project

[mikke]

Hi

I am new to this forum and not an expert on lasers, but I have an idea to measure roughness or profile on a surface.
I don´t know if it will work but maybe you have some ideas about it.
I don´t need to get a value or something I just want to see if their are changes in the surface.

My idea is to put a laser in a angle to the surface and measure the reflection.
I might add that the surface is not smooth in fact it has small small grains on it that are 5-6µm and would like to detect differences in that surface. Let´s say that we have a grain that is the double 10-12µm.

Feel free to have opinions about my project and tell me what kind of tools I could use to test my theory.
Thanks
M

 

[kiyoukan]

grab an old optical mouse that uses a led there is a small camera and with it it can auto detect surfaces.

 

[mikke]

Good idea. But how can I detect differences in the surface roughness?
I might also add that I can not touch the surface because then it would be ruined.

 

[HIMNL9]

Not touching the surfaces, for detect "roughness" or irregularity, you can use a camera or webcam with a very good macro lens in front (microscope type), and 3 light sources placed around the camera at 120 degrees ..... illuminating the surface in sequence with the 3 lights inclinated (radent ?) from 3 different angles, you can be able to obtain 3 images where the shadows formss a 3D "map" of the roughness ..... then, anyway, you still need a software that can combine the 3 images and extrapolate a 3D model from them, if you want to make it in automatic ..... just as idea .....

 

[mikke]

Hi HIMNL9
I have worked with cameras on this but I have not found a optic good enough to see the grains clearly. The grains on the surface are 5-6µm and about 3-4µm apart.
The idea is good but the optical side is the problem.

 

[kiyoukan]

might i ask why are you asking this on a laser forum?
There are better places to ask this kind of stuff.

 

[Limecat]

This is a shot in the dark but you can characterize the roughness of a surface by directing reflected light into a lens which will map spatial frequences at the focal plane of a lens, with smoother surfaces having less spacial components. I haven't tried working out the math but that might be an idea. I could be wrong of course lol

 

[HIMNL9]

So, you was thinking to use lateral illumination with a laser, cause it's monochromatic and can give better results than a common light source ? ..... but probably the "optic factor" still cause you problems, for the same reasons ..... maybe some monochromatic cameras with VERY good optics can be turned in microscopes, but this reduce a lot also your field of vision and focus depth ..... there are already camera microscopes able to take 2000x for biology uses, but their focusing capability on a variable surface can be a nightmare .....

Uhm ..... with micron scale, maybe holographic principles can work (but i'm not sure, i never tried this before, just speculating).

I mean, basically, holograms are produced from the interference patterns generated from 2 different light sources (or, better said, from the same light source divided in 2, so the wavelenght is identical), shooted to an object from 2 different directions ..... maybe the same principle can be used for detect interference schemes generated from objects in the microns scale ? ..... anyone that works in laser field have already experimented with this ?

 

[kiyoukan]

why not just use a usb microscope with 3 lasers spaced equally around the head at same angle.
they seem to focus quickly.
Almost seem like you want a laser record player but instead of sound you want an image of the surface.

 

[benmwv]

It would help very much if you would specify the item you are trying to detect imperfections on (your secrecy makes me suspicious, stolen diamonds? ). The very simplest way would be to simply take a low power laser pointer (405nm works well) and focus it to a small point on the object a few inches from the apeture then hold up a sheet of paper behind it so that the reflection is visible on the paper. if you focus the dot on the object to 0.1 mm and the spot on the paper is 100mm then it is pretty much a 1000x magnifier. You will of course need a holder for the laser so it wont wobble, and the resulting image wont be 3D but should be adequate to see the difference between the grain of the surface. The surface would also need to be reasonably reflective for this to work, it is really cool to use this to look at the topography of CDs just as long as you dont accidentally use a too powerful laser and corrupt some of the data on it.

 

[Cyparagon]

Does that mean if I focus a laser to a point and place the paper on the other side of the room with NO optic, that the air itself is a magnifier?

That's not quite how it works.

 

[benmwv]

Im not quite sure what you mean.

I guess if you wanted the projection to go long distance you could refocus it but that would be completely unessecary. The size of the projection on the paper behind it is determined by the angle of focus of the laser.

basicly it works like this, the spot on whatever shiny surface where the laser is hitting has small imperfection which when the laser bounces off and gradually become a bigger circle that lands on the paper behind it are easily seen as darker places. these places are darker because the light that should be there if the surface was perfect was bounced of at a different angle because of the tiny imperfections.