It could be a hardware issue... a failing/bad screen inverter. This is a small, narrow printed circuit board mounted to the bottom of the LCD panel which inverts the DC power provided to the laptop to AC power which the Cold Cathode Fluorescent bulb requires. This of course, would depend on whether or not your laptop had an LED lit or CCFL lit screen. You only provided the make of the laptop, and not the model, so there isn't any way to determine which type of illumination is used here. But... there is a great chance that your laptop has a CCFL bulb and not LED... which would point to the inverter as being a possibility.
Now. If you have an inverter, it could be the inverter, or the power connection to the inverter, or the power connection to the bulb, or even the bulb itself (As in the case of an HP DV2000 I had to repair once, where the issue was a burnt connection at the bulb itself).
So... what do you do? First, determine if it is hardware or software. Download ANY LiveCD Operating system. You could download a Linux LiveCD (Ubuntu, Linux Mint... even Parted Magic would work), burn a disc of the OS, and boot to the disc. Once in that OS, wait and see what the deal is. Does your screen start dimming the same way it did with your install of Windows 7? You know... Unpredictably? If the issue is a hardware based problem, it will happen regardless of what OS you are using... which is the point of this diagnostic step. This would prove it wasn't a power settings issue.
If it is a hardware issue... then it gets to be a bit more dicey. Since the laptop is less than a year old, if you bought it new, it is still under warranty, and should now be sent back to the manufacturer for repair. So... you would call Cryo tech support again, tell them the new diagnostic step you just followed (booting to a LiveCD Operating System and seeing the problem repeat), inform them you strongly believe it is a hardware problem, and arrange to have it shipped back.
If it is NOT a hardware issue, meaning that the backlight works just fine and without issue no matter how long you use your LiveCD Operating System, then reinstall Windows 7 fresh, and make sure your power settings are as you want them.
It is important to note, that there is a non-screen related possibility here. Power. If your power adapter port is damaged at all (where the power cord plugs into the laptop), where any slight movement momentarily interrupts power, this would also cause the screen to dim when it went from the mains to the battery.... and then to go back to being bright again once power was restored to the mains. This could also be caused by a failing power adapter. You'd need a multi-meter to test your power adapter to see if it is putting out a constant supply of power. You can see how much wiggle room you have when you plug the power adapter into the laptop... to also see if this affects whether or not you have a constant supply of power.
If this issue happened due to driver corruption after update, try performing a system restore to an earlier restore point before the issue encountered. In order to perform system restore,in the search box, type System Restore, and then, in the list of results, click System Restore. Administrator permission required If you're prompted for an administrator password or confirmation, type the password or provide confirmation. Then select a earlier restore point and proceed to restore.
Best Answer
From Samsung R580, Ubuntu 10.04 and Brightness control, not written for Mint but may still work :
Edit the
/etc/X11/xorg.conffile and add the RegistryDwords line (only that line) to the Device section: