A complete machine vision system generally includes four core parts: light source, camera, lens, and software. The selection of light source determines the success or failure of the project. Before answering how to choose a suitable machine vision light source, first analyze the role of the light source and the factors that affect the selection of the light source. Understanding these can make a faster and more accurate selection.
1. The role of light source in machine vision system
- Highlight measurement features and simplify image processing algorithms;
- Overcome the interference of ambient light and improve the image signal-to-noise ratio ;
- Improve the positioning, measurement, and recognition accuracy of the visual system and the operating speed of the system;
- Reduce the complexity of system design
2. Factors affecting light source
Contrast
The most important task of lighting in machine vision applications is to create the greatest contrast between the features that need to be observed and the image features that need to be ignored, making it easier to distinguish the features.
brightness
When choosing between two light sources, the best choice is definitely the brighter one. Because when the light source is not bright enough, the following three situations will occur:
- The camera’s signal-to-noise ratio is not good enough;
- The image contrast is not enough, and the possibility of noise appearing on the image increases;
- If the light source is not bright enough, the aperture must be increased to reduce the depth of field , and natural light will also randomly increase the impact on the system.
robustness
The way to test a light source is to see if it is minimally sensitive to the position of the part. When the light source is placed in different areas or at different angles in the camera's field of view, the resulting image should not change. A light source with strong directionality increases the likelihood of specular reflections in bright areas, which is not conducive to subsequent feature extraction . In many cases, a good light source needs to have the same effect in actual work as it does in the lab.
Light source is predictable
When light is incident on the surface of an object, the reflection of the light source is predictable. The light source may be absorbed or radiated, and the light may be completely absorbed or partially absorbed. The light that is not absorbed will be reflected, and the angle of the incident light is equal to the angle of the reflected light.
Due to the differences in the surfaces of objects, it is necessary to observe the surfaces of objects in the field of view and analyze the reflection of the incident light source.
The location of the light source
The goal of a light source is to make the feature of interest reflect the light differently from the background around it. Predicting how the light will reflect off the surface of an object can determine the location of the light source.
3. Lighting method
Understanding the role and influencing factors of light sources will help us make specific judgments. Next, we need to choose the best lighting system from a variety of lighting solutions to achieve better image recognition results. The following two methods are commonly used:
- Observational Experiment
Try illuminating the object with different types of light sources at different positions and then observe the image through the camera;
- Scientific analysis
Analyze the imaging environment and recommend the best solution.
4. Tips for selecting light source
- Need more contrast between foreground and background? — Consider using a black and white camera and a color light source.
- Ambient light problem? - Try using a monochromatic light source with a filter
- Shiny curved surface? - Try using diffuse dome light
- Shiny, flat, but rough surface? - Try using coaxial diffuse light
- See the shape of the surface? - Consider using dark field (low angle)
- When testing plastics - try using UV or IR light
- Need to see features through a reflective surface? - Try using a low-angle line light source (dark field)
- Combining light sources can sometimes solve the problem
- Strobe light can produce light 20 times brighter than continuous lighting