First of all you need to find out which ACPI interface controls the brighntess, for that run this command in the terminal

grep acpi_video /var/log/Xorg.0.log

Above command should display something like this

[
7.385] (--) intel(0): Found backlight control interface acpi_videoX

Now find out what is your ACPI interface. It may be acpi_video0" or "acpi_video1"

If above command does not display anything then your ACPI is intel_backlight. You can use it as your ACPI for nest steps.

Now set your laptop brighness using brightness keys whether it does't work, to aspect ed brightness you use. Now you have to find out actual brightness value which you set. For that enter this command in terminal cat

/sys/class/backlight/acpi_videoX/brightness

You need to replace your laptop ACPI to bolded place.

Now you will see the actual brightness level. Remember it for next steps.

Now we will have to create a file at /etc/init/ for configurations.

sudo gedit /etc/init/fixbrightness.conf

Now paste this to opend text file without quotes.

"description "Sets brightness after graphics device is loaded"

start on graphics-device-added
task
exec /bin/echo BRIGHTNESS_VALUE> /sys/class/backlight/acpi_videoX/brightness"

You have to replace BRIGHTNESS_VALUE with actual brightness you get and acpi_videoX with your laptop's ACPI. Save the file and reboot the laptop and find out weather the problem is fixed. Thank you.