Screen backlight does not turn on during bootup, but turns on when accessing recovery partition I have a late-2011 15" MacBook Pro running Mavericks.  I normally don't reboot my computer and can leave it logged in for days on end.  I decided to reboot my computer and when it was booting, the screen backlight would not turn on, although I could faintly see the login screen where I could type my password.

I tried resetting the SMC, but that didn't do anything.

I also tried resetting the PRAM, but I'm guessing because I have a firmware password and the computer is FileVault-encrypted, instead of actually resetting the PRAM, I have the prompt to type in my firmware password (also really faint since the backlight isn't on).  If I type in my password, it boots into the recovery partition and the display backlight finally turns on. However upon reboot, the backlight will stop working again.

I tried using Disk Utility to check for any permissions/disk errors but didn't find any issues.

I haven't tried logging in with the faint login screen, only because I can't find my mouse cursor to select my account to login.  I've tried rebooting with both the power cable connected and disconnected.

How can I get my screen backlight to turn back on again?
I had the same problem. The dimmed screen happened during the reboot after turning on file vault. I was able to solve the problem by going into a darkroom. This allowed me to see the cursor and to log in to an account. Once I logged in the back light came on and everything was fine. 

I had first tried to solve the problem by zapping the pram. This had no effect.