Contrast transfer function

About: Contrast transfer function is a research topic. Over the lifetime, 934 publications have been published within this topic receiving 26533 citations.

Correction of aperture aberrations of hybrid lens

Abstract: The possibilities of aberration correction in the case of a single lens are limited. It is well known that, if classic glasses are used, it is impossible to compensate spherical aberration. It can be, however, minimized by proper choice of the ratio between the first and second surface radii of curvature (referred here as (zetz) ). It is possible also, in cost of uncorrected spherical aberration, to compensate III order coma. Additional possibilities of aberration correction occur, however, if a thin diffractive structure is deposited on one of the lens surface. Such lens is usually referred as a hybrid (diffractive-refractive) lens. The diffractive structure typically corresponds to the holographic lens generated by the interference of two spherical waves. The ratio of these waves radii of curvature is treated as a parameter (called here (beta) ) describing fully aberration properties of this structure. A focusing power of the diffractive part typically is only a small fraction ((eta) ) of total focusing power of a hybrid lens, so the diffractive part acts mainly as aberration corrector. Aberration properties of hybrid lens are determined by two parameters: (zetz) and (beta) so it is possible to achieve simultaneous correction of aperture aberrations: spherical aberration and coma. In the paper formulas describing the III-order aberration coefficients were used for calculating the values of parameters (zetz) and (beta) assuring such correction for several values of parameter (eta) and different locations of object plane. The calculations were performed with help of the MATHCAD programme. Basing on the results a number of hybrid lenses (collimating and imaging) were designed. Their imaging quality was then evaluated by numerical calculation of aberration spots. Estimated values of such image characteristics as the aberration spot moment of inertia or third order moment of the spots distribution enable to compare the imaging quality.

Optical Pickup with a Spherical Aberration Compensator Using a Crossed-Stripe Liquid Crystal Device

Abstract: To increase recording density in an optical disk system, a two-lens system which enlarge its numerical aperture (NA) has been developed. When NA becomes larger, however, the spherical aberration due to disk thickness error increases. We propose an optical pickup with a spherical aberration compensator using nematic liquid crystal (LC). Our LC device is composed of two LC cells each of which has X and Y stripe electrodes, respectively. We discussed how the spherical aberration due to the disk thickness error can be compensated by combination of two crossed cylindrical phase distributions induced by the LC cells, and confirmed the compensation effect in a design example by simulation. We have also fabricated the crossed stripe LC device actually, and have obtained the experimental result which shows that amount sufficient to compensate the aberration is achieved.

Local symmetry breaking of a thin crystal structure of β-Si3N4 as revealed by spherical aberration corrected high-resolution transmission electron microscopy images

Abstract: This report is an extension of the study for structural imaging of 5-6 nm thick β-Si(3)N(4) [0001] crystal with a spherical aberration corrected transmission electron microscope by Zhang and Kaiser [2009. Structure imaging of β-Si(3)N(4) by spherical aberration-corrected high-resolution transmission electron microscopy. Ultramicroscopy 109, 1114-1120]. In this work, a local symmetry breaking with an uneven resolution of dumbbells in the six-membered rings revealed in the reported images in the study of Zhang and Kaiser has been analyzed in detail. It is found that this local asymmetry in the image basically is not relevant to a slight mistilt of the specimen and/or a beam tilt (coma). Rather the certain variation of the tetrahedral bond length of Si-N(4) in the crystal structure is found to be responsible for the uneven resolution with a local structural variation from region to region. This characteristic of the variation is also supposed to give a distorted lattice of apparently 2°-2.5° deviations from the perfect hexagonal unit cell as observed in the reported image in the work of Zhang and Kaiser. It is discussed that this variation may prevail only in a thin specimen with a thickness ranging ~≤ 5-6 nm. At the same time, it is noted that the average of the bond length variation is close to the fixed length known in a bulk crystal of β-Si(3)N(4).

Spherical aberration from trajectories in real and hard-edge solenoid fields

Abstract: For analytical, real and hard-edge solenoidal axial magnetic fields, the low-energy electron trajectories are obtained using the third-order paraxial ray equation. Using the particle trajectories, it is shown that the spherical aberration in the hard-edge model is high and it increases monotonously with hard edginess, although the focal length converges, in agreement with a recent field and spherical aberration model. The model paved the way for a hard-edge approximation that gives correct focal length and spherical aberration, which is verified here by the trajectory method. In essence, we show that exact hard-edge fields give infinite spherical aberrations.