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.
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. 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.
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.
Sony SLT concept
Sony has introduced cameras with plastic pellicle-like mirrors, which it describes as "Single-Lens Translucent" cameras. These cameras divert a portion of incoming light to a phase-detection autofocus unit, while the remaining light strikes a digital image sensor. Sony "SLT" cameras employ an electronic viewfinder (EVF) allowing exposure value, white balance and other settings to be verified and adjusted visually before taking a picture, although typically the EVF displays far less dynamic range than the sensor. The refresh rate of the viewfinder is limited by the time it takes the sensor to make a usable exposure; thus in low light the frame rate of the viewfinder may be as low as four frames per second. "SLT" cameras also lack a real-time view at high shooting rates, when the viewfinder shows the last picture taken instead of the one being taken — a phenomenon comparable to certain older SLRs that can only achieve their maximum burst rate in mirror lock-up.
Advantages and Disadvantages
Advantages of a pellicle mirror:
- The user has an uninterrupted view through the viewfinder while making an exposure.
- There is no vibration from mirror movement, reducing shake and audible noise.
- Shutter lag may be diminished, and pictures taken at a faster rate, compared to systems employing a reflex mirror.
- Continuous phase-detection autofocus during video, live view or continuous shooting mode.
Disadvantages of a pellicle mirror:
- The pellicle mirror causes a 1/3-stop loss of light. (Some light has to go to the viewfinder.)
- The mirror has to be kept perfectly clean, or the light sensor and other electronics (as well as the image quality) will suffer.
- Owing to its thinness, the pellicle mirror is rather fragile. This makes it difficult to clean.
Possible disadvantages of a pellicle mirror: (These are matters of taste that could mean something to one photographer, and nothing to others.)
- As the viewfinder doesn't go dark, there is no visual indication that the shutter has fired. This could be a problem if one is in a noisy environment (rock concert, war, etc) where the shutter cannot be heard.
- Pellix QL (1965)
- F-1 High Speed (LE for the 1972 Olympics)
- EOS RT (1989)
- EOS 1N RS (1994)
- F2 HS
- F3 HS (Introduced for the 1998 Nagano Olympics)
- Eric P. Goodwin, James C. Wyant (2006). "Plate and Pellicle Beamsplitters". Fundamentals of Interferometry. SPIE Press Book. p. 8. ISBN 978-0-8194-6510-8.
- Roger Hicks (1984). A history of the 35mm Still Camera. Focal Press. ISBN 0-240-51233-2.
- Brian Coe (1979). Cameras. AB Nordbok Göteborg. ISBN 0-517-53381-2.
- Ivor Matanle (1996). Collecting and using Classic SLRs. Thames & Hudson. ISBN 0-500-27901-2.
- Canon Camera Museum
- Paul Comon, Art Evans (1990). Nikon Data. Photo Data Research. ISBN 0-9626508-0-3.