RMS contrast of image interpretation
You defined to compute an RMS. I don't know why you choose it, but OK. How can we probe it as a contrast measure?  If you are considering images with positive pixel values, and consider that multiplying them by a scale factor $a&gt;1$, you extend the range, hence the contrast, or if  $a&gt;1$, you shrink the range, hence the contrast again. How does the RMS behave? A quick computation should give you an answer.

Indeed, those are hypotheses. Honestly, I would not consider RMS as a proper measure, as it does change with a value offset, which to be is substantially iso-constrast.