Comment 7 for bug 1411514

I can confirm this on Vivid with the proprietary nvidia driver (version 346) installed. With switchable graphics disabled, I have an acpi_video0 backlight that does nothing. That is, the contents of /sys/class/backlight/acpi_video0/backlight changes, but the screen's brightness does not. If I enable switchable graphics, then I have only a /sys/class/backlight/intel_backlight, and it works.