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 dist
ance of a few centimeters behind the Ronchi ruling; therefore, reasona
bly well-defined modulated fringes are obtained into the CCD sensor. M
oreover, for small wave aberrations and coherent illumination, the fir
st Talbot image of the ruling may be imaged over the CCD to increase t
he sensitivity of the test. By means of the experimental setup suggest
ed herein, we obtain slightly modulated fringes. These fringes are dem
odulated using a highly sensitive digital phase-locked loop. Consequen
tly, the transversal aberration along the direction perpendicular to t
he grating is found continuously across the entire image; that is, no
unwrapping process is required, given that it is implicit within the p
hase-locked loop technique. Additionally, the same scheme with minor m
odifications may be used for null testing of aspheres.