Photographic Recording

(optical recording), the use of photographic methods to record electrical signals that carry information about sound and/or images.

A photographic film (still or motion-picture), plate, or some other material may serve as the carrier in the recording system, and the recording is produced by a light or electron beam. During the recording process, the carrier may be moved relative to the stationary beam, or the recording beam may be displaced relative to the stationary carrier. The intensity or shape of the beam incident on the carrier is varied to correspond with the recorded signal. Subsequent photographic processing (development and fixing) produces a signal track, on which the coded signal is registered in the form of corresponding alterations in the optical density or reflection coefficient of the various sections of the carrier. Various photographic recording systems may use incoherent light (a light beam without spatial coherence), an electron beam, or coherent light (a laser beam).

Photographic recording with incoherent light is the most common system type. It is used for sound recording, for example, in making sound films, as well as for recording television images from the screen of a kinescope; the latter process is used by television studios to store television programs. In practice, photographic recording with incoherent light is primarily used in systems that modulate a line recorded on the carrier by means of an electromechanical modulator with a moving mirror, which is controlled by a permanent-magnet device; such systems use an external light source of constant intensity.

As shown in Figure 1, an optical image of a diaphragm with a narrow rectangular slit is produced on the carrier, such as a motion-picture film, by means of an objective lens. In turn, an optical image of a diaphragm with an M-shaped aperture, illuminated by a recording lamp, is formed in the plane of the first diaphragm by means of an imaging lens, a mirror, and condenser lenses. As the mirror oscillates to correspond with changes in the recorded signal, the image of the M-shaped aperture oscillates relative to the narrow slit, which results in a change in the width of the non-illuminated sections of the carrier. The sound track obtained after the film is developed is called a bilateral variable-area photographic sound track.

Reproduction of the recorded information from the signal track is accomplished by passing a light beam through the track. In the reproduction process, the signal track moves relative to the beam with a speed equal to the velocity of the carrier relative to the recording beam during the recording process. The light passing through or reflected from the signal track strikes a photoelectric transducer, such as a photocell, which converts the encoded signal into an electric signal.

Figure 1. Diagram of a light-modulating device with mirror modulator, used for photographic recording: (1) recording lamp, (2) and (6) condenser lenses, (3) diaphragm with M-shaped aperature, (4) imaging lens, (5) modulating mirror oscillating about axis a, (7) diaphragm with narrow rectangular slit, (8) objective lens, (9) light track projected onto the film, (10) motion-picture film, (11) section of the signal track photographically recorded

Advantages of systems using electron beams and coherent light rather than incoherent light include higher quality in recording high frequencies and greater density in the recorded signal. For this reason further development of such systems shows more promise for image recording.