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Lighting Equipment Questions

Question Asked:

I want to purchase a colored ring light for darkfield inspection of a white surface. The surface has a micro texture (not too different from sandpaper), it is this texture that I plan to inspect. How do I go about selecting a wavelength of light to improve detection on a white surface. With white light in near darkfield I get a specular reaction from the surface.

2 Answers

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Perry West - President
Automated Vision Systems, Inc.
perry [at] autovis [dot] com
(408) 267-1746

John -- Scott is right when he said that wavelength will likely not have any effect on the specular reflections. A lot depends on the microtexture. If it's truly irregular like sandpaper, there is not going to be any illumination angle that will eliminate all glints. If it's a manufactured texture about which you know the geometry, there may be an illumination angle or range of angles that will work to keep the specular reflection out of the camera. The challenge may be to get the illumination within this angle range across the entire field-of-view. The other option Scott offered is the polarizer-analyzer approach. Using a polarizer over the light source and an analyzer (2nd polarizer) over the camera lens, you might be able to tune the analyzer position to remove the glints. However, if the texture results in multiple reflections off the texture facets, even this might not work.

Robert McConnell
WAY-2C Color Machine Vision
rkm [at] way2c [dot] com
(781) 641-0605

I agree with Scott and Perry, Since the surface you are inspecting is white it will reflect all visible wavelengths equally. Therefore, simply changing the color of the illumination is unlikely to solve the specular reflection problem. What you will have to do is change the geometry of the illumination, perhaps abandoning the idea of a ring light. One thing you did not mention is what kind of features you are trying to detect. That will have a bearing on what lighting geometry will give the best results.

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