Aperture

J.Seydel, Ultrasonic Synthetic-aperture Focusing Techniques in NDT , Research Techniques in NDT, London, 1983, v.6. 

In order to ensure perpendicular beam incidence on the cylindrical specimen, the circular B-scan profiles were acquired by high frequency (narrow beam) transducers in a synthetic circular aperture array. From these profiles two-dimensional reflection tomograms were reconstructed using a filtered backprojection technique. Straight line propagation was assumed. Several artificial discontinuity types in a cylindrical Plexiglas (Perspex) specimen were compared with similar artificial discontinuities in a cylindrical A/Si-alloy [2]. Furthermore, examples of real discontinuities (an inclusion and a feed head) in the cylindrical AlSi-alloy are presented. 

Fig. 1. Circular aperture array with uniformly distributed transducer elements (No. 1, 2. . N).

First, by circular array imaging A synthetic circular aperture array was used to obtain circular C-scan images. These images displayed the location of different discontinuity types, although only shadow images were obtained. 

By using absolute probes with selective resolution, the distortions caused by the probe (probe aperture) could be minimized. This results in images which come very close to the actual physical conditions. 

Let us consider a diffuser G Figure la) which is illuminated with an He-Ne laser. The illuminated area is limited with a rectangular aperture A. The observation plane IToiTl, ), parallel to the reference plane n(x,y), is located at a distance D from it. 

The encircling probe was characterised with its mirror in water. As we did not own very tiny hydrophone, we used a reflector with hemispherical tip with a radius of curvature of 2 mm (see figure 3c). As a result, it was possible to monitor the beam at the tube entrance and to measure the position of the beam at the desired angle relatively to the angular 0° position. A few acoustic apertures were verified. They were selected on an homogeneous criteria a good one with less than 2 dB of relative sensitivity variations, medium one would be 4 dB and a bad one with more than 6 dB. 

The principle of the acquisition system is to translate the probe into a tube (including hemispherical drilled holes) step by step, every 0.04 mm, after a forwards and backwards 360 rotation of the tube trigging every 0.2° angular step a 360° electronic scanning of tube with the 160 acoustic apertures. During the electronic scanning the tube is assumed to stay at the same place. The acquisition lasts about 30 minutes for a C-scan acquisition with a 14 kHz recurrence frequency. 

The analysis software rebuild a C-scan of every three reflectors and for every 160 acoustic apertures, and plots (see figure 5) ... 

The system can also numerize the A-scan from the back-wall echo of the specific target, giving the central frequency, relative bandwidth and sensitivity of the 160 apertures. 

Fig.l, Change of the reflected signal with scanning of a sample as parallelepiped with internal apertures. 1 - sample Ks 1 2 - sample iNs 2 3 -difference of the reflected signals 4 - difference of the reflected signals after mathematical processing. The area from 12 up to 14 corresponds to an additional aperture (defect). 

Figure A3.5.L The fast ion beam photofragment spectrometer at SRI International. L labels electrostatic lenses, D labels deflectors and A labels apertures.

Electron lens systems between each component serve a number of fiinctions. A lens following the source focuses electrons on the entrance aperture of the premonocliromator and decelerates these electrons to the pass energy required... 

The magnification is rather chosen to be about 500 to 1000 where is the numerical aperture of the objective (see the next section) The eyepiece is then necessary to magnify the real image so that it can conveniently be inspected. 

The smallest resolvable stnicture is thus addition, the aperture of the illumination system... 

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