How does One Way Mirror Film work?

One Way Mirror Film – How does it work?

One way mirror film is a great way to add privacy, and is commonly used to control viewing through glazing. There are many different types of one way mirror film available, with varying shades and grades.

One way mirror film is a simple solution, and is frequently used in reception areas, offices, security huts, in the home and many more.


How does one way mirror film work?

A one way mirror film can give glass the capability to act as a one way mirror, but it is the surrounding lighting conditions that actually dictate whether the mirror effect occurs, and the strength of the mirror.

Standard glazing (without one way mirror film) already produces somewhat of a one way mirror effect. The side with the majority of the light is the side that is most reflective and difficult to see through, whereas if you’re looking from the other darker side, you will be able to see through.

Take your windows at home as an example, when it’s dark outside and you have lights on inside, passers-by can easily see in and you will just see your reflection when looking out. When it’s light outside during the day, the reverse effect occurs, where you can see outside but passers-by struggle to see in.


As shown in the images above, one way mirror film works in the exact same way, but further enhances the one way mirror effect.

The greater the light difference, the stronger the mirror effect will be. When the ratio is smaller (similar light levels on both sides) we get ghosting, whereby it partially works both sides. With one way mirror film we only need a 1:4 ratio of visible light for the effect to work, which means that it “activates” much sooner than untreated glass.

The key point is that the film is only part of the process, as lighting conditions play a controlling role.


Are different versions of one way mirror film available?

Many window films can produce a one way mirror effect. As a general rule, the darker and more reflective the film, the stronger the one way mirror effect will be, and the lower the lighting difference you will need before it becomes apparent.

Standard Solar Films, such as ARC RSI20, work best with a 1:4 light ratio in order for the one way mirror effect to activate. This is much lower than untreated glass.

Dual Reflective Films, such as ARC DRE05, are “mirrored” one side, and neutral on the other. These films are effectively the same as the above film, with an additional “tint” on the internal surface. Not only does this make the internal surface less reflective, the one way mirror effect is also activated with a lower light ratio of just 1:3.

Lighter films, such as ARC DRE35 which are specifically designed for homes, are almost unnoticeable from the inside looking out, but still have the ability to deliver strong one way privacy during the day.


Controlling the one way mirror effect

As a general rule, the greater the difference in light between the two sides, the stronger the one way mirror effect will be. When fitted to an external window, outside is usually brighter during the day, and will therefore see a mirror. Of an evening, we get the reverse effect. This is why it is sometimes called “Daytime Privacy Film” – as it only increases privacy during the day.

However, one way mirror film can be controlled to some extent, by artificially adjusting the lighting conditions either side of the mirror. Obviously we are at the mercy of the sun for external windows, but internal panes offer more control. As long as one side is darker than the other, by the correct amount, the mirror will be apparent.

This is why for example in FBI programmes where there is an interview room, there is always a very dark room that they view from. The interview room itself is usually very well lit. Although they go overkill on the lighting difference required, they are artificially creating conditions to ensure that the window always acts like a total one way mirror.

For internal partitions, spotlights can be added to one side to increase brightness, and dimmer switches can be used to increase or decrease lighting either side of the mirror.


“The mirror effect is working in reverse!”

The side with the most light will always be the side which sees the mirror effect. This means that the effect can reverse, depending on lighting conditions.
This is particularly the case on external windows, whereby during the day the mirror is seen from outside, but of an evening (when it is dark outside and the lights are on inside) the mirror is seen from the inside.

There is no way around this – as it is the lighting which dictates the strength of the mirror and the direction it works in.


Can one way mirror film work in one direction all the time?

Unfortunately no. One way mirror film will always be dictated by lighting conditions.