'Inverting' an intensity variation

This is somewhat hard for me to explain, but what I'm looking for is a technique to correct a fixed-pattern intensity variation in an image. The pattern comes from (say) a projection system and is not inherent to the actual image.

I can produce a 90%-average white frame that exposes these variations, which amount to perhaps as much as 10%, so that the range of values will be from about 85% to 95% in different areas. I then need to make an inverted mask / alpha that undoes those variations. So if applied to the original white frame it would result in a pure image with no variations. Once created, the mask will be applied externally, not within a graphics program.

If I've managed to convey that, how would you approach the creating of this transparency / alpha mask, given the original white frame with variations? My tool of choice is PSP 9 (no laughs), but a solution using PS would be fine.