How film works

Movies work by taking advantage of the persistence of vision phenomenon of the human eye, that is, when images with continuous movements are played quickly, the human eye will automatically form a sense of movement.

The key to movie cameras and projectors is to play back multiple frames quickly. The core of modern movie cameras and projectors is the invention of the film grabber and the light-shielding film. Although the film on the film reel is continuously rotating, the film actually moves intermittently when passing through the optical path. The film grabber is inserted into the edge of the film. The film is quickly pulled into the perforations. When the film is aligned with the light path window, the film stops moving and is still there and is illuminated by the light. At this time, a still frame of picture is projected on the screen. This picture is captured after the playback time is reached. The film claws pull the film to move again, pulling the film of the next frame to the light path window, stopping the playback, and repeating the process of putting down frame by frame.

The film transport part of the projector is composed of film transport gears, pulleys, film doors, frame offset adjusters, intermittent motion mechanisms, shutters, and film supply and rewind devices. The film transport gear is a mechanism that transports or brakes the film by extending the teeth around it into the film teeth. The pulley is a device used to change the moving direction of the film, limit the left and right positions of the film, stabilize the moving speed of the film and slow down the vibration of the film. The function of the film gate is to make each frame stay in a strictly determined position and receive light in a certain area. During the screening process, if the film frame is not aligned with the film door hole, it can be aligned by adjusting the frame Nog adjuster knob. The film makes intermittent movements of stop and move in the film gate. In 16mm projectors, gripping claws are currently used to make the film do intermittent motion. Its working condition has a decisive impact on the projection effect and the life of the film. The function of the main blade in the shutter is to block the light hitting the film door once when the film changes frame. This prevents people from seeing traces of movement when the film is changed; the function of the secondary leaf is to block the light again when the frame stays in the film gate, which can reflect each frame twice, increase the flicker frequency, and reduce the time it takes for the human eye to observe the screen. A flickering sensation caused by too low a frequency of light and dark changes. The film supply device is used to support the film supply clamp, feed the film evenly to the film transport part, and provide appropriate resistance moment to the film supply clamp to prevent it from rotating freely due to inertia. The film rewinding device rewinds the film that has been screened, and often uses friction transmission to transmit power.

The function of the light-shielding film is to automatically block the main light path when the film is moved by the claw, so that the screen is temporarily black, so as to prevent the moving film from being projected onto the screen, affecting the playback effect and making the audience dizzy..., Once the film grabbing claw pulls the film to position it and the picture is still, the visor will rotate to the gap position that does not block the light, so that the still picture can be projected. The playback speed of modern movie players is 24 frames per second, but this frequency is still too low for many people, and they will feel flicker. Therefore, in order to increase the flicker frequency to eliminate the flicker, although modern movie players play pictures at 24 frames per second However, the light visor will block each picture twice, so the number of flickers increases to 48 times per second, and most people will not feel the flicker. In fact, today's TVs also use a similar principle: Although PAL-DK TVs have 25 images per second, each image is divided into interlaced odd and even fields for scanning, so the flicker frequency of TV images is 50 times per second, which is generally not visible to the human eye. I can't feel the flicker anymore.

The stable playback speed of the movie at 24 frames per second is achieved by a stable speed motor. The commonly used low-cost stable speed motor is an AC synchronous motor, which is synchronized with the frequency of AC power because the power supply frequency of AC power is very Stable, so the rotation speed of the AC synchronous motor is also very stable. After the reduction gear, it can be guaranteed to be accurate at 24 frames per second.

The roll of film is sent from the film supply tray to the film window for illumination to reflect the picture, and the optical sound band signal is converted into audio current through the sound reproduction section. Then rewind the film that has been shown. It consists of film feeding and retracting boxes (or film arms), film feeding, intermittent, and retracting film transport gears, intermittent motion mechanisms, various pulleys, film doors, and frame adjustment devices. Narrow film mobile projectors often use intermittent film grabbing mechanisms instead of intermittent film transport gears. The core part of the movie projector is the intermittent motion mechanism, which changes continuous rotation into intermittent rotation, and cooperates with the light shield to make the film stay stably at the film window when the light passes through, and is clearly imaged on the screen; the light is blocked Pull it down quickly to avoid revealing traces of the film’s movement. The standard movie moving speed is 24 frames/second. In order to reduce the flicker effect, the visor blocks light twice per frame, increasing the flicker frequency to 48 times per second. The most widely used intermittent motion mechanism is the Malti cross.

There is a guide pin 2 on the continuously rotating cam 1. Every time it engages with the radial slot of the cross carriage 3, the cross carriage rotates 1/4 turn, so each time the transmission shaft rotates (by a～e), the intermittent film transport gear 4 rotates 90 degrees and pulls the film one frame. When the cam is in the process of e → a → b, the cross carriage is stationary. Some narrow film projectors use optical intermittent compensation instead of mechanical intermittent mechanisms, which is beneficial to protecting the film, but the brightness of the picture is compromised. The film rewinding mechanism slows down as the diameter of the film roll increases to reduce tension.