The light source is an important factor affecting the input of the machine vision system, which directly affects the quality of the input data and at least 30% of the application. Because there is no universal machine vision lighting equipment, for each specific application case, the corresponding lighting equipment should be selected to achieve the best effect. Several common lighting methods are described below.
The light is directed towards the object, resulting in a clear image. This type of light is effective when high contrast images of objects are required. But when it is used on bright or reflective materials, it will cause a mirror-like reflection.
General lighting for machine vision generally uses ring or point lighting. Ring lights are a commonly used general-purpose lighting method that can be easily mounted on lenses to provide adequate lighting for diffuse surfaces.
Dark field lighting is related to the appearance of objects to provide low viewpoint lighting. Use the camera to take a picture of the mirror so that it is within its horizon. If the light source can be seen in the horizon, it is considered to be bright-field illumination. On the contrary, if the light source cannot be seen in the horizon, it is dark-field illumination.
Therefore, whether the machine vision light source is bright field illumination or dark field illumination is related to the orientation of the light source. Typically, darkfield illumination is used for illumination of a portion with protrusions on the outer surface or illumination with a change in surface texture.
Light coming from the back of an object with a uniform field of view. The side profile of the object can be seen through the camera. Machine vision backlighting is often used to measure the size and orientation of objects. Backlighting creates strong contrast. When using the backlight technology, the appearance of the object may be lost. For example, the diameter of a coin can be measured using backlight technology, but the front and back of a coin cannot be identified.
The formation of coaxial light is the change of divergent light that comes out through the straight wall and hits a beam splitter that makes the light downward, and the camera looks at the object through the beam splitter from above. This type of light source is especially helpful for detecting highly reflective objects, and is also suitable for detecting objects with inconspicuous areas under the influence of shadows from the surrounding environment.
Continuous diffuse lighting is used for objects whose surface is reflective or whose surface has a cluttered point of view. Continuous diffuse lighting uses a hemispherical uniform lighting to reduce shadows and specular reflections. This lighting method is useful for lighting fully assembled circuit boards. This light source can achieve uniform illumination over a 170 solid angle range.
In addition to the several common lighting techniques described above, there are also lighting techniques used in special occasions, such as strip light lighting that requires concentrated brightness in line scan cameras; for example, parallel light used in cooperation with telecentric lenses in fine-scale measurement lighting technology; such as frequency bright lighting technology to reduce the blur of the measured object in high-speed online measurement; another example is the structured light lighting technology that can automatically measure the distance from the camera to the light source and the polarized lighting technology to reduce stray light interference.
In addition, many complex test environments require two or more lighting techniques to work together. Therefore, rich lighting skills can solve many problems in image acquisition in machine vision systems, and the selection of light source lighting skills may be critical to the success of a vision system.
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