A pellicle mirror (diminutive of "pellis" - 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 enables its use for multiple purposes simultaneously, while the thinness of the film greatly reduces reflections commonly associated with glass beam splitters.[1]
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.[2]
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.[3]
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-pass 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. [4] The Pellix pellicle mirror was a 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.[5]
The next 35mm SLR camera to employ the pellicle mirror was the Canon F-1 High Speed, made available in the event of the 1972 Olympic games, the object being rapid series photography, difficult at the time to obtain with a moving mirror. The mirror design was the same as in the Pellix.[5] In 1984, Canon released another version of their then "New F-1", which attained a record 14 frames per second performance, being the fastest analog SLR of that time.
Nippon Kogaku KK, Japan introduced their high-speed Nikon F2H in 1976. The mirror is a pellicle rather than a conventional front surfaced mirror that swings out of the light path when the exposure is made. To identify the F2H, note the shutter speed dial has no T, B or 1/2000; has no self-timer and has a non-removable Type B focusing screen. [6]
Two further Canon models were produced with pellicle mirrors, the EOS RT and the EOS-1N RS, the RT being based on the EOS 600/EOS 630 and the 1N RS being based on the EOS-1N.
As development of SLR cameras has progressed since these early models, fast sequence shooting has apparently become possible using ordinary moving mirrors in high-speed cameras, getting rid of the vulnerable pellicle mirror that was prone to dust and dirt. The mirror mechanism of conventional SLR cameras has improved since the Pellix mirror was introduced; the viewfinder is dark for only a very short time, the shutter lag is small, and the mirror-return is fast enough for rapid shooting. Digital SLR cameras are able to take ten frames or more per second employing an instant-return mirror.
There are other potential uses for pellicle mirrors in modern cameras. One is to use a pellicle to direct a portion of the incoming light to a fast phase difference detection autofocus unit. There are patents in this area. Sony introduced two dSLRs with this capability in August, 2010, though they were not based on pellicle mirror, but rather a plastic[7] half mirror which wasn't as thin and as lightweight as one used originally by Canon.[8]
Sony has recently introduced cameras with pellicle mirrors, which it erroneously describes as "translucent" mirrors. A translucent material passes light, but scatters it so much that little or no distinct image remains. A camera with a translucent mirror would produce an indistinct blob of light at the image plane.
Advantages of a pellicle mirror:
Disadvantages of a pellicle mirror:
Possible disadvantages of a pellicle mirror: (These are matters of taste that could mean something to one photographer, and nothing to others.)
This type of mirror has been used in:
Canon
Nikon
Sony