Currently, I am trying to change the brightness of my Ubuntu system. This is my first time using Ubuntu. I couldn't change the brightness using the fn key. This is because my keyboard don't have the fn key. So i found out through the online source that it is possible to change the brightness my modifying the value of the acpi_video0/brightness file. I tried changing it. It seems that the value can be changed but in the screen has no effect. I have also tried changing commands in the GRUB file but its still the same. My Ubuntu system is too dim now. I need to increase it somehow.

   description: VGA compatible controller
   product: Intel Corporation
   vendor: Intel Corporation
   physical id: 2
   bus info: pci@0000:00:02.0
   version: 06
   width: 64 bits
   clock: 33MHz
   capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
   configuration: driver=i915_bpo latency=0
   resources: irq:324 memory:de000000-deffffff memory:c0000000-cfffffff ioport:f000(size=64)
  • Have you tried changing brightness from System Settings ? Because there is option for that. – Sergiy Kolodyazhnyy Aug 15 '16 at 3:03
  • Yes i have tried. but there is no effect though. – anonymous5671 Aug 15 '16 at 3:04
  • How about these things ? wiki.ubuntu.com/Kernel/Debugging/Backlight – Sergiy Kolodyazhnyy Aug 15 '16 at 3:07
  • Have you tried acpi_backlight=vendor in GRUB ? – Sergiy Kolodyazhnyy Aug 15 '16 at 3:08
  • yes when i tried acpi_backlight=vendor the backlight folder is empty. I even tried acpi_backlight=video acpi_backlight=vendor acpi_backlight=native but its still the same. – anonymous5671 Aug 15 '16 at 3:10

When we check the path /sys/class/backlight/, it will show us the folder name of ACPI containing /sys/class/backlight/acpi_video0/brightness file. However, after the change of kernel development, this backlight mode on kernel has been disappeared like this.

root@joe-UBTPC:/home/jarurote# ls -al /sys/class/backlight/
total 0
drwxr-xr-x  2 root root 0 พ.ค.  31 17:02 .
drwxr-xr-x 66 root root 0 พ.ค.  31 17:02 ..


So, we require xrandr command to view and set the screen on Ubuntu 18.

root@joe-UBTPC:/home/jarurote# xrandr
Screen 0: minimum 8 x 8, current 1600 x 900, maximum 32767 x 32767
**eDP1** connected primary 1600x900+0+0 (normal left inverted right x axis y axis) 440mm x 250mm
   1600x900      60.00*+  59.82  
   1440x900      59.89  
   1400x900      59.96    59.88  
   1368x768      60.00    59.88    59.85  
   1360x768      59.80    59.96  
   1280x800      59.81    59.91  
   1152x864      60.00  
   1280x720      59.86    60.00    59.74  
   1024x768      60.00  
   1024x576      60.00    59.90    59.82  
   960x540       60.00    59.63    59.82  
   800x600       60.32    56.25  
   864x486       60.00    59.92    59.57  
   800x450       60.00  
   640x480       59.94  
   720x405       59.51    60.00    58.99  
   640x360       59.84    59.32    60.00  

**VGA1** disconnected (normal left inverted right x axis y axis)

**VIRTUAL1** disconnected (normal left inverted right x axis y axis)


See that the 3rd line of output command shows the output screen of "eDP1".

To adjust the brightness of the screen, we can define the brightness of the screen in a range of [0.3, 1.0]. Suppose we need the brightness of 0.4, the command of xrandr should be like this.

xrandr --output eDP1 --brightness .4

That is it.

PS: Don't adjust the brightness value higher than 1.

  • This depends on the hardware an the kernel drivers (mine: Thinkpad X200T, kernel 5.0-pf6). I have /sys/class/backlight/intel_backlight/brightness (values from 0 to 2655570) and /sys/class/backlight/thinkpad_screen/brightness (values from 0 to 16). Both can control the brightness, but remember their number independently (when I write to one of them, it set's the physical brightness, but the number of the other does not change). Additionally, I can work via setpci on PCI device 00:02.0 register f4. Writing there dosn't affect the /sys/-files, too, but the other way round affects. – Golar Ramblar May 31 at 12:10
  • From man xrandr: "--brightness brightness: Multiply the gamma values on the crtc currently attached to the output to specified floating value. Useful for overly bright or overly dim outputs. However, this is a software only modification". -> xrandr does not change the backlight! (Testet and confirmed on my machine). Use xbacklight = <percentage>. – Golar Ramblar May 31 at 12:12
  • Thank you for comments. However, that can not be applicable in my PC. It doesn't show anything on a folder of /sys/class/backlight/ as given in my answer. Therefore, I also look for an alternative way for solving the problem by xrandr on the x-screen. – Jarurote Tippayachai Jun 4 at 8:53
  • Noted my Ubuntu also uses the kernel version of 4.15.0-50-generic SMP x86_64 on HP Compaq Pro 4300. Although I have done following [askubuntu.com/questions/1034305/…, my PC does not work also. – Jarurote Tippayachai Jun 4 at 9:02


You might first want to try xbacklight:

man xbacklight:

xbacklight - adjust backlight brightness using RandR extension

and set it e.g. via

  • xbacklight = 80 (set to 80%).

More basic ways:

Then, there are more basic ways to do it:

On my machine (Thinkpad X200T, kernel 5.0-pf6), I have

  • /sys/class/backlight/intel_backlight/brightness (values from 0 to 2655570) and
  • /sys/class/backlight/thinkpad_screen/brightness (values from 0 to 16).

Both can control the brightness, but remember their number independently (when I write to one of them, it set's the physical brightness, but the number of the other does not change).

Additionally, I can work with writing directly to PCI-registers with setpci on PCI device 00:02.0, register f4:

  • setpci -s 00:02.0 f4.b=<brightness>

(be careful when using setpci. You can crash your system when writing to the wrong places! It is machine dependent what you need to to.)

Writing there also does not affect the readout of the /sys/clas/backlight/*/brightness-files, but writing to the /sys/clas/backlight/*/brightness-files does change the readout of setpci -s 00:02.0 f4.b.

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