Laser Etching Detection on Clear Plastic

tud623 · ‎11-06-2020

I am looking for information/physics for improving detection of a laser etching on a clear plastic. Laser etching is similar to engraving, only smaller in depth, where a laser pretty much just removes material to create letters, numbers, etc. This can be seen with the human eye but only at certain angles, as line of sight perpendicular to the etch is hard to see.

There are several issues with trying to detect this with a camera system at an angle, but the lets focus on the main issue: detecting the letters in general.

From a physics standpoint, if a light source is angled opposite to the camera (where the light is reflecting off the plastic and going back into the camera, this seems to make the edges of the letters to cast a shadow. Is this the correct thinking of why at an angle, the letters are easier to detect?

What are the ways that contrast of the etch can be increased? I have tried some filtering, already but I am unsure on the combination of filters. What type of background would work the best (glossy versus matte, color, etc.)? It seems to me very reflective would be the worst and a dark color.

I am looking for any information that would help this detection. Thank you!

1100fps · ‎11-06-2020

Color will do you no good. There are no spectral elements (at least in visible) that will be enhanced with a filter.

It might be possible to use polarizers to positive effect. This would require reflecting the light source off of the subject directly into the camera. With a polarizer on the camera, crossed with the polarizer on the light source, the field should be dark, since the plastic will preserve polarization state, but the etched area will randomize the polarization, and show up bright. The light source will need to be as large as the camera field if view.

Alternately, edge lighting of the plastic could work, The etched bits would scatter light. This works best against a dark Background.

If the camera is looking through the pastic at a dark background, the sheet can be illuminated from the backside to provide a darkfield image that will highlight the etch.

There may be some tricks with retroreflection film, that will help, but i would need to know more about the setup.

The laser etching process usually relies on local plasma discharge to create the etch effect, a time exposure during the etch process might be useful, depending on how the motion system works.

When cameras look at a flat field, The focus (Depth of field) or perspective can be corrected by tilting the cameras with respect to the lens and meeting the Scheimpflug condition

Have fun!

Robert