer images of surface defects. The design issues such as poor contrast and sharpness of acquired images due to low scattering efficiency and non-uniform illumination were addressed as well. PCBs for the installation of the LED arrays and their power supply were also optimized. These were manufactured on aluminum substrate to help regulate heating of the inspection platform. This feature makes the system more compact, operable at low power, and easy for modification.