The basic elements of light source selection in machine vision

In machine vision, it's important to get a high-quality, processable image. The success of the machine vision system must first ensure that the image quality is good and the features are obvious. Conversely, if the image quality is not good and the features are not obvious, the machine vision system will become unreliable or robust, and even lead to project failure. Therefore, the choice of light source skills is a must, the following introduces the basic elements of light source selection:

1. Light source color <br> Good light source color selection can produce the maximum contrast between the features that need to be observed and the image features that need to be ignored, that is, there is enough gray level difference between the feature and the surrounding area. This makes it easy to distinguish features. In order to maximize the distinction between the object being viewed and the background, a complementary color source is typically selected. For example, when the object being viewed is green, selecting a background of the red source can improve contrast.

When the observed object is mixed with impurities that we do not want to see, the background light source color is usually selected to be the same color as the magazine, so that the impurity interference can be visually filtered out.

Ultraviolet light is a good choice when detecting tiny dust particles. For example, in the detection of wafer dust particles, ultraviolet light is usually used.

2. Lighting methods <br> Good lighting should ensure that the features that need to be detected are highlighted in other contexts. There are many lighting methods, such as forward lighting, back lighting, coaxial lighting, etc. Depending on the application background, different lighting methods are directly related to the success or failure of the project.
When it is necessary to highlight the contour of the object, back illumination is usually used, that is, the object to be observed is located between the light source and the high speed camera.

Multiple ways of forward illumination Multiple ways of forward lighting <br> In addition, in some special applications, flexible use of customized lighting is also important. For example, in order to save the space of the light source and the high-speed camera and at the same time achieve the effect of the light source from a certain angle, a mirror can be used, and the seam tracking system independently developed by Ke Tianjian fixes the high-speed camera and the laser line emitter in a small Inside the box, the direction of the laser line is changed by a mirror so that the laser line and the high-speed camera are at a large angle to the optical axis for laser triangulation of the weld.

Weld seam tracking lighting system Weld seam tracking lighting system
3. Other elements <br> The brightness and robustness of the light source are also non-negligible elements in the selection of the light source.
Brightness: When choosing two sources, the best option is to choose the brighter one. When the light source is not bright enough, there may be three bad conditions. First, the signal-to-noise ratio of a high-speed camera is not sufficient; because the brightness of the light source is insufficient, the contrast of the image is inevitably insufficient, and the possibility of noise appearing on the image also increases. Secondly, the brightness of the light source is not enough, and the aperture must be increased, thereby reducing the depth of field. In addition, when the brightness of the light source is insufficient, the random light such as natural light will have the greatest influence on the system.
Robustness: refers to whether the light source is least sensitive to the position of the component. When the light source is placed in different areas of the camera's field of view or at different angles, the resulting image should not change. A highly directional light source increases the likelihood of specular reflections in the highlighted area, which is detrimental to subsequent feature extraction.
A good light source needs to be able to make the features you need to look very obvious. In addition to being able to capture parts with a high-speed camera, a good light source should produce maximum contrast, brightness and is not sensitive to changes in the position of the part. The light source is chosen, and the rest of the work is much easier. Of course, the specific method of light source selection lies in the practical experience of the test.

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