Okay, but I'll mention once again my general thought - to what exact wavelenght this method is sensitive? If it's about 500-800 then it will be completely useless!
Never forget that we are dealing with full spectrum light source! If it would be narrow spectrum say 265 UV LED, then such a method could be awesome.
In our exact case (B9 printer with B9R resin) we need to know not only light intensity but also a wavelenght for that "map". Moreover I expect such a intensity distribution to be different acros the different wavelenghts.
If we are talking about B9R resin then 350-550 nm is of our interest, if others like MJR, FTD and so on, then 300-450 nm.
I can't see any better light mask testing method then one which we "got under our nose" (local idiom here - means something that is close, near, always on the view, another words - hidden on the light). The photopolymers themself! They got perfect spectral response to... their own spectral response!
One could print array of pillars with slightly lowered settings and see where he have lower and higher power IN THE DESIRED SPECTRUM of resin absorbance. But then we need on the software level to adjust accordingly. Now we have "hot spot" method. That is cool, but possbily something like dot matrix could be better, more intuitive and easier.