I'm not there so I can't guess what it is. It could be backlight issues (not uncommon with all models) to LCD driver issues. At no point am I telling you it's this or that.
This needs a tech to diagnose and give a repair estimate. Many folk freak when I write it's a 5 year design but it is and it did over 5 years. Get estimates. It's not a setting.
I have a 6 year old Samsung 55" 6400 series LED TV that has developed an intermittent problem where the right half of screens backlighting flickers. I can tell the second I power it on, because the left side has bleed in from corners, and left side stays dark. As picture appears, from the middle over to left side flickers. It is only 6 years old! I tried messing around with ECO sensor on/off as Samsung suggested, and tried their screen diagnostic, but no joy. I replaced all cabling, and went direct into cable box, so as to eliminate the simple stuff. It's the TV, any idea's that deal with this particular model? Heck, I paid $2500 for this thing
Thank you, Larry T.

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