Imaging Of Phase Objects Research Articles

Diffraction of electromagnetic waves by transparent edges*

We have measured the diffraction pattern of the edge of a transparent sheet for plane-polarized microwaves (λ = 3.0 cm) for angles of incidence from 0° to 45°. The material was expanded polyurethane (Styrofoam) with index of refraction 1.0150 (that of air being 1.0003). The thickness of the sheet, and thus of the rectangular edge, varied from 1.7 to 13.5 wavelengths. Our initial purpose was to study the diffraction of microwaves by phase objects, as an analog that might indicate general ways of improving the contrast in images of phase objects obtained with an electron microscope and, in particular, ways of explaining electron edge diffraction. The high contrast of as much as 2 to 1 of irradiance of the edge pattern for material of such high transparency (99.5% for thickness of 3.4 wavelengths) was surprising. We have explained the major features of our results on the basis of Huygens’s principle and Fresnel’s equations for reflection and refraction at the surface of the edge of finite thickness.

Phase Contrast Microscopy in Partially Coherent Light

In an earlier paper, results were reported of the effects of partial coherence on the phase contrast images of phase objects when the source was (i) circular and centred on the axis, and (ii) annular. In the first part of this paper, some further results obtained with the circular source being shifted away from the axis are presented. Further, extension is made to introduce a second similar source placed symmetrically about the axis. This second source may either (i) be completely incoherent with respect to the first one or (ii) have a point-to-point coherence. The effects of partial coherence are studied for both the above cases. In the second part of the paper, images of more general but periodic phase objects have been considered. Results for singly periodic and square-topped phase gratings are presented for axial full-cone and annular illumination.

Diffraction Images of Circular Phase Objects in Partially Coherent Light

Images of circular phase objects in partially coherent light have been studied. It has been shown that recognizable contrast is obtained in the image of phase objects formed by an ordinary microscope even when the cone of the illuminating beam is sizable and the microscope is focused perfectly.