Machine vision uses machines to replace the human eye for measurement and judgment. Machine vision systems mainly include cameras, lenses, light sources, image processing systems, and execution mechanisms. As an important component, the light source directly affects the success or failure of the system. In the visual system, images are the core. Choosing the appropriate light source can present a good image, simplify the algorithm, and improve system stability. If an image is overexposed, it will hide many important information, and if shadows appear, it will cause edge misjudgment. If the image is uneven, it will make threshold selection difficult. Therefore, to ensure good image effects, it is necessary to choose a suitable light source.
At present, ideal visual light sources include high-frequency fluorescent lamps, fiber optic halogen lamps, xenon lamps, and LED flood light. The most common applications are LED light sources, and here we will provide a detailed introduction to several common LED light sources.
1. Circular light source
LED light beads are arranged in a circular shape at a certain angle to the center axis, with different illumination angles, colors, and other types, which can highlight the three-dimensional information of objects; Solve the problem of multi-directional lighting shadows; When there is light shadow in the image, a diffuse plate can be selected to evenly diffuse the light. Application: Screw size defect detection, IC positioning character detection, circuit board soldering inspection, microscope lighting, etc.
2. Bar light source
LED light beads are arranged in long strips. It is often used to illuminate objects at a certain angle on one or more sides. Highlighting the edge features of objects, multiple free combinations can be made according to the actual situation, and the irradiation angle and installation distance have good degrees of freedom. Suitable for larger structures to be tested. Application: Electronic component gap detection, cylindrical surface defect detection, packaging box printing detection, liquid medicine bag contour detection, etc.
3. Coaxial light source
The surface light source is designed with Beam splitter. Suitable for detecting engraved patterns, cracks, scratches, separation of low and high reflective areas, and elimination of shadows in surface areas with varying roughness, strong or uneven reflection. It should be noted that the coaxial light source has a certain amount of light loss that needs to be considered for brightness after beam splitting design, and is not suitable for large area illumination. Applications: contour and positioning detection of glass and plastic films, IC character and positioning detection, chip surface impurity and scratch detection, etc.
4. Dome light source
LED light beads are installed at the bottom and diffuse through the reflective coating on the inner wall of the hemisphere to evenly illuminate the object. The overall illumination of the image is very uniform, suitable for detecting highly reflective metals, glass, concave and convex surfaces, and curved surfaces. Application: Instrument panel scale detection, metal can character inkjet detection, chip gold wire detection, electronic component printing detection, etc.
5. Backlight source
LED light beads are arranged in a single surface (emitting light from the bottom) or in a circle around the light source (emitting light from the side). Commonly used to highlight the contour features of objects, suitable for large-scale illumination. Backlight is generally placed at the bottom of the object, and it is necessary to consider whether the mechanism is suitable for installation. Under high detection accuracy, it can enhance the parallelism of the light output to improve detection accuracy. Applications: measurement of Machine element size and edge defects, detection of beverage liquid level and impurities, light leakage detection of mobile phone screen, defect detection of printed posters, edge seam detection of plastic film, etc.
6. Point light source
High brightness LED, small size, high luminous intensity; Commonly used in conjunction with telephoto lenses, it is a non direct coaxial light source with a smaller detection field. Application: Detection of invisible circuits on mobile phone screens, MARK point positioning, scratch detection on glass surfaces, correction and detection of LCD glass substrates, etc.
7. Line light source
The arrangement of high brightness LED adopts light guide column to focus light, and the light is in a bright band, which is usually used for linear array cameras. Lateral or bottom illumination is adopted. The linear light source can also disperse the light without using a condensing lens, and Beam splitter can be added in the front section to increase the irradiation area, which can be converted into a coaxial light source. Application: LCD screen surface dust detection, glass scratch and internal crack detection, fabric textile uniformity detection, etc.
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