Machine vision is applied across a wide range of industries, including electronics, automotive, packaging, printing, food, and medical. Consequently, the products we inspect are diverse: varying in shape and size, color and material, and requiring different inspection environments and specifications. Faced with such a wide variety of products and diverse requirements, how do we choose a light source? Let's first look at the characteristics of common light sources.
Currently, ideal visual light sources include high-frequency fluorescent lamps, fiber optic halogen lamps, xenon lamps, and LED light sources. LED light sources are the most widely used, and this section will introduce several common LED light sources in detail.
1. Ring light source
The LED beads are arranged in a ring at a certain angle to the central axis, and are available in different illumination angles and colors to highlight the three-dimensional information of objects; solve the problem of shadows in multi-directional lighting; and for images with light shadows, a diffuser plate can be added to ensure uniform light diffusion. Applications: screw size defect detection, IC positioning character detection, circuit board solder inspection, microscope illumination, etc.
2. Bar light source
LED beads are arranged in a long strip. This is often used to illuminate an object from one or more sides at a certain angle. It highlights the edge features of the object and multiple strips can be freely combined according to actual needs. The illumination angle and installation distance offer good flexibility. It is suitable for larger structures being measured. Applications include: gap inspection of electronic components, surface defect inspection of cylinders, printing inspection of packaging boxes, and contour inspection of medicine bags, etc.
3. Coaxial light source
The surface light source employs a beam splitter design. It is suitable for surfaces with varying roughness, high reflectivity, or unevenness, for detecting engraved patterns, cracks, scratches, separating low-reflectivity and high-reflectivity areas, and eliminating shadows. It should be noted that the coaxial light source, due to its beam splitting design, experiences some light loss, which needs to be considered for brightness, and it is not suitable for large-area illumination. Applications: glass and plastic film contour and positioning detection, IC character and positioning detection, wafer surface impurity and scratch detection, etc.
4. Dome light source
The LED beads are installed at the bottom, and the diffuse reflection from the hemispherical inner wall reflective coating evenly onto the object. The overall illumination of the image is very uniform, suitable for detecting highly reflective metals, glass, uneven surfaces, and curved surfaces. Applications: dashboard scale inspection, metal can character marking inspection, chip gold wire inspection, electronic component printing inspection, etc.
5. Backlight
LED beads are arranged in a single surface (bottom illumination) or in a ring around the light source (side illumination). This is commonly used to highlight the outline of objects and is suitable for large-area illumination. Backlighting is generally placed at the bottom of the object, and the suitability of the installation mechanism needs to be considered. For higher detection accuracy, the parallelism of the emitted light can be improved. Applications include: measurement of mechanical parts dimensions and edge defects, detection of beverage levels and impurities, detection of light leakage in mobile phone screens, detection of defects in printed posters, and detection of edge seams in plastic films.
6. Point light source
High-brightness LEDs are small in size and have high luminous intensity; they are often used in conjunction with telecentric lenses. They are a type of non-direct coaxial light source with a relatively small field of view. Applications include: inspection of hidden circuits in mobile phone internal screens, mark point positioning, glass surface scratch detection, and LCD glass substrate calibration inspection.
7. Line light source
High-brightness LEDs are arranged using light guide columns to focus the light, creating a bright band of light. This is typically used in line scan cameras, employing side or bottom illumination. Line light sources can also be used without a focusing lens to diffuse the light, increasing the illumination area. A beam splitter can be added to the front to convert it into a coaxial light source. Applications include: dust detection on LCD screen surfaces, detection of scratches and internal cracks in glass, and detection of fabric weaving uniformity.
For specific applications, selecting the best lighting system from numerous options is crucial for the stable operation of the entire image processing system. Unfortunately, no single, universal lighting system can adapt to all situations. However, due to the diverse shapes and colors of LED light sources, we have identified some methods for selecting visual light sources. The main methods are as follows:
1. Observational Experiment Method (Look and Experiment - the most common): Try illuminating the object with different types of light sources at different positions, and then observe the images through a camera;
2. Scientific Analysis – the most effective method for analyzing imaging environments, recommending the best solution.
