That's quite simple, you just need to remove the polarized layer from your computer screen and use new polarized sheets to make polarized glasses out of 3D glasses for example.
Only the person wearing the special glasses can see what's on screen, others will just see a blank monitor.
Better try that first on a used monitor you don't care about though...
Do you have an old LCD monitor at home? You can turn it into a “spy” monitor. While everybody sees it as a monitor with white screen, you will have special “magic” glasses to make it work. Here is an instruction on how to make a privacy monitor out of an LCD monitor.
Don't forget to watch video with this awesome creation.
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