Ray tracing is utilized to obtain the expression of the wave amplitudes at the observation plane in the multiple-grating interferometer. The white-light condition as well as the broad-source condition are found both for the simple three-grating interferometer and for the multiple-grating interferometer with grating lines oriented in arbitrary directions. Finally, a variable-frequency grating interferometer, composed of four identical gratings, which can be operated under white-light broad-source illumination is proposed.
