One-Way / Two-Way Mirror
Do-It-Yourself Mirror Effects
Do-It-Yourselfers seeking a one-way mirror effect (sometimes called two-way mirror or observation glass) should be aware that light conditions are an important factor in achieving this result. The 'observer' side of the glass must be darker than the side of the glass that is being observed, otherwise you will be able to see back in. You can also use window films to create a standard mirror on flat glass.
For more information on creating mirror effects with window films, and which films to use, please Click Here
Apex Window Films recommends "Black Steel 5" as the optimal reflective film to create a one-way mirror effect. The '5' indicates the visual light transmission or "VLT" number for the film. The lower the number the less light that passes through, and in the case of the Black Steel line of solar films, the most reflective. Higher VLT number, in the case of Black Steel 15, 25 and 35, are progressively less reflective; the upside is that they permit more light transmission which may be preferred given that the observation side of the glass must be dark in order to achieve this effect.
Please click here for more information on VLT