How to improve defects detection on surface with unequal contrast?

Hello everybody, I am new in image processing...

I am working on a MV project and I have to find all the anomalies on the images that look familiar as this one:

As you can see from picture, contrast of anomalies (defects) is not so good. I have to improve contrast of them so that the thresholding after will be easy. Any suggestions how can I do that?