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

Question

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.

Answer 1

Did you try running the computer from a copy/clone backup of the system?
How about a testing with second Admin user account? Or, secure erase &
a complete reinstall of the system from scratch? (With full backup in archive.)
Check to see if there is a weak or dead PRAM battery on logic board?
In your other similar thread, there were issues in replying to there based on
problems with this site. A different reply was botched, and lost, so there it
is. Then a followup didn't appear. But it may still, though not what I wanted.
There still can be a hardware problem, not matter what someone else may
have done with the settings while using the computer. Several builds of the
early 20-24" Intel-based iMac had various graphic issues. Hardware failure.
So that is unfortunate that even after significant time attempting to resolve
the issue or the symptom, you haven't gained much traction. The time may
have arrived to consider my suggestion to seek Apple trained specialists
in an AASP, to look into this matter from a diagnostic standpoint. And be
sure you have complete backup (clone, plus timemachine, etc) beforehand.
Good luck & happy computing!