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So I went to the Apple store to compare my phone with the display models. It was roughly 50-50, with half the phones somewhat better and half worse. Looking it the screen at full brightness, it appears that Apple placed the LEDs used for backlighting the screen at the top, the area where it's somewhat dimmer. It appears to be a combination design flaw and quality control issue, with the light not properly diffusing right at the edge. My guess is that it's a result of using a low-quality display. The other thing I noticed is that no two screens had the same white point, although some screens were similar to each other. My screen has a slight reddish hue; one at the store had a horribly blue cast to it.

I should also add that I spoke with a Genius when I first arrived and he said that he and the other Geniuses did not see cause for replacement with my phone, adding that some could not discern a difference between the top and bottom portions of the screen. I thus opted to look at the display models in order to avoid getting needlessly angry and frustrated over a potential non-issue, and reached the conclusion noted above.

I will also add that I'm not not entirely satisfied with the outcome of my little experiment and may attempt an exchange in the future.

Just do the exchange in the future if you get a big scratch, dead pixel, or something or a loose button

my screen is also darker in the top left, annoying but just gonna keep it till after sept 30 to see if theres any changes
 
Just swapped mine out at the genius bar. The genius dude took on look, said "that's not normal" and got me a new one. I did look at most of the display model iPhones and most have a slight dimming at the top, but it was nothing like what my previous phone had.

This new one seems normal. It has a very very slight dimming, but nothing that bothers me.

My old one looked like it had a light brown/gray smudge running down the screen.

Also, there were about 30-40 people in line outside the store waiting to buy iPhones... I guess they got some new ones today.
 
Yea my phone is slightly dimmer in a spot near the earpiece. These forums actually helped show the dimmer spots. By holding the phone sideways while browsing the forums, the grey area where our avatar/user name is shown is a good hue to emphasize the dimness. Just hold the phone sideways with the ear piece on the left and look at a word in a post (a word nearer to the earpiece) and scroll up and down alittle with your thumb or finger, keeping your eye on the word you picked.
Your peripheral should pick up on these dim areas easily if there are any. Make sure they aren't smudges tho
 
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