Marble slab

[SimonAldred]

You might want to consider installing a light baffle below the line. This would act has a light absorber and give you a low level uniform background. There are some extremely absorbent light absorbing materials on the market - relatively inexpensive.

[desalando]

Hello Simon,at the moment we can't install that. 
The background ist relatively good. I think some image processing makes the work there. Any ideas?

[SimonAldred]

You could create a mask from the image and use this to extract just the region of interest you are looking at. I've not had chance to look at your script but I might start with something like this.

[desalando]

Hello Simon,
We have always different marbles. Size and color are changing:

Some marbles natural holes on them. We want to detect these holes. too So, i don't want use "fill holes"
Marble slab with holes

[BruceAmmons]

Since it is a line scan camera, the background image is fairly constant.  You could create a background image with no marble, then subtract that from the marble image.  This will cancel out the background and make it almost zero.  You can then do a manual threshold using a fairly small value.  This would keep the holes and other features of the marble.

Bruce

[desalando]

Hello Simon,
"Line profiling" in your script and suggestion of using mask gave me ideas to solve my problem. Thank you.