27" not bright enough

Recently, I bought a 27" LED Cinemy Display (Model MC007ZM/A) for my MacBook Pro (Model 5,1/Late 2008). It is connected over the mini display port.

But even when I close the drapes, the new display is so dim, that it is really exhausting for my eyes to work with it. It is considerably dimmer than my old internal notebook display. Approximately the same brightness as if I turn down my laptop's brightness 6 steps (with the F1 key). Of course, I have already done a calibration and turned the brightness to maximum in System Preferences / Displays and turned off auto adjustment (which also does not help).

Is there any way to make it brighter? I'm getting a headake. 😟

Thanks

Jan