DIRECT-PHASE DETECTION OF MODULATED RONCHI RULINGS USING A PHASE-LOCKED LOOP

A recently developed technique for fringe analysis based on a digital phase-locked loop is applied to detect the ray aberration of modulated Ronchi rulings. The Ronchi ruling is placed outside the caustic, and the shadow of the ruling is imaged directly into a two-dimensional CCD video array for further processing. The CCD array is placed at a distance of a few centimeters behind the Ronchi ruling; therefore, reasonably well-defined modulated fringes are obtained into the CCD sensor. Moreover, for small wave aberrations and coherent illumination, the first Talbot image of the ruling may be imaged over the CCD to increase the sensitivity of the test. By means of the experimental setup suggested herein, we obtain slightly modulated fringes. These fringes are demodulated using a highly sensitive digital phase-locked loop. Consequently, the transversal aberration along the direction perpendicular to the grating is found continuously across the entire image; that is, no unwrapping process is required, given that it is implicit within the phase-locked loop technique. Additionally, the same scheme with minor modifications may be used for null testing of aspheres.