The incoherent optical correlator was extended to perform rotation-invariant pattern recognition by using a set of two real-positive filters presenting a single-order circular harmonic function.
© 1992 Optical Society of America
Adolf W. Lohmann and David Mendlovic, "Circular harmonic filters for a rotation-invariant incoherent correlator," Appl. Opt. 31, 6187-6189 (1992)