Patent GB-1358753-A (GB)
Title: Acoustic to optical image converter using an acoustic grating
IPC: G01H9/00
Filed: 1971-07-26
Abstract: 1358753 Acoustic-to-optical image converter AMERICAN EXPRESS INVESTMENT MANAGEMENT CO 26 July 1971 [27 July 1970] 34995/71 Heading H4D [Also in Division G2] In a system wherein an acoustic wave field generated in a medium is imaged by reflecting light from a deformable surface subjected to the field, a grating formed of a material having an acoustic impedance different from the medium is immersed in the medium adjacent but spaced from the deformable surface to superimpose the acoustic wave field on a spatial frequency carrier. This enables image-forming information carried by the reflected light diffracted at the surface to be separated readily by spatial or electrical filtering from "noise" represented by the undiffracted light. As shown in Fig. 1 ultrasonic energy from source 13 is passed through (or, alternatively, reflected from) an immersed test object 12 and passed through grating 20 to form an acoustic interference pattern on a light-reflective, elastic, surface 16 of, e.g. plastics or glass, which pattern represents the acoustic wave field alternating at the acoustic frequency. For holographic applications sinusoidal or temporally coherent ultrasonic energy preferably is used. For focused acoustic imaging, acoustic lens 15 may be used. Light from laser 21 is directed through beam-splitter 22 on to surface 16 and the surface-reflected light is spatially filtered by a binary mask in the Fourier transform plane 27 of lens system 25. The mask passes image frequencies which, because of grating 20, are at spatial frequencies offset from the central portion of the spectrum at which is concentrated the undiffracted light. These image fre. quencies are directed by mirror 18 through a light modulator 17 driven at the frequency of source 13 so as to shift the optical frequency by an amount equal to the acoustic frequency. A stationary light-intensity pattern is then produced on the target of a storage-type television camera in image plane 26 by combining the light from modulator 17 with a reference light beam reflected at beam-splitter 22. The camera drives a cathode-ray tube display. In the Fig. 3 embodiment the deformable surface is the free surface of liquid (e.g. silicone oil) 45 retained by a thin polyester membrane 46. Stationary deformations of the surface are produced by the acoustic wave field and are detected by reflecting arc or laser light from the surface and focusing it at plane 61. Spatial filter 60 for extracting the desired image light is optional if the target of a television camera is arranged slightly out of plane 61 at a position where the light diffracted by surface deformations associated with the grating 47 are 90 degrees phase shifted with respect to the undiffracted light, and the electrical signal produced by scanning the target is electrically filtered and demodulated to extract the desired frequency components comprising image information. The filtered and demodulated signal is fed to a display, e.g. a cathode-ray tube. The grating of Fig. 4 comprises air-filled tubes or channels 51 in a solid material (e.g. plastics) layer 50 or sandwiched between two layers. The Fig. 5 grating comprises wires 52 between plastics sheets 53. An alternative grating is made by forming depressions in a sheet of, e.g. plastics material which is then bonded to the polyester membrane retaining the deformable liquid layer of Fig. 3. The lines of the grating may be parallel or randomly arranged.