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Building a one-way mirror

I've moderated usability studies in about a dozen labs over the years. When HumanCentric moved into our new building in 2008, I was part of the team that designed our labs, and I was responsible for the physical layout and facilities. As part of this process, we consulted with a professional acoustician, and I've incorporated those suggestions into the tips below.

In a separate article, I discuss whether you need one-way mirror in your usability lab.

What is a one-way mirror?

A one-way mirror is glass that has been coated with a thin layer of metal so that it's reflective, like a mirror. It's called a one-way mirror because in one direction it's a mirror, but in the other direction it's a window. There's some debate about the proper name. Many people call it a two-way mirror because either side can be used as a mirror by changing which room is lit and which room is dark. They're commonly used in usability labs and focus group rooms, but also in other locations like observation rooms in police stations and day care facilities.


The basic design is two panes of glass separated by an airspace. The pane on the participant side is mirrored, and the pane on the observer side is clear glass. There are two design issues involved: light and sound. Reducing light transmission is the easy part; reducing sound transmission is much more difficult.

Why two panes? Acoustics

If all you wanted to do was control light then you could use a single mirrored pane and be done. The second pane and air gap help reduce sound transmission. You want a STC of 35 as a bare minimum, 40-45 is better. There are many lab design techniques for reducing sound transmission between the study room and observation room. In this article, I focus on just the issues specifically related to the design of the one-way mirror.

Choosing your glass



One-way mirrors work when the participant side is bright and the observer side is dark, it's as simple as that. If the moderator wants to turn down the lights because they have a headache that day and the observer wants brighter lights to write on paper you risk turning your one-way mirror into a window and allowing your participants to see into the observervation room.

Participant room lighting should be as bright as you can make it without giving the moderator and participants a headache.

Observation room lighting is more complicated. Here are some tips:

Additional sources

You may also be interested another article of mine, Does your usability lab need a one-way mirror?