Rear projection applications usually require the built-in depth to be minimized. This can be done using optical mirrors to fold the light path.
Optical mirrors are always first surface mirrors, i.e. mirrors without glass in front of the reflective layer, in comparison with standard “bathroom” mirrors. Standard mirrors cannot be used because the glass will create reflections and ghost images.
Optical mirrors are available in two different types: glass, or as a film or foil stretched over a metal frame.
The glass based mirrors are available in special coated versions for improved reflection and with cleanable surfaces. The film versions do not offer a cleanable surface and are very sensitive to scratches.
Big glass mirrors are heavy and difficult to handle and if angled forward (as is usual) they will deflect due to their mass, which means image distortion.
Due to the much lower mass of the foil based mirrors, these do not suffer from deflection, but in general glass based mirrors have better performance.
Reflection from high quality optical mirrors is typically around 90%; the perceivable light loss due to mirrors in the light path is negligible.