Image analysis fraction Area vs IntDen

I have been given some fluo images to analyze, I have run a batch measure over all of them.
I plotted the data (see attachment) and cannot precisely explain what is the most likely cause of the different response for fraction_area vs IntDen across the different groups (EV, KD, OE, SCR). is it possible the images were not collected during the same session? what other technical or non-technical reasons would explain it?
corr_matrix.pdf (199.8 KB)

Hi @edoardo_pedrini, and welcome to the forum!

The measurement parameters are explained in the user guide, in particular:

Integrated density The sum of the values of the pixels in the image or selection.

Area fraction […] For non-thresholded images is the percentage of non-zero pixels. Uses the heading %Area .

Your results can be explained by different raw intensities in the image. As an illustrative example, take these three images:

image image image

… and compare their measurement values:

IntDen %Area RawIntDen
1 119764 6.320 119764
2 39184 3.160 39184
3 80580 3.160 80580

Note that image 2 and 3, while having the same amount of non-zero pixels, have different integrated density.