IMac screen dims itself automatically; all attempted fixes fail; how can normal brightness be restored?

Question

Machine and OS:
2007 white plastic iMac with 20" screen and 2 GHz Intel Core Duo, running OSX 10.6.8.
The problem:
The screen dims itself for no known reason. It is especially noticeable when waking from sleep: for the first second or two after waking up, the screen is at normal brightness -- but then after a second the computer seems to "remember" some setting or command, and the screen unquestionably dims itself to be about 20% or 25% darker.
Because the screen for a brief moment after waking is at full brightness, that proves to me that there is nothing physically "broken" about the display that prevents it from illuminating fully; it could be at full brightness if it "wanted to," but some kind of software or firmware glitch is telling it to be dimmer.
The fixes I have attempted so far -- ALL of which have FAILED (so please don't recommend them again):
- Reset the NVRAM or PRAM by holding down Command-option-P-R upon startup. For some reason this technique doesn't cause the second chime, so it doesn't appear to be zapping the NVRAM or PRAM or whatever, and consequently has no effect on the dimness, which persists unchanged after restarting.
- Reset the NVRAM using the terminal command ' sudo nvram boot-args=”-p -r” ', as recommended on this page: https://discussions.apple.com/thread/2172681?start=0&tstart=0. I can't tell if this really did reset the NVRAM (it doesn't cause a second chime upon restart), so I'll just have to assume that it "worked" -- but except for one instance, it had no effect on the screen dimness. (That one instance went like this: after issuing the Terminal command and restarting, the screen did go back to full brightness -- but only until the first sleep. After waking from that sleep, the screen once again dimmed itself as before. And no, I have not been able to repeat this momentary "success"; on all subsequent attempts following the exact same steps, no brightness has ever returned at all since.)
- Reset the System Management Controller (SMC) as recommended on this page: http://support.apple.com/kb/HT3964 . Basically this just entailed shutting down the computer waiting a few seconds, then unplugging it, waiting a few more seconds, then plugging it back in, then waiting a few more seconds, and then finally starting it up again. Result? No change: screen still dims itself.
- Pressed the "brighter" and "dimmer" keys on the keyboard; those do make the brightness go up and down, but the top brightness setting is much dimmer than it should be (and used to be). All the way to brightest setting = still noticeably dimmer than normal.
- Used  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.
- Readjusted the "Calibration" in the System Preferences > Displays, which had no noticeable effect; 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 significant 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.
- Tried going to System Preferences > Universal Access > Seeing and adjusting the "Enhance Contrast" slider and trying out "Black on White" vs. "use Greyscale"; none of that helped, and did not seem to be relevant to the problem.
- Tried going to System Preferences > Energy Saver, and clicking the "Automatically reduce the brightness of the display before display sleep" check box on (and then closing the preference pane) and then off again (following the advice at http://support.apple.com/kb/TS2309?viewlocale=en_US) -- but it had absolutely no effect whatsoever on the brightness level.
Final note: The keyboard and mouse are both Apple products and work perfectly, and the machine has no other performance issues. Just the dimming, which started a few weeks ago inexplicably and without warning.
I can't imagine what is causing this problem, and I've absolutely run out of ideas about how to fix it. One thought I had was that maybe the screen is dimming itself due to detected ambient illumination, but I'm not sure 2007 iMacs even have that feature, and also the screen remains dim whether the room is dark or bright. Maybe there is an ambient light detector somewhere that is malfunctioning?
Help! How can I solve this? The screen is not so dim as to be unusable, but it's annoying enough that I'll have to get a replacement machine if this is not fixed eventually.

Answer 1

Does an external display also act in a similar manner as the internal one?
Some earlier Intel-based iMacs had graphic processor card issues; not sure what
the symptoms of the majority of them were, but some had odd patterns, others
went black, and some would range from OK to totally unusable. Among others.
So there is a chance the graphic processor may be going out; or just the circuitry
that controls the backlight. An inverter may be weak or other circuit to the display
could be wearing out.
http://www.apple.com/retail/geniusbar/
If you have access to an Apple Store with Genius bar, they can perform some kinds
of diagnostic tests there even though the computer is out-of-warranty; that may be
of help to narrow down the cause of the symptoms you've noticed. They may not be
able to repair it, however, at an Apple Store; as it is rather old by today's standard.
An Apple Authorized Service Provider could do repairs and testing on older models
the Apple Store's Genius and other may not be set up to handle due to vintage.
Find an Apple Authorized Service Provider
Visit an Apple Retail Store
There is a country locator page to help find either of the above in regions outside
of the US & Canada, not sure how the above links would work; and it appears my
bookmarks to the country locator page may not be accessible now to post here.
You may be able to get an idea about what kinds of parts support the display
function by name by looking into an iFixit.com repair guide for your iMac series.
Hopefully the parts supporting the display are the reason it is dimming down, as
a graphic processor failure likely would exhibit other behaviors than just dimming.
PS: I see you've added content to your thread after I'd started working on this
& the answer likely is a hardware repair; professional testing is worth the time
and you seem to have at least one other thread on the same topic....
Good luck & happy computing!
edited 2x