If the light color is similar with inspected object, the color on the surface of object can be filter, if the color is some different, special the complementary color can strengthen the color on the surface of object. So we can use this principle to filter some colors or strengthen some colors to get good image result.
Like the below, the object is can cap, and on the surface of the can, it is printed red color “WELCOME” letters, if we need to inspect the surface defects of the can, then the red letter will disturb the inspection. If we use white color machine vision light, then the image result like picture 1, and get very clear letter on the surface, but if we use red color machine vision light, the image result like picture 2, and there is no letter on the surface of can, so it will not disturb the inspection of the surface.
Below picture is beer bottle cap, we need to check the printed code on the surface of the cap, but the problem is there are many different colors of the caps, below caps is example used green and blue colors, so it is better to choice the color of machine vision light like the background, so that the background color can be filtered or reduced and the printed code can get clear image result.
Image result after filter the background images of beer bottle cap
Printed borad solder joint location
Spectral reflection of metal surface
Different material has different chemical content, when different wavelength of color comes to the object surface, the light will be reflected or absorbed, we can simple say different color has different reflective rate. Below picture is the reflective rate of gold, silver, copper, aluminum under the situation of visible light and IR light.
From the picture, we can see that the four materials have stronger reflective under the illumination of longer wavelength, for the short wavelength, the reflective rate of silver and aluminum is much higher than gold and silver, so according to this principle, we can use short wavelength to distinguish the materials.
Application example 1
Below picture is die bonding inspection project, the object is printed circuit of CCD sensor, printed board is covered silver material, and the solder joint is gold, the inspection is to judge the solder joint is good or not, or the solder joint is in the right position or not, according to the material reflective principle, we choose blue color coaxial light in this application, the reflective rate of silver board is higher, from the image result, it is brighter, solder joint reflective rate is lower, from the image result, it is darker, and it has clear distinguish, so it is easy to get stable image result.
Application example 2
Picture 1 is printed circuit board of transfer structure, the touch pin bottom side is copper material, the touch pin up side is silver material, the inspection is to measurement the pin length.If use natural light, both side of the material is white color, and it is very hard to inspect as the impacted, if use the blue color machine vision light, the bottom side can be black, so it is easy to distinguish the up side from the bottom side, so that the measurement is accuracy.Picture2 is solder joint inside the multi-layer metal material, total 5 layers, the bottom 4 layer material is silver-cooper-silver-silver, the upper layer is silver lines, so we still use blue color machine vision light to get the image result like the picture 2.