Grain Structure

[Klemmensen, Cory]

Hi All,
I am new to ImageJ and image analysis in general.  I am trying to compare grain structure of various materials using SEM micrographs (greyscale).  I would like to import micrographs, set a threshold to group grains by grey-level, and then be able to determine the grain sizes in each group.  Any help very much appreciated.

Thanks,
Cory