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Technical Discussions
Philippe Rubbers
SCM, South Africa, Joined Nov 1998, 22

Philippe Rubbers

South Africa,
Joined Nov 1998
06:51 Mar-14-2002

Dear Sir

I would like to know if SAFT (Synthetic Aperture Focusing Technique) used in ultrasonics is the same as SAR (Synthetic Aperture Radar) and if SAFT requires coherent pulses to work.

If it does not use coherent pulses, where can I get more information on the equations used (since it cannot use diffraction equations).

If it does use coherent pulses, are there any manufacturers of industrical UT equipment which make coherent pulsers.

Best regards
Philippe Rubbers ( Philippe.Rubbers@eskom.co.za )

M. Schickert
R & D,
Institute for Materials Research and Testing, Germany, Joined Jan 2000, 18

M. Schickert

R & D,
Institute for Materials Research and Testing,
Joined Jan 2000
00:05 Mar-18-2002
SAFT/SAR: SAFT has been developed in analogy to SAR (also adopting principles from optical holography and geophysics), so the algorithms are similiar except that in SAR usually far field approximations are made to accelerate computation. For synthetic focusing, all ultrasonic imaging techniques (SAFT, SAR, holography, tomography) require coherent superposition of received signals, although special incoherent methods exist.

Coherent Pulses: The term "coherent" applies to continuous waves in the first place. When using short pulses as a container for a broad frequency spectrum, the temporal relation of pulses determines the phase relation of the waves and therefore their coherence. So all one has to do in an imaging experiment is to assure fixed temoral relationship between the transmitted and each received signal. This can be done by triggering A/D-conversion by the transmitted pulse. For increased accuracy, the instruments clocks involved should be coupled. It should be noted that coherent imaging also requires coherent propagation of pulses in the medium imaged. Problems might occur in, e.g., fluctuating and strongly scattering media.


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