In the first part of our series, by saying "Your camera can only see what you illuminate," we focused on the power of LED technologies and basic configurations (ring, bar, dome, etc.). However, choosing the right hardware is only the first step; true mastery lies in how you direct that light onto the object you are inspecting. Sometimes we want to strike the light perpendicularly to see all the details, while other times we prefer to leave the surface in darkness, waiting only for the defects to shine. In our second installment, we take a deep dive into the world of Lighting Techniques, which are shaped entirely by the character of the surface and the type of defect we are looking for. Let's look together at how hidden defects emerge one by one when you find the right angle and strategy.
Bright Field: In the Bright Field lighting technique, the light source is directed straight at the object's surface, and the camera captures the light reflected off that surface. This method is particularly suitable for acquiring a general view and conducting basic surface inspections on matte surfaces with low reflectivity. While it provides high-contrast and stable images, it can cause information loss due to glare on highly reflective or shiny surfaces.
Dark Field: In the Dark Field lighting technique, light is directed at the object's surface at extremely low/oblique angles, meaning the directly reflected light is not captured by the camera. As a result, only fractures, scratches, deformations, and micro-defects on the surface scatter the light up into the lens, making them highly visible against a dark background. It is a highly effective and preferred method for defect detection, especially on metal and highly reflective surfaces.
Front Light: The Front Light technique relies on applying light onto the object from the same general direction as the camera. This method provides a balanced view of the surface's overall structure and details. It is widely used for highlighting features in low-contrast scenes and for general surface analysis.
Backlight: In the Backlight technique, the light source is placed behind the object, causing the camera to perceive the object as a dark silhouette. This method allows for the extraction of the object's outer contours and geometric shape rather than its internal surface details. Because it offers high precision in edge detection, dimensional measurement, and shape analysis, it holds a crucial place in industrial applications.
On-Axis Lighting (Coaxial Lighting): The On-Axis Lighting technique is based on directing the light onto the object along the exact same optical axis as the camera. This setup provides highly homogeneous illumination, minimizing shadows and glare, particularly on flat and reflective surfaces. It is the preferred choice for high-precision applications such as semiconductor inspection, glass inspection, and print verification.
Directional or Correctional Light: In this technique, light is controlled and applied to the surface at a specific, optimized angle. This approach increases the contrast of surface defects, making them stand out much more clearly. By optimizing the lighting angle, unwanted reflections are reduced, rendering defect detection far more reliable.
Tunnel Light: In a Tunnel Light system, the object is illuminated homogeneously within a structure that has a highly reflective inner surface. Through multiple reflections, the light distribution is balanced, and shadow formation is virtually eliminated. It offers significant advantages on industrial production lines by ensuring stable and repeatable imaging, especially for cylindrical, shiny, and highly reflective surfaces.
Requirement-Based Lighting: Rather than being a single fixed technique, Requirement-Based Lighting refers to evaluating and combining different lighting methods according to the specific needs of the application. Multiple lighting techniques can be integrated to meet varying requirements simultaneously, such as edge detection, surface defect analysis, or 3D measurement. The primary goal of this approach is to craft the optimum lighting combination that maximizes overall system performance.
In Conclusion; As we can see, in industrial machine vision, there is no one-size-fits-all lighting formula; there is only the "most optimal" technique tailored to the material structure of the inspected part and the specific character of the targeted defect. Sometimes we search for a microscopic scratch hidden in the shadows created by a Dark Field, and other times we rely entirely on the razor-sharp silhouettes provided by a Backlight.
Formulating the right Requirement-Based strategy is the most critical move you can make to push the error margin of your AI and vision system closer to zero. Now that we have successfully set up this optical chess game on the lighting stage, it is time to move on to the lead actors that will capture that flawless light. In the next part of our series, we will bring the topics of Camera Types and Lens Selection—which work in perfect harmony with these lighting setups—to the table.
See you then!
