Samsung S9 Anti Spy/Privacy tempered Glass Screen Protectors

They come with a feature where any person viewing your phone from an angle greater than 30 degrees cannot see any content on the screen. They will only see a dark screen thereby preventing them from spying on your phone's content. 

However, it should be noted that Anti spy tempered glasses by design reduce the brightness of the screen because of the PET film applied on the glass. However, the display quality is unaffected.