Thank you so much for your reply.
Actually the image seen above the white spots are grains and the black spts are voids, Its basically Saccaing electron microscopic image .White spots are shown in those area where the electron repulse back. While the black because of voids in the sample they passes by and i guess hit the background.
Here now i want to calculate the voids ,and grains in these sample images , their number particle analysis and want to calculate the porosity of the voids black spots.For which i want to have a good thershoulding and each sector seperated from each other so it can give me a good idea..
However as you can see in the images the black spots have some thin layer almost same intensity as white and in some region they are very dark which is fine. I want to seperate them and then do several stuff with it,but my thershoulding in this case i guess need to be perfectly fine to do the analysis. which i am having big time issue as i am new to iamge j. These are not only the image i have more 50 60 images to deal with.
Different days sample whos voids number shape changes with time to time.