Lens phase contrast with

This opens perspectives for obtaining phase contrast information in a microfocus tomographic system Recently we have developed a desktop X-ray microtomographic system [4] with a spot size of 8 micrometer (70 KeV) and equipped with a (1024) pixel CCD, lens coupled to a scintillator. The system is now commercially available [5], The setup is sketched in Figure 1 In this work we used the system to demonstrate the feasibility for phase contrast microtomography. 

High-resolution electron microscopic studies employed a modified JEOL-JEM200CX (8) operated at 200 kV with objective lens characteristics Cs = 0.52 mm, Cc = 1.05 mm leading to a theoretical point resolution as defined by the first zero in the phase contrast transfer function of 1.95 A at the optimum or Scherzer (9) defocus position (400 A underfocus). 

Preliminary examination of the latex involved centrifugation and optical microscopy. Only a marginal tendency to fractionate was noticed after 2 hr of centrifuging several 10-ml samples. The approximate diameter of particles separable by normal centrifuging was near 0.5 fi. An optical microscope was equipped with an oil immersion lens (1000 X) and a phase contrast stage. The polymer particles were noticeable but only marginally visible. Their diameters were near the threshold of reso-... 

Inclusions of the order of 2 nm diameter or larger can also be made visible by a phase contrast mechanism. When a crystal is oriented so that no strong, low-order diffracted beeuns are operating, all the Fourier coefficients Kg of the potential are negligibly small except the mean inner potential Kq, which is effectively the refractive index of the crystal for electrons (as explained in Sections 4.1 and 4.2). If the mean inner potential 0i of an inclusion is different from the mean inner potential Kq of the matrix, then the inclusion can be considered as a phase object. In the light microscope, a phase object is usually barely visible at exact focus but if the objective lens is slightly defocused, it will be seen with high contrast. 

Figure 14.1. Schematic diagram showing the principle of image formation and diffraction in the transmission electron microscope. The incident beam/o illuminates the specimen. Scattered and unscattered electrons are collected by the objective lens and foeused back to form first an electron diffraction pattern and then an image. For a 2D or 3D crystal, the electron-diffraetion pattern would show a lattice of spots, eaeh of whose intensity is a small fraetion of that of the incident beam. In praetiee, an in-focus image has no eontrast, so images are recorded with the objeetive lens slightly defocused to take advantage of the out-of-focus phase-contrast mechanism.

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. 

The situation is somewhat comparable to positive and negative phase contrast in optical microscopy. The lens imperfections have been used to Introduce a phase shift of jt/2 in the same way as the quarter wavelength ring in the optical microscope. Only beams passing through the "window in which the condition sin X =- I is met, interfere with the required phase relationship that causes the image to represent maxima in projected potential as dark areas. 

In phase contrast scattered beams are allowed to pass through a large objective aperture and recombine with the unscattered beam to form the image. This would give no contrast if the objective lens was perfect, and perfectly in focus. The lens is not perfect, and often defocused, causing the scattered beams to be phase shifted. 

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