Back surface mirrors

Back surface mirrors are used to protect the front mirror layer 

Back or rear surface mirrors are mirrors with a coating on the backside. This type of mirror is very robust against external influences, as the mirror glass is on the front and the mirror coating on the back can be additionally protected by protective films, for example. The disadvantage of rear surface mirrors, however, is that they have more absorption losses and secondary reflections, resulting in double images during image reproduction. Although these can be reduced by additional reflection-reducing layers, they cannot be completely eliminated.