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Light Source Selection for Machine Vision System

The direct light and diffuse light of machine vision system

For example, in the case of a bullet-shaped LED light source, the direct light has a small light distribution angle and relatively concentrated light energy, resulting in high brightness. When the target is made of diffuse reflection material, the target will be brighter. When the target is made of specular reflection material, the target surface will form a bright spot (actually the image of the light source), with poor uniformity and not conducive to detection; diffuse light is a secondary light source formed by covering a layer of diffuse semi-transparent material after the direct light.

As the transparency of the diffuse material decreases, the divergence angle of the transmitted light increases, and the uniformity of the scattered light of the machine vision system continues to improve, but at the same time, the brightness will decrease significantly. For targets made of specular reflection materials, a uniformly luminous surface can avoid the formation of hot spots on the target surface, thereby obtaining an ideal detection image.

The complementary and adjacent colors of the machine vision system - color mixing law

The three primary colors of light are red, green, and blue. None of the three primary colors can be produced by mixing other two primary colors, while other colors can be mixed in certain proportions with these three colors.

The continuous change law of color light: In the mixed color composed of two colors of light, if one color of light changes continuously, the mixed color will also change continuously.

The law of complementary colors of machine vision: A uniform mixture of the three primary colors can obtain white light. If you mix red light and green light first, you can get yellow light, and then mix yellow light and blue light, you can also get white light. These two colors are called complementary colors. There are three basic complementary colors: red-green, green-cyan, and blue-yellow. An important characteristic of complementary colors is that when colored light shines on an object with complementary colors, it will be absorbed. If a yellow object is illuminated by blue light, it will appear black.

The sub-law of color light mixing: Mixing any two non-complementary color lights produces intermediate colors. The color depends on the relative energy of the two color lights, and the brightness depends on their distance on the hue order.

The substitution law: lights with the same color and appearance have the same effect regardless of their spectral composition in color light mixing. All colors of the same appearance on the visual are equivalent. That is, similar colors remain similar after mixing.

If color light A = B/C = D, then: A + C = B + D. The substitution law of color light mixing shows that as long as colors are similar in sensation, they can be interchanged, and the machine vision effect is the same.

There are more than four basic laws of color light mixing. These rules can guide the design of machine vision lighting systems. For example, different spectral light sources and illuminator machine can be selected according to the color of the target, and complementary color and brightness addition rules can be used to highlight the brightness of the target and weaken the background, thereby achieving the final effect of highlighting the target.

The foreground light and background light of the machine vision system

The foreground light, that is, the illumination mode where the light source and camera are on the same side of the target; the background light, also known as the back light, refers to the illumination mode where the light source and camera are next to the target.

Foreground light, which is important in machine vision measurement, is helpful to display the surface detail characteristics of the object and can be used for various surface inspections.

Backlighting has two different purposes: imaging transparent objects observed by projection and contour imaging of opaque objects. Thin glass is a transparent product observed under backlight. Non-coaxial spotlights will highlight surface defects (scratches, chisels) and external defects (bubbles, inclusions); backlighting is more commonly used to represent the contour of opaque parts. The contour is an easy-to-handle image because it is two-dimensional and binary. Flexible parts feeding machines usually use backlight images to determine the position of mechanical parts selected by robots in assembly.

There are many illumination modes for foreground light, while the illumination modes for background light are usually relatively single. Background light is used to create a bright background, and the opaque or semi-transparent target forms a strongly contrasting dark area. Background light is more suitable for inspecting defects in films and measuring external dimensions.