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
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
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
- Silvering - the glass used in one-way mirrors
differs from regular mirrors in that the silver coating is thinner
(generally 50% to 75% of mirror silvering), and it lacks the coat of
paint typical on mirror glass. Tell your glass supplier that you want
glass for an observation room and they will know what you mean. We used Mirropane.
Be careful about the cleaning solutions you use on the glass because
some can dissolve the coating; check the manufacturer's instructions.
- Tinting- tinting helps further reduce light
transmission. The mirrored glass we bought came pre-tinted. You can also
buy untinted glass for the mirrored pane and then apply a tinting film
to the unmirrored pane. This has the advantage that you can test out
different tints and select one that has a good balance of visibility vs
protection from light, but removing and re-applying tints is more
expensive and time-consuming.
Tempering makes glass stronger and for safety reasons is often
recommended for interior windows, but you do NOT want to use tempered
glass. The tempering process warps the glass, producing a fun-house
effect. The warping catches your eye when you move, continually
reminding your participants that they are being observed through a
- Laminate - you can use either regular glass or laminate.
Laminate is safer because it is more likely to stay in one piece if
cracked, and it is slightly more soundproof than plain glass.
- Thickness - the specific thickness of the two panes
is less important than that the two panes be different thicknesses
and/or materials to help reduce sound transmission. Different
thickness/material panes have different profiles of the sound
frequencies that they transmit, and so two different panes will block
slightly different frequencies. You could use 1/4" and 3/8" panes, or
1/4" glass and 1/4" laminate.
- Size - typical panes come in a maximum size of
about 6 x 10 feet. You can connect multiple panes of glass with a thin
ribbon of epoxy or silicone or similar substance and the seams
will be barely visible from a distance.
- Weight - glass is about 3.5 times as dense as
gypsum (drywall) at the same thickness. Make sure your wall can support
the weight of both panes. Larger openings will weaken the wall more.
- IGUs - you do not want to use IGUs
(insulated glass units in the US, or double glazed units in Europe).
The maximum air gap of these factory-manufactured double panes is about
1.5", but you want a 2.5"-4" gap for sound insulation purposes (see
- Argon and krypton - these gases are more dense than
air. In home and building construction they are commonly used in IGUs
to provide additional heat insulation. You don't want them in your
usability lab because they have little value for sound insulation, both
because of the sound transmission qualities of the gas and because they
require factory-sealed IGUs with too small a gap between panes.
- Triple panes - if a double-paned setup is good,
triple-paned would be better, right? Wrong. You want to maximize the
airspace between your panes of glass. Assuming your wall thickness
limits how far apart you can place your panes, adding a third layer of
glass means having two smaller gaps instead of one big gap. The added
sound insulation of the third pane is a LOT less than the loss of sound
insulation due to cutting your airspace into two sections.
- Orientation of the mirror - you'll want the
mirrored side of the mirrored pane to close to and facing the
participant. It's tempting to put the mirrored surface facing into the
airspace to protect it from scratches and cleaning fluids. However,
glass is slightly reflective, and this may result in a double-image
effect where you can see reflections on both the glass and silver
sufaces at the same time.
- Gap between panes - the larger the gap, the better
sound insulation. At less than 2. the air acts like a solid and
transmits more sound. Every extra inch of gap adds an extra 3dB of noise
reduction. Sound studios may use a 6. to 10. gap, but this is necessary
because they use angled panes, and is achieved by building two separate
walls. You'll want to maximize the gap between the panes, within the
contraints of the thickness of your walls. For a usability lab, 2.5. to
4. should be sufficient. When discussing this with your glass
contractor, they may speak in terms of gap between the glass frames,
which isn't exactly the same as the gap between the panes.
- Angled panes - angling glass prevents reflecting
sound back to its source and slightly reduces sound transmission. In a
recording studio, controling sound reflection is extremely important,
but it's not an issue for a usability lab. More important is that
angling the pane reduces the volume of air between the panes. Sound
studios, with a 6. to 10. gap, can afford to loose some air volume. If
you are installing into a typical office wall, maximizing the volume of
air between the panes is more important than an angle. If you have a
wide gap and would like to angle one pane, a 1/4" difference between the
top and bottom of the unmirrored pane should be sufficient.
- Wall framing - you can frame out, plaster and paint
the opening for the one-way mirror before installation begins. Paint
the frame a medium or dark color so that participants don't see it
illuminated with the light from the participant room. Make sure you run
all your cables (power, network, audio/video) before you frame the
- Sound absorbing material - you can fill the gap
between panes with sound-absorbing material like neoprene and foam. We
decided this was unecessary.
- Glass frame - your choice of frame should be guided
by the ability to seal all air gaps around the panes and to maximize
the gap between panes. We used a snap-in frame that only needs mounting
screws on one side so that it can be placed closer to the outer edge of
- Seal adjacent panes - if you need to purchase multiple panes to fill in your space, seal the seam with a ribbon of epoxy or
silicone. It will block sound and be barely visible.
- Seal gaps - when installing the frame, caulk around
the inside and outside of both frames to make sure there are no air
gaps to allow the passage of sound. Close any seams in the framing with
caulk or epoxy.
- Dust and prints - be extra careful that there are
no fingerprints or construction dust on the pane interiors or frame
between the panes, because once that mirror is up you are never going to
be able to clean it!
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:
- Dimmable lights - you'll want to be able to dim the
lights during sessions, but your observers may go crazy if they can't
have bright lights between sessions or during setup.
- Blackout curtains - cover any windows with blackout
curtains. True blackout curtains are extremely expensive and not
necessary for most rooms. Our labs are in a wooded area and don't get
much direct sunlight, so we used drapes we bought at a home linens
store, and added light-blocking liners. Mount the drapes several inches
higher than the top of the window to minimize vertical light spillage.
Velcro can help attach panels together and close gaps between the
curtains and the wall. Pick a dark color for the drapes.
- Paint - paint the walls of the observation room and the gap between the panes a medium or dark color. We used gray.
- Doorways - be very careful of doors that can be seen from the participant room that might be opened during a session. Some strategies:
- Place the door on the same wall as the one-way mirror, if your observation room is wider than the participant room.
- Keep the hallway outside the door dim by turning off the lights or removing bulbs
- Most doors can be hung to open on either side; hang the door such that the door itself blocks the hallway from view when opened
- Build a wall to block sight of the door. In our lab you enter into a short hallway so the door doesn't have to be closed at all
- Hang a curtain on the door
- Ceiling lights - avoid lights that can be seen from
the participant room. Mount the lights above the one-way mirror so that
they are blocked from view by the wall, or use deeply recessed canister
- Monitors - orient monitors in the observation room away from the one-way mirror, and turn down the brightness.
- LEDs and other point sources - avoid lights in the
observation room like LED status lights and backlit LCD displays. You
can cover lights with static-cling tinting film sold for tinting your
own car windows. Watch out for backlit logos on the lid of your laptop.
- Light-colored objects - avoid placing anything
light-colored near the one-way mirror because the light spilling in from
the participant room will increase its visibility. This includes
observers wearing light-colored clothing, white note paper, and white
You may also be interested another article of mine, Does your usability lab need a one-way mirror?