[ubuntu] back light in "Fujitsu-Siemens Esprimo M9400" remains on its minimum level

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http://ubuntuforums.org – Hi, I have installed Ubuntu 12.10 on my Fujitsu-Siemens laptop but I can`t increase the back light via Fn key function. Only when Ubuntu is loading "Fn" works correctly to decrease or increase LCD light but when I restart ubuntu, again back light remains on its minimum level and I have a dark screen! Help me please! (Hardware)