One-way mirrors are treated on one side so that an observer can see out, but the observed can not see in. In order for these windows to work efficiently, the observed side of the mirror must be well-lit so that the subject of observation sees only their own reflection when looking into the window.
Silver Window Film
One type of treatment for a one-way, privacy window is a silver window film, which provides the mirrored surface that most people think of when picturing a one-way mirror. This film combines the mirror surface with a dark tint in order to accomplish maximum privacy. This type of film most often is used on buildings with a glass exterior. The silver window film blocks 99 percent of ultraviolet light while still letting visible light in, allowing it to work for office buildings. You might notice this at work during the day when such a building only reflects your own image back to you, but at night allows you to view the interior of the building. This is caused by the interior light being greater than the light outside of the glass.
Gold Window Film
Gold window films are another way to create a one-way effect that provides a more obvious treatment. The gold tint allows for 10 percent more visible light to penetrate than silver window films do. This type of film also requires more light on the observed side of the mirror in order for the window to provide the expected privacy. Consider blinds at night if using this treatment in a residential building or you will not get much privacy. If you desire a more subtle way to create daytime privacy, the silver film may be better than the gold.
Bronze Window Film
A good go-between for the silver and gold window films is the bronze treatment. While it's still not as subtle as the silver treatment, it is a less glitzy film than the gold. Bronze films allow for a 73 percent reduction of the sun's glare and a 65 percent heat reduction, therefore requiring less effort to keep your home cool in the summer when used on exterior windows. Like the silver and gold films, it reduces 99 percent of harmful ultraviolet radiation. Again, remember that the one-way effect only works if the interior of your home is darker than the exterior, regardless of the type of film you use.