Lighting-viewing methods

Abstract

The image acquisition sub-system consists of one, or more, light sources, lenses and possibly mirrors, filters, prisms, polarisers, etc. It also contains one, or more, opto-electronic transducers (i.e., cameras and/or scanners). Simply knowing what ''building bricks'' are available is not enough; we must be able to construct an integrated system. Object presentation and transport are important and require that we address mechanical handling issues: grasping the object, as well as moving it before, during and after image capture. The type of viewing background is important, as are numerous seemingly mundane tasks, such as eliminating ambient light, building vibration-proof optical mountings and keeping optical components clean, dry and cool. In most applications, the placement of the camera relative to the object under inspection and the lights is critical; the image quality derived from the same object may vary from very poor to excellent, depending on the lighting-viewing conditions. This chapter introduces the catalogue of Lighting Viewing Methods (LVMs) in Chap. 40. The latter presents qualitative descriptions of a variety of ''front end'' configurations for a vision system. There can be no rigid formula-based approach to designing a lighting-viewing system. This does not, in any way, diminish the need for sound mathematical analysis, where it can be applied appropriately (e.g., lens design). There can be no equation to tell us when it is most appropriate to use, say, diffuse, or dark-field illumination, monochromatic or polarised light, or a combination of these. The system designer must use his/her judgment and experience. Most importantly, he/she must look at the widget and its surroundings! Two diagrammatic representations of the spatial relationship between the lights and camera are presented here. These are useful for recording and summarising the results of experimentation, which the author asserts is the only practical approach to designing the image acquisition sub-system. This short chapter provides an introduction to the Catalogue of Lighting-Viewing Methods in Chap. 40.