Default Screen Galaxy S5

Hi everyone, just a quick question. When my screen is set to +3 brightness or so I can see that in some menus (for example settings) the colour at the top of the screen is darker than on the bottom!! Please note, you can only see this when the screen is very bright. The more you are looking down on the screen the brighter it gets. When I look at Internet and the backcolour is white it also is darker at the top than on the bottom of the screen.

Is this normal or do I have a defective screen?

Gr. Marc