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Kinoton movie projector in operation.]] movie projector.]] A movie projector is an Opto - Mechanical device for displaying Moving Pictures by projecting them on a Projection Screen . Most of the optical and mechanical elements, except for the illumination and sound devices, are present in Movie Camera s. PHYSIOLOGY According to the theory of Persistence Of Vision , the perceptual processes of the Brain and the Retina of the Human Eye retains an image for a brief moment of time. This theory is said to account for the illusion of motion which results when a series of Film images is displayed in quick succession, rather than the perception of the individual frames in the series. Persistence of vision should be compared with the related phenomena of " or " Scan Rate " in the eye. Instead, the eye/brain system has a combination of motion detectors, detail detectors and pattern detectors, the outputs of all of which are combined to create the visual experience. The frequency at which flicker becomes invisible is called the Flicker Fusion Threshold , and is dependent on the level of illumination. Generally, the frame rate of 16 frames per second (fps) is regarded as the lowest frequency at which continuous motion is perceived by humans. (Interestingly this threshold varies across different species; a higher proportion of Rod Cells in the retina will create a higher threshold level.) It is possible to view the black space between frames and the passing of the shutter by the following technique: Close your eyelids, then periodically rapidly blink open and closed. If done fast enough you will be able to randomly "trap" the image between frames, or during shutter motion. This will not work with television due to the persistence of the phosphors nor with LCD or DLP light projectors due to the continuity of image, although certain color artifacts may appear with some Digital Projection technologies. Since the birth of Sound Film , virtually all film projectors in commercial movie theaters project at a constant speed of 24 fps. This speed was chosen for financial and technical reasons - it was the slowest speed (and thus required the least film stock and was cheapest for producers) at which a satisfactory reproduction and amplification of sound could be conducted. There are some specialist formats (eg Showscan and Maxivision ) which project at higher rates, often 48 fps. -base prints catching fire in the projector. (A dramatic rendition of a nitrate print fire and its potentially devastating effects is famously found in '' Nuovo Cinema Paradiso '', which revolves around the goings-on of a projectionist.) PRINCIPLES OF OPERATION Kinoton FP30ST movie projector, with parts labeled. (Click thumbnail for larger text.)]] Projection elements As in a Slide Projector there are essential optical elements: Light source An Incandescent Lamp or an Electric Arc Light produces illuminating Photon s. The traditional carbon arc or modern Xenon Arc light source produces sufficient heat to burn the film should the film remain stationary for more than a fraction of a second. Xenons were introduced in the 1950s and are now the more common source, being easier and safer to maintain for the most part. Reflector and condenser lens A curved reflector redirects light that would otherwise be wasted toward the condensing lens. A positive curvature Lens concentrates the reflected and direct light toward the film gate. Douser (Also spelled dowser.) A metal blade which cuts off light before it can get to the film - usually this is part of the lamphouse, and may be manually or automatically operated. Some projectors have a second, electrically-controlled douser that is used for changeovers (sometimes called a "changeover douser" or "changeover shutter"). Some projectors have a third, mechanically-controlled douser that automatically closes when the projector slows down (called a "fire shutter" or "fire douser"), to protect the film if a failsafe causes the motor to shutdown. Dousers protect the film when the lamp is on but the film is not moving, preventing the film from melting from prolonged exposure to the direct heat of the lamp. It also prevents the lens from scarring or cracking from excessive heat. Film gate and single image A single image of the series of images comprising the movie is positioned and held flat within an aperture called the gate. The gate also provides a slight amount of friction so that the film does not advance or retreat except when driven to advance the film to the next image. Shutter A commonly-held misconception is that film projection is simply a series of individual frames dragged very quickly past the projector's intense light source; this is not the case. If a roll of film were merely passed between the light source and the lens of the projector, all that would be visible on screen would be a continuous blurred series of images sliding from one edge to the other. It is the shutter that gives the illusion of one full frame being replaced exactly on top of another full frame. A rotating petal or gated cylindrical shutter interrupts the emitted light during the time the film is advanced to the next frame. The viewer does not see the transition, thus tricking the brain into believing a moving image is on screen. Modern shutters are designed with a flicker-rate of two times (48 Hz) or even sometimes three times (72 Hz) the frame rate of the film, so as to reduce the perception of screen flickering. (See Frame Rate and Flicker Fusion Threshold .) Higher rate shutters are less light efficient, requiring more powerful light sources for the same light on screen. |
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