Grey Lines on Acer A500 Tab

I have purchased 2 Acer A500 32GB Tablets and the main one I have used has developed grey horizontal lines which can be seen on the screen when turned off. The LCD screen which should be normally black has light grey lines.

These resemble word text on a page ie lines  

I am concerned this may become a problem in the future.

Please advise if this is normal or should I seek repairs under warranty

Thanks