A pellicle mirror 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.

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

Canon Pellix

In the mid 1960s through the lens exposure metering was the new technology entering the SLR camera market. The Pellix was Canon's answer to that challenge. This technology was first successfully realised in a 35mm SLR camera in the remarkable Tokyo Kogaku KK Topcon RE-Super, launched in 1963, but...

, launched by Canon Camera Company Inc. Japan in 1965. The object was to accomplish exposure measurement through the lens (TTL)

Through-the-lens metering

Through-the-lens metering is a photographic term describing a feature of cameras capable of measuring light levels in a scene through their taking lenses, as opposed to a separate metering window...

, which was pioneered by Tokyo Kogaku KK, Japan

Topcon

Topcon is a Japanese manufacturer of optical equipment for ophthalmology and surveying. Their headquarters are in Itabashi, Tokyo. They are affiliated with Toshiba, which holds 40% of Topcon's stock.-History of Topcon:...

 in the 1963 Topcon

Topcon

Topcon is a Japanese manufacturer of optical equipment for ophthalmology and surveying. Their headquarters are in Itabashi, Tokyo. They are affiliated with Toshiba, which holds 40% of Topcon's stock.-History of Topcon:...

 RE Super. It employed a CdS

CDS

-Computing and electronics:* Cadence Design Systems, American Electronic Design Automation software company* Chromatography data system, software to control chromatography instruments* Cockpit display system* Compact Discs...

 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. 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.

The next 35mm SLR camera to employ the pellicle mirror was the Canon F-1 High Speed

Canon F-1

The Canon F-1 is a 35 mm single-lens reflex camera produced by Canon of Japan from March 1971 until 1976's introduction of the mildly updated F-1n, while in 1981 a New F-1 was launched. The new Canon FD lens mount was introduced along with the F-1, but the previous Canon FL-mount lenses were...

, 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

Nikon

, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...

 introduced their high-speed Nikon F2H

Nikon F2

The Nikon F2 is a professional level, interchangeable lens, 35 mm film, single-lens reflex camera. It was manufactured by the Japanese optics company Nippon Kogaku K. K. in Japan from September 1971 to June 1980...

 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.

Two further Canon models were produced with pellicle mirrors, the EOS RT

Canon EOS RT

The Canon EOS RT is a 35mm single-lens reflex camera produced by Canon from 1989 to 1992. The camera is essentially an EOS 630/EOS 600 with a pellicle mirror. It was the first autofocus camera to feature a pellicle mirror, followed by the EOS 1N RS five years later...

 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 half mirror

Beam splitter

A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.In its most common form, a rectangle, it is made from two triangular glass prisms which are glued together at their base using Canada balsam...

 which wasn't as thin and as lightweight as one used originally by Canon.

"Translucent" mirrors

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 and Disadvantages

Advantages of a pellicle mirror:

• The viewfinder does not go dark.
• The user has an uninterrupted view through the viewfinder while taking photographs.
• There is no mirror shake - this is good for macro photography, where even the slightest shake of the camera can cause a blurred image.
• Pictures can be taken at a faster rate.
• The mirror does not have to go up and down for every image. The EOS 1N RS, for example, can take 10 pictures every second. The system can be more quiet. (no "mirror slap")

Disadvantages of a pellicle mirror:

• The pellicle mirror causes a 1/3-stop
  F-number
  In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
  
  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, obviously) will suffer.
• Cleaning a pellicle mirror is difficult.

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.
• Ripping through a 36-exposure roll of film in a little more than three seconds can be a practical and/or financial problem.

This type of mirror has been used in:

Canon

• Pellix QL (1965)
• F-1 High Speed (LE for the 1972 Olympics)
• EOS RT (1989)
• EOS 1N RS (1994)
  

Nikon

• F2 HS
• F3 HS (Introduced for the 1998 Nagano Olympics)
  

Sony

• Sony SLT A33
• Sony SLT A55
• Sony SLT A65
• Sony SLT A77