persistent lcd dimming issue when using external monitor

i have mid 2009 15'' mbp running lion hooked 23'' samsung monitor samsung monitor primary monitor , mbp 15'' display secondary. i've been experiencing dimming issues on 15'' when using both screens concurrently. i'll have word doc on 15'' , browser , ppt open on 23'' in , 15'' monitor dim @ random. can turn brightness function keys minutes later auto dim , i'm finding myself fighting dimming of screen. i've checked in preferences , don't think i've hit settings causing this.

ideas on how fix this? getting rather annoying...

 

is happening while on battery? if so: preferences --> energy saver --> uncheck "slightly dim display while using power source"

 

Forums Macs Notebooks MacBook Pro

• iPhone
• Mac OS & System Software
• iPad
• Apple Watch
• Notebooks
• iTunes
• Apple ID
• iCloud
• Desktop Computers
• Apple Music
• Professional Applications
• iPod
• iWork
• Apple TV
• iLife
• Wireless