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One Way Mirror Film – How does it work?

A one way mirror film is a fantastic way to add privacy and control the view through glazing. There are various types, shades, and grades. They are a simple addition, and are regularly used in offices, reception areas, security huts, and even in the home.


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 will work and the strength of the mirror.

As you know from at home, standard glass already produces somewhat of a one way mirror effect. The side with the most light is the side that will see a mirror, whereas the other side will be able to see through. During the day you can see outside but passers-by struggle to see in. Of an evening, the effect is reversed, as it is dark outside and the lights are on inside. People can see in and you see your reflection.

One way mirror film works in exactly the same way, by enhancing this property. 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, need a 1:4 light ratio 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 above film, with an additional “tint” on the internal surface. This not only makes it less reflective in appearance on the internal surface, it also means the one way mirror effect activates with a lesser difference in light, with a ratio of only 1:3.

Lighter films, such as ARC DRE35 are designed for the home, are almost unnoticeable from the inside, yet can still deliver strong one way privacy during the day.



Controlling the 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 the mirror effect 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 the mirror work in one direction all the time?

Unfortunately no. It will always be dictated by lighting conditions.


Considering a one way mirror film for your window?

Contact one of our team today on 0333 800 2400 to find out what options are available.

We’re perfectly placed to help, whatever your requirement. Take a look at our extensive range of products and services, or gain more insight into our solutions by reading our case studies and articles.

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