The problem in detail
The brightness hotkeys works well in increasing or decreasing the brightness, but if the brightness is less than 100%, after a reboot the brightness level and actual backlight will mismatch; where I mean the actual backlight is not at its maximum while the brightness level shows 100%. To avoid such problem I should set the brightness level at 100% every time before shutdown.
I’ve tried all suggested answers for brightness problems, including
acpi_backlight in grub and
echo values in rc.local, but no change. Recently I found that pressing brightness hotkeys while brightness settings window is open sometimes make the brightness slider disappear!
I’ve had this problem after 11.04, I mean in either 11.10, 12.04 and 12.10, and saw in various netbooks and laptops. I guessed it should be related to unknown graphics driver; because I had a known driver in 11.04 and no problem, but an unknown one in all computers with this problem:
So I just tried 13.04 via USB and faced the same problem although the graphics was known:
A possible cause
In the folder
/sys/class/backlight there are two links (to folders):
acpi_video0 which contains brightness files with the maximum 15;
intel_backlight which contains brightness files with the maximum 125. I wonder that in this folder
brightnessis 125 (at maximum) but
actual_brightnessis 70, even if the backlight is at its actual maximum.
Does anyone know about this or any possible solution?