Thanks a lot for your help, I succeed now to assess pixel area with a certain threshold im Gimp.
I Have another question , I tried to do the same process on another software called Image J , in order to find out the most convenient software to do this measurement.
But I don't succeed to determine the number of pixel from a certain range on a certain selection from my image in the Image J software.
For the moment I set up first the threshold , then it colored me all the area from my image, then I go in Analyse Particles and then I can't found out how to seperate my areas to do indepandant calcul ...