One-way mirror

[2][3] The glass is coated with, or has been encased within, a thin and almost transparent layer of metal (window film usually containing aluminium).

The result is a mirrored surface that reflects some light and is penetrated by the rest.

This allows a viewer in the dark side to observe the bright room covertly.

When such mirrors are used for one-way observation, the viewing room is kept dark by a darkened curtain or a double door vestibule.

These observation rooms have been used in: Smaller versions are sometimes used in: The same type of mirror, when used in an optical instrument, is called a beam splitter and works on the same principle as a pellicle mirror.