In the previous part, we laid the foundations of lighting techniques (such as dark field, bright field, etc.). However, the success of these techniques is strictly tied to the character of the surface the light strikes. The physical interaction between light and the object—reflection, scattering, absorption, or transmission—is the primary factor dictating which technique we should choose. Let's examine the most appropriate lighting strategies based on surface types together, so we can understand the "language" of the materials.
The selection of lighting systems depends directly on surface properties such as the roughness, reflectivity, transparency, and geometric structure of the inspected object. Since the interaction of light with the surface occurs through different physical mechanisms, the right lighting approach is the most critical design parameter determining image quality.
Glossy and Reflective Surfaces: Light undergoes specular reflection at an angle equal to the angle of incidence. The excessive glare caused by direct lighting can hide surface details. To prevent glare and provide a homogeneous image, Dome Light (Dome/Tunnel Lighting) is preferred. For detecting fine scratches, dust, and micro-defects on the surface, Dark Field lighting—where the reflection is directed away from the camera axis—is a highly effective solution.
Matte and Diffuse Surfaces: Since they scatter and diffuse light, they exhibit a much more balanced optical behavior. Bright Field is the most common method to provide general surface inspection on these surfaces. It is frequently used in the quality control processes of materials like paper, cardboard, and textiles.
Rough, Textured, or Wrinkled Surfaces: The angle of the light plays a critical role in accurately analyzing surface geometry. Bumps and dents are highlighted using unidirectional or bidirectional lighting techniques (such as bar lights). On irregular and partially reflective surfaces, Tunnel Light, which distributes light homogeneously, minimizes shadows and glare, providing stable imaging on complex structures like telephone keypads.
Transparent or Translucent Materials: Since light passes through the surface exhibiting transmission, the Backlight technique is used. This method, which shows the object as a dark silhouette, offers high accuracy in edge detection, precision dimensional measurement, and internal defect analysis for products like glass and plastic films.
Complex and Three-Dimensional Surfaces: To solve the shadowing problem in structures containing depth, On-Axis (Coaxial) Lighting is used. Because the light is directed along the exact same optical axis as the camera, narrow gaps, deep recesses, and protruding areas can be imaged clearly without casting obscuring shadows.
In summary; there is no "one-size-fits-all" in industrial lighting; there is only the right strategy that adapts to the physical properties of the material and effectively reveals its flaws. Whether you are inspecting a sheet of glass or a metallic gear, without the right lighting, even the most expensive vision systems are practically rendered blind.
Now that we have come to the end of this detailed guide on the lighting stage, we can transition to the lead actors that will capture that perfect light.
In the next part of our series, we will bring the topics of Camera Types and Lens Selection—which will work in perfect harmony with these lighting configurations—to the table. Stay tuned!
