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i have just replaced the LCD on an ipad 3rd gen. I have completed the repair and now i have noticed that one side is darker in screen brightness to the other side. Why is this?

How much darker? Can’t read it dark or just annoyingly dark? If its just a bit darker, it could be as simple as a screw or something else is stuck under the LCD causing it to look funny. If its noticeably darker, then you blew one of the backlight filters. There are 2, one for each side of the screen. Its very common to blow those filters, especially if you used metal tools to disconnect the LCD or digitizer.