Brightness goes down in Lenovo Thinkpad Z61t?

I have a llaptop Lenovo Thinkpad Z61t and lately i have a problem.
When working in my llaptop brightness goes down and screen goes dark unexpectly.
I have to restart my llaptop 3 or more times in order to work again.
Brightness goes too low but if i put a light in my screen i can see the desktop. I try to raise brightness with keyboard commands but it don't work.
Could someone help me please.

Added (1). I thought this was a Windows problem but i updated from windows 7 to 8.1 (with a flashlight) but it's still the same.

Guest

The backlight CCFL tube or its inverter has failed. Seek repair or search on YouTube how to replace. Parts must match, not difficult for most people.
"* Dark screen may be caused by "Inverter or Backlight lamp", Inverter is Much easier to replace, we advise you to confirm or replace the inverter first.
* Replace with "known good inverter" before attempting backlight lamp." http://www.amazon.com/...B00K8S8P6A

Starrysky