I have a Lenovo Z500 notebook with an Intel and Nvidia GPU. I have Ubuntu 18.04 alongside Windows 10 in legacy mode installed.
To get the brightness adjustment keys and slider to work, I have set
GRUB_CMDLINE_LINUX="acpi_backlight=vendor" in the grub file.
However, the brightness is much lower than on Windows.
I have now changed the line to:
GRUB_CMDLINE_LINUX="acpi_osi=Linux acpi_backlight=vendor". That seems to work until shortly before the login screen. The screen is bright like it should be while booting. Then it dims for a second and goes completely black afterwards.
The interesting thing is, in recovery mode, it works fine. The brightness nice and bright. My guess is, that in normal mode, Ubuntu loads something while booting, which breaks it.
I am not 100% sure, but I believe that I did not have the max. brightness issue a few years ago with a lower version of Ubuntu.
I have no idea where to start searching and hope that someone of you could help me.