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.

Geometrical-Optical Calculation of Frequency Response for Systems with Spherical Aberration

Abstract: The usefulness of the geometrical-optical treatment of frequency response is considered. With this treatment calculations have been made, using numerical integration, for several cases of defect of focus, primary and secondary spherical aberration. The results are compared with those obtained from diffraction treatments of the same cases. Provided the wave front aberration exceeds about 1 wavelength, it appears that geometrical optics will give an accuracy in the calculated frequency response of about 5% of its value for zero frequency, over the range of frequencies encountered in many photographic objectives.

Optical disk apparatus using mechanism for controlling spherical aberration

Abstract: In the past there has been a problem that on a detection surface the influence of interference causes a defocusing signal to degrade, narrowing the range in which spherical aberration can be stably detected. Accordingly, a diffraction grating is used to focus the inner and outer sides of luminous flux on separate optical detectors before the optical flux is focused on an optical detector and defocusing signals are independently calculated to find the difference therebetween, thereby providing a spherical aberration signal. This makes it possible to detect spherical aberration signals more stably.

Finite conjugate spherical aberration compensation in high numerical-aperture optical disc readout

Abstract: Spherical aberration arising from deviations of the thickness of an optical disc substrate from a nominal value can be compensated to a great extent by illuminating the scanning objective lens with a slightly convergent or divergent beam. The optimum conjugate change and the amount and type of residual aberration are calculated analytically for an objective lens that satisfies Abbe's sine condition. The aberration sensitivity is decreased by a factor of 25 for numerical aperture values of approximately 0.85, and the residual aberrations consist mainly of the first higher-order Zernike spherical aberration term A60. The Wasserman-Wolf-Vaskas method is used to design biaspheric objective lenses that satisfy a ray condition that interpolates between the Abbe and the Herschel conditions. Requirements for coma by field use allow for only small deviations from the Abbe condition, making the analytical theory a good approximation for any objective lens used in practice.

Possibility of a phase contrast electron microscope.

Abstract: In-focus phase contrast has been demonstrated in the electron microscope using an arrangement analogous to that of the Zernike phase contrast light microscope. Thin carbon films with a central hole were placed in the back focal plane of the objective lens so that the scattered electrons were selectively phase-shifted by the film. The maximum phase contrast was obtained when the film thickness was adjusted to give a retardation of about pi/2 to the scattered electrons and appears to be due to the elastically scattered electrons. The observed contrast was about one-half that calculated taking into account the scattering of both the object and the phase plate and making the assumption that the inelastic scatter was incoherent. Improved phase contrast should be obtained if the nonscattered intensity is reduced by a beam stop and if phase-shifting can be accomplished by a small electrostatic lens rather than by a film. An objective lens ofthe smallest available spherical aberration is required. The in-focus phase contrast arrangement may provide useful contrast for thick (>2000 A) unstained objects in the l.0-MeV microscope. A combination contrast mode is recommended for conventional (100-kV, microscopes where amplitude contrast and enhancement of phase contrast are provided by filtering out the inelastic scatter.

Surface profilometry by a parallel-mode confocal microscope

Abstract: Confocal imaging by a parallel-mode confocal microscope is based on the real-time incoherent-holography technique. Besides the image amplitude, the image phase is inherently reconstructed. In this paper we prove that the phase image component can be converted into the height map, and, in this way, we measure the surface profile with the precision of several nanometers. Small height differences can be measured inside one optical section of the specimen surface, while the measurement of large differences needs to connect more optical sections. The two procedures are demonstrated experimentally even for the surface with large and steep height changes. The ambiguity in the height determination from the image phase component is overcome by means of the depth discrimination property of the microscope. The axial resolution is improved using broadband illumination.