Equalizing intensities for a set of images

Hello,
I am currently writing a function that mimics movement of a particle in fluid as a function of time, following mie theory. I was able to successfully obtain images which represents the coordinates of the particle in 3D, per frame. However, the mean intensity values of the frames vary quite a lot from one another.
Any ideas on how to keep this parameter fixed? I would like to include this to my existing function so it can generate each frames (i=1:100), check the intensity (to match that of the first, i=1), and adjust accordingly.

The reason I need them to be the same intensity is to make it easier to normalize the images later (for my research).I tried using

for i=1:size(filelist,1)
        I=imread(['newframe-' num2str(i) '.tif']);
        
        J=imadjust(I,[],[0.2 1]);
        imwrite(J,['intframe-' num2str(i) '.tif'])
Externally (after having the frames already), but still get different intensity values. Any help on how to keep this constant?

Thank you
Regards,
Nitsorn