Some times acpi can't adjust the brightness.
Next thing we need to try is, pass the following parameter to Linux kernel from grub configuration file.
Now if you are lucky, you can control brightness using the same old technique.
Current brightness is provided by file /sys/class/backlight/intel_backlight/actual_brightness.
Collect current brightness.
Max brightness is provided by file /sys/class/backlight/intel_backlight/max_brightness.
Now write a number, from 0 to MAX, to the file /sys/class/backlight/intel_backlight/brightness using echo.
Thanks to https://wiki.ubuntu.com/Kernel/Debugging/Backlight