External display gets brighter when Macbook pro lid is closed

Hello all,

I've been trying forever to increase the brightness of my external display when connected to my Macbook pro 15".

Funny fact, I realize that as soon as I close the lid of the Macbook pro and thus only use the external display, it actually gets much brighter.

Any idea on how to fix this so that I can benefit the same full brightness when using both displays?

Thanks