It depends how different your backgrounds are and how well they respond to equalization. Here, I've picked a background where I know what color it should be: white.
Then, I created a curves adjustment layer, pointed the white-point eyedropper at the location that should be white, and clicked.
(Excuse the terrible circle and arrow art)
This image didn't respond badly to the change, but if you don't have an absolute, you may need to open up the info panel, note the settings at a given location, and then for each succeeding image, tweak until you get the same readings. If you note that the tweaks are very similar for each image, then save the curve and load it into each successive file and edit to taste.
By the way, doing this adjustment in Adobe Camera RAW may produce better results if you are shooting RAW simply because you are working with more data and the decision to change the luminance is done against more bits, reducing the probability of banding or other nasty artifacts.