Refractive lenses operation

The operation of a transmission scanning acoustic microscope requires the lenses to be set up so that they are accurately confocal. This requires holders that can be moved relative to each other along three axes, with rather fine adjustment, and that are rigid to better than a wavelength even when a specimen is vibrating between them. The separation must first be set. If ro is the radius of curvature of each lens, ciq the aperture radius, and n the refractive index, then the focal planes of the two lenses will coincide when the separation between their front surfaces is... 

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. 

The substitution of the crystalline lens for a transparent intraocular lens (lOL) has become an integral part of almost all cataract operations. Because the implantation of the lOL is done through a small incision, foldable lenses have been developed, some of which are PHEMA hydrogels (refractive index, 1.44 water content, 38%o) or HEM A copolymers with... 

Past strategies for increasing the storage capacity of optical disks were based on a reduction of A and an increase in NA, as can be seen from Table 12.1. In principle, a reduction in the spot size can be achieved with the aid of solid immersion lenses. This as yet not practically exploited technique, operating with a hemispherical or a Weierstrass superspherical lens placed near the recording medium (< 100 nm), yields a reduced spot size, S, as is evident from Eqs. (12-2) and (12-3), respectively, where n denotes the refractive index of the lens [8],... 

Microfocusing devices that are based on refraction operate in the same way as visible light optics but there are some differences (Schroer et al. 2005). Firstly, the X-ray refractive index of a material is smaller than in vacuum or air and, therefore, an X-ray focusing lens has a double concave shape. Secondly, because the refractive index of all materials is very close to unity for hard X-rays, the deflection is usually very small and many lenses have to be placed in series to achieve reasonably short focal lengths. In order to keep absorption to a minimum, these compound refractive lenses (CRTs) should be made from low-Z materials such as beryUium, carbon, aluminum, and silicon. CRTs with parabolic shapes made from polycrystalline aluminum by a pressing technique have proven to be well suited for microanalysis and full field microscopy applications for 20-120 keV X-rays (Lengerer et al. 1998). 

Because the voltage on the electrodes is designed to change as the square of the distance from the center of the lens device, a square change in the LC index of refraction occurs if the LC is operated in the linear portion of its characteristic curve. Figure 5.5 illustrates LC molecules in their rotated positions as results of the different voltage levels generated by the resistor bias network in the device, n represents the extraordinary index of refraction of the LC material, is the ordinary index of refraction, and dn = > 0 is... 

Although the physics of these devices is simple, construction and operation of the X-ray interferometer had to await the arrival of large, perfect single crystals such as semiconductor silicon. The interferometer makes use of the fact that the refractive index for X rays is slightly less than unity, about 1 — 10 . This makes conventional optical techniques difficult because the focal length of a lens would be several kilometers, but for interferometry, where optical paths are balanced, the physical dimensions need only be accurate to X — n) 50 /rm. 

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