non-linear brightness control

The brightness control on my Inspiron 1525 is not linear.  Some of the lower settings are brighter than some of the higher settings and the highest setting is dim. The display and lcd inverter were recently replaced  due to a cracked screen.  Has anyone else experienced this issue and if so is there an fix. 

The display has been replaced twice because I initially thought that the replacement display was the problem but the second display from a different manufacturer (part number matches original display) exhibits the same problem.

Thanks