One-way mirror
A one- way mirror (also semitransparent mirror , Venetian mirror , Spanish mirror , spy mirror or police mirror ) is generally understood to be an optical component (such as a glass plate) that largely reflects incoming light from one side and lets through a small portion and the incident light from completely absorbed on the other side. This would make it possible, for example, to observe people on the opposite side while the other person only recognizes his own reflection.
According to today's physical understanding, a medium that brings such properties with it is not possible (see, however, optical isolator ). In order to still obtain glass panes with (largely) these properties, one makes use of a weakness of the human eye (or all optical recording devices), namely that only a limited area of the existing brightness spectrum can be recorded, while other areas appear overexposed or underexposed .
functionality
A real one-way mirror is a float glass that has been coated with a thin metal oxide layer by vacuum cathode atomization (sputtering) . In contrast to a real mirror, this layer is thin enough to allow part of the incident light to be transmitted. This means that only a certain proportion of the light is reflected, the rest penetrates to the other side. Normal glass panels such as window panes also have this beam-splitting property, but not with the necessary ratio of reflection and transmission.
One-way mirrors, which are used for the actual one-sided, optical shielding of a room, have a very low degree of light transmission and a high degree of light reflection, i.e. hardly any light through to the other side. This automatically initiates the conditions that must exist for a one-way mirror to function as intended:
- The room to be monitored must be very brightly lit so that sufficient light can reach the other side. But that also means that a lot of light is reflected back. People in the monitored room mainly see their reflection.
- This effect must be minimized on the observing side, because the observer does not want to perceive his own reflection as much as possible. That is why the observer's room must be illuminated as little as possible so that little light is reflected. As a result, of course, even less light reaches the monitored room, which means that the observer can no longer be seen from there.
The lighting situation therefore causes the two decisive effects that ensure that the observer cannot be seen, namely on the one hand the little light that comes from the observer and on the other hand the large amounts of light that are reflected by the one-way mirror and fade the observer.
Evenly lit rooms
Even rooms of the same brightness can be effectively protected from prying eyes: A partially transparent mirror layer is used on a substrate made of colored glass, which also has an anti-reflective coating on the side facing away from the mirror. While the light in the control room has to pass through the partially absorbing glass pane directed there twice (before and after the reflection), this is only the case once for the light passing through the mirror layer from the other room. As a result, the pane looks dark from the control room and does not obstruct the view. Most of the light in the room to be observed is reflected, however, through the mirror layer on this side - the surface appears bright, contours in the control room are outshone.
Alternatives
High-resolution video cameras, the camera image of which is displayed on a large-format monitor, are now inexpensive and allow unrestricted one-way observation.