The Magic Window is a portable "S-Band" vision window used by S.H.I.E.L.D. The Magic Window is lightweight, flexible and can be rolled into a tube for easy transport.
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Contrary to popular belief, the Magic Window does not use X-Ray technology, but rather has S-band receptors embedded within the sheet itself, somewhat like a radar system.
It is placed against a surface, where it can then be used to see what is on the other side of that surface, though only so much as a thermal sensor would be able to surmise, and not the actual identity of any individuals on the other side.