In machine vision, capturing a high-quality, processable image is essential for the system's success. The performance of any machine vision setup depends largely on the quality of the image and how clearly the features of interest are defined. If the image lacks clarity or the relevant features are not well-defined, the system may become unreliable or fail altogether. Therefore, mastering the art of light source selection is crucial. Below are the key elements to consider when choosing an appropriate lighting solution.
Light Source Color
Selecting the right color for the light source plays a vital role in enhancing contrast between the object of interest and the background. A good choice will maximize the difference in grayscale values, making it easier to distinguish the desired features from the surrounding area. Typically, complementary colors are used to achieve this effect. For example, if the object is green, using a red light source can significantly improve contrast.
When dealing with objects that contain unwanted impurities, the background light source is often chosen to match the color of the impurities, helping to visually suppress them. In some cases, such as detecting tiny dust particles on a wafer, ultraviolet (UV) light is highly effective, as it can make even the smallest particles visible.
Lighting Methods
Proper illumination ensures that the features you want to detect stand out clearly. There are various lighting techniques, including front lighting, back lighting, and coaxial lighting, each suited for different applications. For instance, back lighting is ideal when the goal is to highlight the contour of an object, as it places the object between the light source and the camera.
Custom lighting solutions are also essential in specialized scenarios. For example, in a seam tracking system developed by Ke Tianjian, a mirror is used to redirect the laser line so that it forms a large angle with the optical axis, enabling accurate laser triangulation for weld detection. This approach helps save space while maintaining optimal lighting conditions.
Other Important Considerations
The brightness and reliability of the light source are critical factors in its selection. Brightness affects the signal-to-noise ratio, image contrast, and the depth of field. Insufficient brightness can lead to poor image quality, increased noise, and reduced system stability. Additionally, natural light can interfere with the system if the light source is not bright enough.
Robustness refers to how sensitive the light source is to changes in the position of the object. A reliable light source should produce consistent results regardless of where the object is placed within the camera’s field of view. Highly directional sources may cause unwanted reflections, which can complicate feature extraction.
A high-quality light source should enhance the visibility of the features you need to detect, provide maximum contrast and brightness, and remain stable even when the object’s position varies slightly. Once the right light source is selected, the rest of the system design becomes much more straightforward. However, the best practices for light source selection often come from real-world testing and experience.
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