SYSTEME DE DEMODULATION OPTIQUE

摘要

Optically screened input information is recorded as excursions on the surface of a thermoplastic film. The excursions basically consist of a periodic carrier wave and a modulating signal containing the input information. Highly coherent light is directed incident upon the image bearing surface whereby the signal modulated deformations thereon cause the light to become diffracted. The diffracted light is then passed through a projection system including a spatial filter which is positionable in the focal plane of the system. The filter comprises a thin opaque mask having two clear apertures therein, which, when the mask is positioned in the systems focal plane, are coincident with the two first diffracted order focal spots, the two apertures being of a size capable of passing the modulated side bands of the input signal. The mask also has a third aperture centered about the optical center line of the system and being of a size such that the Airy disc of the zero diffracted order just fills, or preferably, slightly more than fills the central aperture when the two first diffracted order focal spots are centered in the other two apertures. The optical path length of one of the light beams passing through the filter is retarded in relation to the other beams whereby interference between the light beams is produced at the scan plane of the system.