Axial illumination

Wade, R.H. Frank, J. Electron microscope transfer functions for partially coherent axial illumination and chromatic defocus spread. Optik 1977, 49 (1), 81-92. 

In summary, a planar interface causes the mean focal position to be significantly deeper that the usually quoted paraxial focal depth, and there is also a large spread in the axial illumination, resulting in a large depth of focus. When the system is coupled to a confocal collection aperture, the collection efficiency curves are relatively complex, with a generally rapid fall in the collection efficiency with focal depth. For a given (large) focal depth, there will be an optimum numerical aperture for efficient collection of Raman intensity. The results of this theory will be compared to measured spectra in Section III, which describes an experiment to map the stress distribution within the diamonds of a diamond anvil cell. 

In integrated photoelasticity it is impossible to achieve a complete reconstruction of stresses in samples by only illuminating a system of parallel planes and using equilibrium equations of the elasticity theory. Theory of the fictitious temperature field allows one to formulate a boundary-value problem which permits to determine all components of the stress tensor field in some cases. If the stress gradient in the axial direction is smooth enough, then perturbation method can be used for the solution of the inverse problem. As an example, distribution of stresses in a bow tie type fiber preforms is shown in Fig. 2 [2]. 

Darkfield microscopy is one of the oldest modes of microscopy. Here, axial rays from the condenser are prevented from entering the objective, through the use of an opaque stop placed in the condenser, while peripheral light illuminates the specimen. Thus, the specimen is seen lighted against a dark Held. 

Fig. 7. Schematic diagram of forces exerted on a cell when using an inverted microscope with (A) epi-illumination (i.e., laser focused through the objective) or (B) transillumination (i.e., laser focused through the condenser). is the axial force, and Fl is the lateral trapping force. Curved arrows represent the laser beam waist and point in the direction of light propagation.

An alternative readout system is a scanning differential phase-contrast microscope with a split detector as shown in Figure 16.5. The optical configuration is compact and easy to align. The memory medium, in which the data bits have been recorded, is located at the focus of an objective lens. The band limit of the optical transfer function (OTF) is the same as that of a conventional microscope with incoherent illumination. The resolution, especially the axial resolution of the phase-contrast microscope, is similar to that obtained by Zemike s phase-contrast microscope. The contrast of the image is much improved compared to that of Zernike s phase-contrast microscope, however, because the nondiffracted components are completely eliminated by the subtraction of signals between two detectors. The readout system is therefore sensitive to small phase changes. 

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