Abstract: Variable shear A.C. interferometer based on a rotating radial grating. Light is passed through the rotating radial grating to produce multiple sidebands diffracted at various angles. These multiple sidebands produce overlapping (sheared) images of the input wavefront at a detector plane causing intensity variations proportional to the phase difference between the sheared wavefronts. An array of detectors is positioned to monitor the images on the detector plane and the outputs of each are processed to obtain the local slope of the wavefront averaged over each detector. From this data, complete wavefront slope information in one direction (X) can be obtained. By providing a second interferometer to make slope measurements in the Y direction, complete information can be obtained from which the entire input wavefront can be reconstructed.