A pellicle mirror in a camera is a stationary one, replacing the flipping mirror in a SLR. The object of employing a pellicle mirror is to split the image forming light beam from the camera lens into two separate beams, one passing directly through it to the film or image sensor and the other one being redirected to the finder focusing screen; and in doing so, getting rid of the mirror movement mechanism. The consequence of splitting the light is reduced exposure light intensity and finder brightness. The usual intensity ratio is two to one in favour of the exposure, which works out to about 66% (⅓ stop loss) to the film or sensor and 33% (⅔ stop loss) to the finder.
The first SLR to employ a pellicle mirror was the 1965 Canon Pellix using a semi-silvered 0.020mm thick Mylar foil, primarily to accomplish TTL exposure metering by swinging a CdS cell into the light pass behind the mirror. Later it has been used in professional SLR cameras to enable fast sequence exposures, which would be slowed down by a flipping mirror. SLR high-speed cameras reach from five to ten or more exposures per second and retain vision in the finder.
The term is derived from the name used for a very thin fish skin. Artificially a clear substrate may be made of a plastic foil or possibly an optically flat glass plate. A thin coating is applied to the substrate making it semi-reflective while allowing more than half the light passing, ideally undisturbed, through it.
The following cameras employ a fixed pellicle mirror:
- Canon Pellix
- Canon F-1 High Speed
- Canon New F-1 High Speed
- Canon EOS RT
- Canon EOS 1n RS
- Nikon F High Speed, some variants
- Nikon F2 High Speed
- Nikon F3 High Speed
In 2010 Sony rediscovered the pellicle principle to redesign Sony α system cameras with electronic viewfinder, and to combine that reconception with the advanced phase-detection autofocus sensor modules which were developed for the Sony α DSLRs. Sony calls the pellicle translucent mirror. The cameras using it are called SLT (single lens translucent). The high sensitivity of Sony's image sensors makes a new kind of flare visible in maybe less than 1% of the exposures. It's called "ghosting" since this flare shows "ghosty" transparent doublings of lights and contours especially on high-contrast exposures. Sony reacted by giving its newest advanced models of the Sony α SLT series a pellicle with changed semi-reflective coating.