It should be remarked that there is a different meaning of the term “pinhole mirror”.  A highly reflective metallic mirror on which many small elliptic apertures of are randomly distributed is also called a “pinhole mirror”(Hikari no Enpitsu).  This kind of pinhole mirror is a sort of a half-mirror and used as a finder mirror of a SLR camera, where the reflected light goes to the finder and the light passed through the elliptic aperture goes to a photometer.  It is very advantageous that the ratio of the through-light to the reflected light can be controlled accurately but this system was applied only to Nikon F3 SLR.