Laptop Screen Dimming
I use a Thinkpad T400 laptop. One of the installed distros is CentOS 7. I use the LightDM login manager and MATE 1.10.
Often, but not not always, and always unpredicatably, when logging out to the login manager the screen will dim considerably. The dimming is significant and I struggle viewing the screen.
The only cure, and this does not always succeed is:
echo 2408475 > /sys/class/backlight/intel_backlight/brightness
I have to toggle to an alternate console to execute that command while barely reading the screen.
I thought the problem was the backlight service, but disabling and masking the service made no difference. Then I thought perhaps a conflict with redshift but likewise, disabling redshift made no difference.
Likewise with whether the laptop was connected to AC or battery.
I am unable to repeat the problem in a consistent manner.
I have been searching the web for any kind of tidbit.
I have Fedora 22 on the laptop with a similar configuration and never experience the dimming.
Otherwise CentOS is a solid, stable desktop.
I embedded the echo command into the Light DM post session script and that seems to help, but I have found no solution or root cause.