thm@thm-lt:/$ cd /sys/class/backlight/
thm@thm-lt:/sys/class/backlight$ ll
total 0
drwxr-xr-x  2 root root 0 Nov 22 16:57 ./
drwxr-xr-x 54 root root 0 Nov 22 16:57 ../
lrwxrwxrwx  1 root root 0 Nov 22 16:57 acpi_video0 -> ../../devices/pci0000:00/0000:00:02.0/backlight/acpi_video0/
lrwxrwxrwx  1 root root 0 Nov 22 16:57 intel_backlight -> ../../devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight/
thm@thm-lt:/sys/class/backlight$ cd intel_backlight
thm@thm-lt:/sys/class/backlight/intel_backlight$ sudo -s
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# ll
total 0
drwxr-xr-x 3 root root    0 Nov 22 16:57 ./
drwxr-xr-x 4 root root    0 Nov 22 16:57 ../
-r--r--r-- 1 root root 4096 Nov 23 19:22 actual_brightness
-rw-r--r-- 1 root root 4096 Nov 23 19:22 bl_power
-rw-r--r-- 1 root root 4096 Nov 23 19:24 brightness
lrwxrwxrwx 1 root root    0 Nov 23 19:22 device -> ../../card0-LVDS-1/
-r--r--r-- 1 root root 4096 Nov 23 19:22 max_brightness
drwxr-xr-x 2 root root    0 Nov 23 19:21 power/
lrwxrwxrwx 1 root root    0 Nov 22 16:57 subsystem -> ../../../../../../../class/backlight/
-r--r--r-- 1 root root 4096 Nov 23 19:22 type
-rw-r--r-- 1 root root 4096 Nov 22 16:57 uevent
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# cat max_brightness 
976
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# cat actual_brightness 
976
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# echo 500 > brightness 
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# cat actual_brightness 
500
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# echo 976 > brightness 
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight# cat actual_brightness 
976
root@thm-lt:/sys/devices/pci0000:00/0000:00:02.0/drm/card0/card0-LVDS-1/intel_backlight#