Development and evaluation of specific lighting scenarios and ergonomic approaches to optimise visual inspection tasks
The purpose of the several studies reported here was to improve the lighting conditions at industrial visual testing and inspection workplaces beyond the specifications given in technical standards. In this context, special emphasis was laid on the examination of the effects of directional and diffuse lighting as well as of sample specific lighting scenarios with respect to the ability to detect three dimensional (3D) details. The sample specific lighting scenarios were designed according to ergonomic principles and adapted to different material surfaces. Landolt rings of various sizes were laser engraved on several surface types, which are representative of the surfaces encountered in industrial final inspection tasks. The sample surfaces were fixed in a special object mount, which allowed defined and reproducible test arrangements. In a first step, the luminance distribution on the samples and the contrast between the Landolt rings and the background of the samples were measured depending on different illumination types and inclinations of the surface types. Additionally, laboratory tests were carried out in which the test subjects' visual acuity was measured while detecting 3D details (e.g. the Landolt rings) on the surfaces of different materials illuminated in different ways. It turned out that all results were strongly dependent on the inclination of the samples rather than on the lighting scenarios applied. In a further study, the time related performance in detecting surface flaws on samples under different lighting scenarios was measured. For this purpose, the average error quota was assessed, i.e. the proportion of wrongly identified Landolt rings among the overall number of optotypes and the response time to the identification of a presented optotype were determined. Both parameters gave evidence of the long term advantage of the ergonomic sample specific lighting scenarios. In a last step, the various illuminations were evaluated through a structured interview of the test subjects. The subjective assessment of the illuminations also indicated a high dependency on the geometric arrangement between light source and the sample under examination. Especially, in the context of long term inspections, the subjects preferred the sample specific lighting scenarios. The diffuse lighting was also assessed predominantly and positively. The illumination with directional lighting and with the lighting according to technical standards was, mostly, rated disadvantageously.