Pellicle mirror

A pellicle mirror (diminutive of pellis, a skin or film) is an ultra-thin, ultra-lightweight semi-transparent mirror employed in the light path of an optical instrument, splitting the light beam into two separate beams, both of reduced light intensity. Splitting the beam allows its use for multiple purposes simultaneously. The thinness of the mirror practically eliminates beam or image doubling due to a non-coincident weak second reflection from the nominally non-reflecting surface, a problem with mirror-type beam splitters.

In photography, the pellicle mirror has been employed in single-lens reflex (SLR) cameras, at first to enable through-the-lens exposure measurement and possibly to reduce camera shake, but later most successfully to enable fast series photography, which otherwise would be slowed down by the movement of the reflex mirror, while maintaining constant finder vision.

The conventional SLR camera has a reflex mirror directing the light beam from the lens to the focusing screen in the viewfinder, which is swung out of the light path when the exposure is made and causing the viewfinder to go dark. This action adds a delay between pressing the shutter release and the actual exposure of the film.

The first camera to employ the pellicle mirror as a beam splitter was the Canon Pellix, launched by Canon Camera Company Inc. Japan in 1965. The object was to accomplish exposure measurement through the lens (TTL), which was pioneered by Tokyo Kogaku KK, Japan in the 1963 Topcon RE Super. It employed a CdS meter cell placed behind the reflex mirror that had narrow slits cut into the surface to let the light reach the cell. Canon improved on the idea by making the mirror semi-translucent and fixed. The meter cell was swung into the light-path behind the mirror by operating a lever on the right-hand camera front for stopped down exposure reading, momentarily dimming the viewfinder. Two thirds of the light from the camera lens was let through the mirror, while the rest was reflected up to the viewfinder screen. The Pellix pellicle mirror was an ultra-thin (0.02 mm) Mylar film with a vapour deposited semi reflecting layer. Since there was no mirror blackout, the user could see the image at the moment of exposure.

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