First, I'd try using a telecentric lens. You are not using one in this image and that is why the edges are more contrasty near the optic axis and blurrier away from it. A telecentric lens will eliminate that problem.
Next, try different backgrounds - both black and white and see which gives you greater contrast.
In addition to that, try different lighting geometries. What do the cracks look like with broad overheard lighting? How about line sources of light (LED light strips/bars) down inthe plane of the glass pointing inward? How about two point sources of light at 45 degree angles shining down at a 45 degree angle?
If all goes well you'll have either dark or bright cracks against a uniform background and you can simply use thresholding. But don't forget to take a uniform image of the background so that you can divide out the background to flatten the image and get rid of lens shading and light non-uniformities.
As you have it now, I think no matter what you do you won't have satisfactory results. There's just no way some/most of the cracks or solid domains between them can be resolved. Of course some of the image you can get them, but not over the whole image.
So fix your image capture conditions and then come back with your improved image. If you don't know how, then contact a reputable machine vision vendor (not just someone who sells parts but someone who actually builds turnkey systems).
