iMac 20" 2007 display dims itself for no reason; "calibration" does not help

Using a 2007 white plastic iMac with 20" screen and 2 GHz Intel Core Duo, running OSX 10.6.8.

Has worked fine since being purchased many years ago.

Recently a user "did something" (she can't remember what) which immediately caused the screen to become a bit dimmer. Whatever action she took, it wasn't something complicated -- she just accidentally struck a single key or clicked on something in the main Finder window. She was not fooling around with System Preferences or anything like that.

Since that moment, I have not been able to successfully get the screen back to full brightness.

Obviously, I have tried pressing the "brighter" and "dimmer" keys on the keyboard, and those do make the brightness go up and down, but the top brightness setting is much dimmer than it used to be. All the way to brightest setting = still noticeably dimmer than normal.

And obviously I have tried using the "brightness" slider in the System Preferences > Displays > Display window. That also does indeed adjust the brightness up and down, but as with the keyboard brightness keys, the highest setting is still too dim.

Readjusting the "Calibration" in the System Preferences > Displays does not help, even when going into Expert mode and adjusting every little aspect of the screen display's default settings. Those all seem to be about color balance anyway, not about brightness. I now have three different new calibration settings, and clicking back and forth between them and the default setting makes no real noticeable difference. Whatever setting I choose, it is still dimmer than it should be, even though in all my attempts at calibration I tried to make it brighter.

Key detail: NEITHER THE SCREEN NOR THE BACKLIGHTING IS BROKEN OR DYSFUNCTIONAL

I know this is true because the following thing happens consistently:

When I put the computer to "sleep," and then wake it from sleep by pressing a random key or moving the mouse, the screen comes back on and momentarily it is at full brightness, but then "remembers" whatever mysterious settings it has, and then visibly dims itself.

So the screen has the physical capability of being fully bright -- it's just that some System setting is telling it to be dim.

Why? How can I change this? It is practically rendering the computer unusable. Help needed ASAP.