Brightness problem after sleep

I just installed Xubuntu LTS Xenial Xerus on my MacBook Pro 1,2 with a 2.16 GHz Intel Core Duo processor and 2GB 667 MHz DDR2 SDRAM The brightness is fine at startup and during use, but after sleep the screen is very dark; I could barely see the screen asking me to enter my password, couldn't read anything else, had to use the power button to shut down. How can I set my computer so that it wakes up with reasonable screen brightness? Not sure if this goes in the Apple forum or here; the computer wakes up brightly when running Mac OS X on the other partition.