Oblique illumination and matched high spatial frequency cut-off for edge enhancement
Authors: Quercioli F., Molesini G.
Autors Affiliation: Istituto Nazionale di Ottica, Largo E. Fermi 6, 50125 Firenze, Italy
Abstract: Referring to edge enhancement effect obtained with detuned interference filters already reported, a model is discussed where the source is made of a narrow annular aperture in the front focal plane of a lens, producing a narrow spatial frequency band illumination with circular symmetry in the back focal plane. Such a plane is made to coincide with the object plane of a 4 -f optical processor. In the Fourier plane of the processor a circular aperture is placed, to perform low -pass spatial frequency filtering whose cut -off matches the spatial frequency of the illuminating configuration. In reasonable operating conditions the overall processing turns out to be described by a directional Hilbert transform of the object transparency along radial lines of the Fourier plane. Computation and experimental evidence are presented for the case of a simple object as a slit, showing up edge enhancement and background attenuation according to the image intensity distribution which consists of two equally energetic terms, the one associated to the geometrical image and the other to the sharp edge effect.
KeyWords: Dark-filed imagery;Detuned interference filters; Edge enhancement effect; High spatial frequency cut-off; Oblique illumination; optical processor; DOI: 10.1117/12.935414