  This paper presents a digital interferometric method to demodulate Placido
fringe patterns. This method uses a computer-stored conic-wavefront as
reference carrier. Even though, Placido mires are widely used in corneal
topographers. This is not however a paper on corneal topography and/or its
clinical use. This paper focuses on the theoretical aspects to phase-demodulate
Placido mires using synchronous interferometric techniques. Placido patterns
may also be applied to test optical wavefronts using a Placido-Hartmann opaque
plate with periodic annular apertures. This test is sensitive to the radial
slope of the measuring wavefront. Another wavefront testing approach may use a
Placido-Hartmann-Shack screen with a periodic array of toroidal lenslets. This
periodic screen is sensitive to the wavefront's radial-slope at the focal plane
of the lenslets. In brief, digital interferometric methods are herein applied
for the first time to demodulate conic-carrier Placido images. (Patent pending
at the USPTO).
