At present, ideal visual light sources include high-frequency fluorescent lamp, optical fiber halogen lamp, xenon lamp and LED light source. Most applications are led light sources. Here are several common LED light sources in detail.

1. Circular light source

The LED lamp beads are arranged in a ring and form a certain angle with the central axis of the circle. There are different illumination angles, different colors and other types, which can highlight the three-dimensional information of the object; Solve the problem of multi-directional illumination shadow; In case of light shadow in the image, it can be equipped with a diffuser to make the light diffuse evenly. Applications: screw size defect detection, IC positioning character detection, circuit board solder inspection, microscope lighting, etc.

2. Bar light

Led beads are arranged in long strips. It is mostly used to irradiate objects at a certain angle unilaterally or multilaterally. Highlight the edge characteristics of the object, which can be combined freely according to the actual situation, and the irradiation angle and installation distance have better degrees of freedom. It is applicable to the tested object with large structure. Applications: electronic component gap detection, cylinder surface defect detection, packaging box printing detection, liquid medicine bag contour detection, etc.

3. Coaxial light source

The surface light source is designed with a spectroscope. It is applicable to the surface areas with different roughness, strong reflection or uneven surface. It can detect engraving patterns, cracks, scratches, separation of low reflection and high reflection areas, and eliminate shadows. It should be noted that the coaxial light source has a certain light loss after spectral design, which needs to consider the brightness, and 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 lamp beads are installed at the bottom to uniformly irradiate the object through the diffuse reflection of the reflective coating on the hemispherical inner wall. The overall illumination of the image is very uniform, which is suitable for the detection of metal, glass, concave convex surface and arc surface with strong reflection. Applications: instrument panel scale detection, metal can character inkjet detection, chip gold wire detection, electronic component printing detection, etc.

5. Backlight

The LED light beads are arranged into a surface (the bottom surface emits light) or arranged around the light source (the side emits light). It is often used to highlight the contour characteristics of objects and is suitable for large-area illumination. The backlight is generally placed at the bottom of objects. Whether the mechanism is suitable for installation needs to be considered. Under high detection accuracy, the parallelism of light can be strengthened to improve the detection accuracy. Application: measurement of mechanical parts size and edge defects, detection of beverage liquid level and impurities, light leakage detection of mobile phone screen, printing poster defect detection, plastic film edge seam detection, etc.

6. Point light

Bright LED, small size, high luminous intensity; It is mainly used with telecentric lens. It is an indirect coaxial light source with small detection field. Applications: mobile phone internal screen stealth circuit detection, mark point positioning, glass surface scratch detection, LCD glass substrate correction detection, etc

7. Line light

The bright LED is arranged, and the light is concentrated by the light guide column. The light is in a bright band, which is usually used in linear array cameras. Side illumination or bottom illumination is used. The linear light source can also diffuse the light without using the condensing lens, increase the irradiation area, and add a beam splitter in the front section to turn it into a coaxial light source. Application: LCD surface dust detection, glass scratch and internal crack detection, cloth textile uniformity detection, etc.

For specific applications, choosing the best lighting system from many schemes is the key to the stable work of the whole image processing system. Unfortunately, there is no universal lighting system that can adapt to various occasions. However, due to the multi shape and multi color characteristics of LED light sources, we still find some methods to select visual light sources. The main methods are as follows:

1. The observation test method (look and experiment – the most commonly used) attempts to irradiate objects at different positions with different types of light sources, and then observe images through the camera;

2. Scientific analysis (the most effective) analyzes the imaging environment and recommends the best solution.