Most bright spots looks like they're surrounded by a dark spot/ring. Do you want to count those as one of the dark spots? Do you want binary maps of both the dark spots and bright spots independently? Once you " identify bright and dark spots in an image" what do you want to do with them? Identify, to me, means segment, in other words make a binary image of where they are, but you must want something beyond that, like a count or brightness or area fraction or something.
Have you tried to obtain a better image by using optical filters over the lens, or changing the illumination or geometry of the image capture setup?
To find the "skiz" of an image, which you called Voronoi, see