My experience with Sapphire, coming from a F3 non-sapphire, now owning a F5X+ Sapphire: You need the direction of incoming light to be more perpendicular to the screen. It is as if light at odd angles is blocked.
Strangely, it only affects the incoming light, not outgoing light - you can look at the screen from any angle and get a clear view, as long as the incoming light is coming from the correct angle.