For further processing (detecting circle for example), I need to convert this kind of pictures (see below) to black and white ones. I have tried to use im2bw playing with the level but I cannot make it work properly. I have also tried to adjust contrast and brightness beforehand in imtool without success. Moreover, I would need to do this step in a (semi-)automatic way. Any hint would be warmly welcome.
I am sure I am missing something as it must be a classical problem but I am a newbie in image processing and can't find my way through...