Marble slab

[desalando]

Hello Nelson,
We need color image of marble slab. The Customer wants color images (to show his customers his marble slabs). We need vision analysis on the marble slab after, too. Yes, with line backlight we would have better edge information but no surface information. Marble slab would be completely dark.

[SimonAldred]

That's why you need a light line above the marble pointing down. Use different colours for below and above and your line scan camera will be able to get all the information you need. So you will have a transmission channel (say red) and a reflected bright field channel (say green).

Simon 

[Nelson-Bridwell]

There are many lighting options.  The simplest might be white LED barlights, shining from above, but angled so that they do not point down directly into the gap between the conveyor.  That way, the background would be black, and the marble would be reasonably bright enough to be able to monitor the color of the marble with a color camera.

[SimonAldred]

Something like this?

[BruceAmmons]

I would think that lighting setup would work.  You could use a white light on top to get good color images of the marble.  The light on the bottom could be red, which is a color that normally would not appear in marble.  It would be simple to separate the red edges from the marble itself.

The red light on the bottom probably wouldn't be necessary, though, unless the background is not black enough.  With the white light at an angle, the background should be fairly dark and easy to separate from the marble.  I would try it without the bottom light first, then add the light if there are issues. The bottom light would also get covered with marble dust, so that would be an issue to deal with.  Probably a lot easier without it.

Bruce

[SimonAldred]

Yes, dust could be a factor I agree. As Bruce mentions - a dark background below the glass might be needed to prevent unwanted scatter / reflections from the floor. There are some very good absorbing materials on the market. Needless to say the dark background may need cleaning every once in a while. 

[desalando]

Hello Bruce,
We have moved the camera as i wrote before. Now it looks good. I have not done white balancing yet.  I'll do white balancing, too. 

[BruceAmmons]

Wow, that was a great solution.  This is now a very simple task to separate the marble from the background.

Bruce

[SimonAldred]

Yes, the image quality looks good and the background is extremely dark. Just what you need!

[desalando]

Hallo Bruce,

I have a question. After we moved the camera we have a better image quality and background is extremely dark. But background is not completely dark. If you look carefully, you can see some parts on background that not total black are (contrast enhanced image_01)
I need to separate marble from this background. To do that i use manual thresholding (thresholding bright objects, see attached script). I can't find an auto thresholding methode for this. 

Manual thresholding doesn't always work for me. Sometimes i have to change lower value for manual thresholding. I don't want this.

Is there a better way to that? I think i might use some gray morphology before thresholding.