Showing papers on "Depth of focus published in 1971"

Ultrasonic visualization by pulsed bragg diffraction

Abstract: An improvement in reflection mode Bragg display systems of the type wherein an acoustic beam is incident on an object which by reflection spatially modulates this sound field with its image information, a laser light beam interacts with such modulated sound field to undergo Bragg diffraction, thereby reproducing the image information in the diffracted light, and an optical system receiving such diffracted light images various desired cross sections of the object with a discrimination limited by the overall depth of focus of the optical imaging system. Control means are incorporated for delivering both the acoustic beam and the light beam in synchronized bursts of controlled duration to allow any of a plurality of cross-section within the acoustic depth of focus to be visualized, and spurious acoustic images eliminated, with a discrimination depth dependent on the duration of the acoustic burst. The light burst is delayed with respect to the initiation of the sound burst, and its duration limited, so as together to insure proper coincidence of the light with the sound energy reflected from the particular object plane selected for imaging.

Power and Focusing Considerations for Recording with a Laser Beam in the TEM 00 Mode

Abstract: Some of the aspects of recording and reading with a focused laser beam in the TEM00 mode are analyzed. The effects of defocusing and laser power variations on the recorded spot size are presented for various peak-to-threshold intensity ratios. It is shown that by suitably choosing the f/number of the optical system minimum recording power is required. The distinction between optical writing and reading depth of focus is made. It is concluded that the effective depth of focus can greatly exceed the optical depth of focus, thus relaxing mechanical stability requirements in a practical laser recording system.

White-light Three Dimensional Microscopy using Multiple-image Storing and Decoding

Abstract: THERE is a need in microscopy for high quality imaging of transparent specimens which have a large axial depth compared with the depth of focus. Because of the necessarily limited axial depth of focus of conventional microscopes, when a biologist needs detailed information throughout the thickness of a specimen, without compromising lateral resolution, he has to take a series of successive photographs on separate plates (or film frames).

Prospects for High Resolution Electron Microscopy

Abstract: The resolving power of the electron microscope which is required for a direct read out of atomic structures in real specimens is considerably smaller than the average interatomic spacing. This is due to the over-lapping of the more or less defocused projection images of those atoms which do not lie in the plane of optimum focus. Thus, with the best present-day microscopes, which have a resolving power of about 0.2 nm and an accelerating voltage of about 100 kV, atom positions can be observed only in monolayer specimens. If more than one layer is present the intensity distribution is only a measure of the projection image density. Because of the objective aperture being directly proportional to the electron wavelength and inversely proportional to the resolving power the depth of focus strongly depends on both these quantities. Approximation formulae for limit of resolution imaging conditions have been derived. They show that in order to obtain a direct read out of the atomic structure of a specimen of only 2 to 3 nm thickness a megavolt electron microscope should be used, and the required resolving power is as low as a fraction of the average atomic spacing. If the resolving power is improved for microscopes in the 100 kV range the depth of focus will decrease so rapidly that the optimum specimen thickness finally turns out to be less than 1 nm. A small amount of chromatic aberration can produce an effective enlargement of the depth of focus for accelerating voltages U