Solving a machine vision application, whether it involves quality inspection, part verification or any number of additional tasks, requires taking several factors into consideration. The most important part of this process is analyzing the target object and its inspection environment, and then specifying the tolerance between “good” parts and “bad” parts. From this information, one can choose the optimal lighting, vision sensor and lens for the application at hand.
Selecting the proper lighting is the most critical part of creating a working vision solution. Robust lighting simplifies the configuring and running of a vision system. Optimal lighting creates adequate contrast between the feature(s) of interest and the background (everything else in the camera’s field-of-view). To be effective, lighting must be consistent and light pollution (noise from changing ambient light levels) must be eliminated. Light selection is an art that involves analyzing the optical properties of the part to be inspected. The four main optical properties are shape, surface texture, color and translucency. The goal is to define, in terms of these optical properties, how the feature(s) of interest differ from their background, and then choose a lighting technique that takes advantage of these differences.
For example, when trying to read a dot peened barcode on an otherwise smooth piece of metal, the optical property that separates the dot peened marks from the flat background is surface texture. Some lighting techniques (e.g., low-angle ring light and on-axis light) are particularly good at generating contrast in these situations and should be tried first.
Common off-the-shelf machine vision lights are available in various sizes, LED colors, and housings. The technologies can be roughly broken down into these three categories: back-lighting, bright-field lighting (area, on-axis, linear array or camera-mount ring lights) and dark-field lighting (low-angle ring or area lights).
Backlighting, a scheme which places the light source behind the object so it shines directly into the camera, creates silhouettes of opaque objects and is useful to analyze shapes or inspect for holes. Backlighting is also used when looking for defects in translucent objects, or for detecting the degree of translucency.
In bright-field lighting, the light source is pointing along a more or less perpendicular line to the target, making smooth objects (like a mirror) appear bright in the camera’s image. Ring lights are the most common bright-field light, and they conveniently mount directly onto the camera and surround its lens with a band of light. Ring lights are often used to detect label presence or inspect date or lot codes.
Dark-field lighting positions the source at a low angle so that the light bounces off smooth objects and away from the camera, making smooth areas of the target appear dark in the image. Meanwhile, rough surfaces wind up reflecting some of the dark-field light into the camera, making rough areas appear bright in the image. Dark-field lighting is a good technique for inspecting raised (or lowered) features and textures. Area lights positioned 45 to 90 degrees off the camera axis can distinguish between rough and smooth surfaces to detect notches in ceramic rings or dents in metal tubing.