[en] Optical tomography provides a means for obtaining spatially resolved measurements from line-of-sight data. Classical flow visualization approaches such as shadowgraphy, schlieren and interferometry provide information about physical observables averaged along the path of the probe beam. Locally resolved measurements cannot be obtained from such data sets, except for axially symmetric or two-dimensional flows. Tomography has been applied to medical and industrial applications. Particularly in medicine, tomographic diagnostics are well developed. Since medical objects tend to be stationary, data acquisition times need not be very rapid and sequential acquisition of projections is sufficient. In fluid mechanics no such luxury is usually allowed, since the most interesting flows are unsteady and rapidly developing. This requires that tomographic data acquisition systems have a very short response time and all projections must be collected in such a short time that the flow is essentially frozen. A new optical architecture is described that is capable providing 36 equally spaced projections about a 180 degree arc in 300 μsec. Each projection passes through a three inch diameter, three inch high cylinder, allowing a full three-dimensional reconstruction of the flow in that region. Holographic interferometry allows direct measurement of optical phase along the pathlength. From these data, the flow may be reconstructed using any of the available reconstruction techniques. The design of the optical apparatus is described, followed by an outline of the experiment, the data reduction scheme and the results obtained with this method

[en] A novel optical architecture (based on holographic optical elements) for making high speed tomographic measurements is presented. The system is designed for making density or species concentration measurements in a nonsteady fluid or combusting flow. Performance evaluations of the optical system are discussed and a test phase object has been successfully reconstructed using this optical arrangement

[en] Flow visualization results from the interactions between light and matter. Classical methods such as shadowgraphy, schlieren photography, and interferometry visualize variation in the index of refraction induced by changes in density, pressure, or temperature. Nonuniformities of these physical observables modify the phase of optical waves, rendered visible by free-space propagation (shadowgraphy), optical processing in the back focal plane of a lens (schlieren photography), or interference with a reference wave (interferometry). The classical methods visualize variations of the index of refraction or spatial derivatives thereof integrated along the light path through the fluid. Three-dimensional space is projected onto a plane with the corresponding reduction in degrees of freedom. Except for axial symmetric or two-dimensional flows, spatial structures cannot be recovered from a single image

[en] In several applications, such as electron beam lithography and X-ray differential phase contrast imaging, there is a need for a free electron source with a current density at least 10 A/cm² yet can be shaped with a resolution down to 20 nm and pulsed. Additional requirements are that the source must operate in a practical demountable vacuum (>1e-9 Torr) and be reasonably compact. In prior work, a photocathode comprising a film of CsBr on metal film on a sapphire substrate met the requirements except it was bulky because it required a beam (>10 W/cm²) of 257 nm radiation. Here, we describe an approach using a 405 nm laser which is far less bulky. The 405 nm laser, however, is not energetic enough to create color centers in CsBr films. The key to our approach is to bombard the CsBr film with a flood beam of about 1 keV electrons prior to operation. Photoelectron efficiencies in the range of 100–1000 nA/mW were demonstrated with lifetimes exceeding 50 h between electron bombardments. We suspect that the electron bombardment creates intraband color centers whence electrons can be excited by the 405 nm photons into the conduction band and thence into the vacuum.