A “one-way” privacy effect can be imparted to existing glass with an application of window film but, just like “one-way” glass, this illusion is dependent on ambient light conditions. It is not actually possible to make “one-way” glass, only to use coated glass as an element in a system to create an optical illusion; I’m here and can see you, but you can‘t see me.
Effectiveness of One-Way Privacy Film Depends on Lighting Conditions
Equal Light on Both Sides of Glass
When you and I are in an evenly lit room, I can see you because of the light that is reflected by you. If we place a pane of clear glass between us, my view is slightly diminished because the glass reflects (8%) and absorbs (4%) some of the light.
If we place a pane of “one-way”glass, or a pane of glass with a reflective applied window film, my view is further diminished as more light is reflected and absorbed, perhaps 55% and 28% respectively, letting only 17% of the light that you have reflected through for me to see. My view is further affected by the new image of myself created by the light reflected on my side of the glass. Since the light is even in the room your view of me will be similar to my view of you.
Unequal Light on Either Side of Glass
Now we’ll build a wall around the clear glass, placing us in separate rooms, and adjust the light in one room at a time.
As the light in your room is turned down it becomes more difficult for me to see you until, when the light in your room is perhaps at 5 to10% as bright as the light on my side, the combination of light reflected and absorbed by the glass, along with the fact that my eyes are still adjusted to the brighter light on my side, I can barely make you out. At this point you may actually see me a bit better than before.
Replace the clear glass with our reflective pane and you will fade from my view when the light in your room is perhaps 35 to 50% as bright as the light on my side. Now we have the “one way” effect immortalized by police line-up scenes in movies. If we turn up the light level in your room and/or reduce the level of light in my room this optical illusion will eventually reverse (which hardly ever happens in the movies).
If the light levels on either side of the glass are fairly even, it is usually possible to achieve a “one way” effect by using multiple layers of different types of film to adjust the amount of light absorbed and reflected on either side of the glass. This type of application is very site specific and may involve a final adjustment in the placement of and brightness of the lighting on either side on the glass.