Patent GB-1149374-A (GB)
Title: Improvements in and relating to the recording of ultrasonic vibrations
Inventor: GABOR DENNIS
IPC: G01H9/00
Filed: 1965-05-12
Abstract: 1,149,374. Televisions' light modulators. COLUMBIA BROADCASTING SYSTEM Inc. 10 May, 1966 [12 May, 1965; 7 July, 1965], Nos. 20025/65 and 28771/65. Heading H4F. [Also in Division G2] In ultrasonic camera apparatus, an ultrasonic generator (3) (Fig. 1, not shown) is provided in a fluid medium in a tank (1) containing an object (50) to be viewed and an elastic screen (5) having a radiation reflecting surface and being matched to the wave impedance of the medium, and there is further provided, external to the tank, a source of laser light means for directing the radiation on to the surface, and means utilizing the Doppler effect for converting the radiation reflected from the surface into a viewable image. The laser beam is split so that a portion thereof is reflected from the screen (5) while the remainder is superimposed on the reflected portion in such a manner that the radiation reflected from at least part of the surface is in phase quadrature with the superimposed radiation so as to convert the frequency variation imparted by the surface to the reflected radiation into amplitude variation. An image of the screen is formed by the two superimposed portions of the laser beam, and the image is scanned by at least one photo-cell (12) point-by-point, the output of the cell going into an A.C. amplifier (52) tuned to the ultrasound frequency. The rectified output of the amplifier is displayed on a cathode ray tube (13) with a memory. The plane screen (5) is preferably replaced by a thin self-supporting part spherical shell (14) (Figs. 4a, 4b, not shown) carrying a specularly reflecting coating. A portion of the laser light is focussed to a point before being thrown on to the mirror (14) by a reflecting prism (15), and is concentrated by the mirror in a lens (9) while the unreflected (or reference) beam is arranged to travel the same optical distance as the reflected beam and to appear to issue from the centre of the mirror (14). The wave fronts of the two beams are spherical and form a system of straight, equidistant lines as their interference pattern. A coherent ultrasonic reference beam is provided by a second ultrasonic generator (18). The optical reference wave may be adjusted so as to be the equivalent of the sound reference wave, and will then act in the same way as the optical reference wave used in the reconstruction process in light holography to form an image which is magnified by the lens (9) and which can be explored in depth by a scanning system (Figs. 5a, 5b, 5c, not shown). In order to utilize the laser power more fully, only a narrow strip of the vibrating screen 14 is illuminated at any time, and the screen is scanned by a narrow laser beam. Light from a gas laser 23 is concentrated into a thin flat beam by means of lenses 25, 26. The beam then falls on a polygonal prism 27 rotating so that one face of the polygon crosses the light beam in the time of a light scan. The beam is then split 50-50 by a prism 29, and the transmitted part is focussed near to the centre of a spherical lens formed of two halves having a semi-reflecting layer 33 between them. The part reflected at 33 serves as the reference beam and is returned by a mirror 35 to the layer 33 where it interferes with the returning transmitted beam which has been reflected back by the screen 14. The two beams then fall again on the beam splitter 29 and a suitably magnified image of the interference pattern is thrown on the light collecting guides 21 of the photoelectric elements 12 through scanning apertures 22 in the band 19. The illuminated zone now moves with the apertures 22 and a cylindrical meniscus lens is provided to ensure that the movement of the zone is linear. In another embodiment (Figs. 7a, 7b, not shown) allowing of utilization of ¢ the total light (as opposed to # in the embodiment of Figs. 6a, 6b, 6c), the beam splitting prism 29 is replaced by a lozenge shaped prism (38) and the light reflected thereby is brought, by prisms (39), to the spherical lens and forms the reference beam there. In a further embodiment (Figs. 8a, 8b, not shown) a spot (P) scans the whole of the screen (14).