Small update: I managed to quite accurately create a brightness mask in LRC. The procedure is as follows:
- Take a shot of a blank B&W negative
- Import with Linear B&W profile (e.g., NegativeLab v2.3 B&W)
- Analyze the variation of brightness (if any) in LRC in the development module by hovering over the frame and looking at the R, G, and B values: They should be the same (since B&W) and constant across the frame
- Crop to see only the blank negative
- Apply a radial mask: My light panel is square, so I will need a circular mask. A rectangular panel will need an oval mask of similar aspect ratio as the panel. Stretch the radial mask across the frame.
- Invert the mask (to add brightness to the frame edges and corners)
- Apply exposure correction (add) while observing both the histogram and the Tone Curves panel. Observe the peak varying in width while dragging the exposure slider back and forth. The sharpest peak occurs when the brightness is constant across the frame. Drag the exposure slider therefore until the peak is as sharp as possible. Done. Verify homogeneity by hovering across the frame again.
- Assign a preset to this mask and copy that over the scanned frames.
The sharpness of the peak can likely be tuned further by playing with the details of the mask but I did not try that. The brightness is homogeneous now in my case. At some point when I find more time, I will drag my light source further out to the 205 mm position that should give only 1% fall-off in the extreme corner of the frame, but I need to find time since this involves a slight mod on my rig.