Display too bright, even with brightness set to minimum

You display is lightened by two backlights (fluorescent lamps) located at the top and the base of the screen. The backlights are driven by an inverter board who receive the electrical power from the logic board. I'm not a iMac specialist and maybe someone here could post the right answer but I think that the inverter board might send too much power to the backlights resulting in an over lightning issue. IMO the logic board or the inverter board is the culprit. You should bring the iMac to an Apple Store and ask a genius to look at it,