Abstract

A treatment is given for obtaining the image from a modulated object in a projection system. A diffraction grating put on the object plane converts the entering plane wavefront from a light source into a periodic wavefront, which gives a carrier wave, and the signal to be projected is given on the carrier as an amplitude, phase, or frequency modulation. The modulated carrier wave forms its spectrum on the entrance pupil of the projection lens. The signal appears as side bands of each carrier spectrum. A narrow opaque mask put on the plane of the spectra acts as a detector, which cuts off some spectral components, and the residual spectra reveal the image of the signal on a screen. A mask whose transmittance varies as a function of position, that is, a frequency selective filter, is used to detect the signal from a frequency-modulated carrier. The method is shown to be useful to analyze the image from an object pattern composed of scanning lines or mesh points, to detect the defects of optical gratings, and to measure the strain distribution of materials under test.

© 1964 Optical Society of America

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