Screen backlight does not turn on during bootup, but turns on when accessing recovery partition
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.