Components of a System

The aim of a machine vision inspection system is typically to check the compliance of a test piece with certain requirements, such as prescribed dimensions, serial numbers, presence of components, etc. The complete task can frequently be subdivided into independent stages, each checking a specific criterion. These individual checks typically run according to the following model:

01.Image Capture
02.Image Preprocessing
03.Definition of one or more (manual) regions of interest
04.Segmentation of the objects
05.Computation of object features
06.Decision as to the correctness of the segmented objects

Naturally, capturing an image, possible several for moving processes, is a pre-requisite for analysing a scene. In many cases these images are not suited for immediate examination and require pre-processing to change certain sizing specific structures etc. In most cases it is at least approximately known which image areas have to be analysed, i.e. the location of a mark to be read or a component to be verified. These are called Regions of Interest (ROIs) (sometimes Area of Interest or AOIs). Of course, such a region can also comprise the entire image if required.

A process called segmentation is used to isolate these objects. Because of the essential role of this step, various segmentation methods are used in machine vision. Once the objects have been segmented, characteristic properties can be computed, such as area, perimeter, position, orientation, distance from each other, similarity to predefined patterns (e.g. for character recognition). Finally, these properties are checked for compliance with the nominal values of the inspection task.