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GE Inspection Technologies

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Technical Discussions
frederic Brossard
frederic Brossard
00:02 Mar-24-1999
Resolution

Dear all,

Could anyone give me a precise definition or where to find
information on the spatial and time resolution of an ultrasonic
sensor (EMAT, OBD, PZT)and eventually which parameters
affect this performance characteristic.

Thanks in advance!


    
 
 
Robert A. Day
Engineering
Milky Way Jewels, USA, Joined Nov 1998, 40

Robert A. Day

Engineering
Milky Way Jewels,
USA,
Joined Nov 1998
40
01:55 Mar-24-1999
Re: Resolution
Frederic -

I have some discussion on this topic at http://www.xdcr.com/profile.html, mostly on how to rather than definitions. There are many definitions and much depends on what you want to do. Medical imaging has different definitions than the industrial and there are several ways to define it. The basic definitions are usually in the procedure for a particular test. ASTM E-317 defines resolution only in time. Resolution definitions usually are therefore specific to the test target used.

Time resolution is defined with flat bottom holes, side drilled holes, wires, or front surface reflection. The actual definition varies but is usually a 3dB, 6dB or 12dB criteria, that is the signal has dropped by this amount before the second signal has arrived. The criteria for front surface resolution verses a flaw is often much greater than this, 40dB or more in some cases.

Beam diameter or spacial resolutions are not usually specified but when they are they are often a variation on the Rayliegh criteria. Since this is an angular criteria it is usually modified to a spacial domain and would involve resolution of two closely spaced flat drilled holes of a size smaller than the beam diameter in the focal plane. Again resolution really is defined as how much the amplitude of the beam falls before the adjacent object begins. The value is very arbitrary and a function of the beam path along with the usual factors. The optical scheme of measuring the point spread function (psd) or modulation transfer function have only been rarely used in ultrasound and not standard targets exist. Measuring the beam diameter essentially measures the psd but for it to be meaningful it must be in the identical conditions of the actual test which is difficult in practice. Hence most measurements are in water or in blocks defined for specific purposes, AWS, ASTM etc.

The exact definition you may wish to use may be none of the standard ones and you will probably have to do some research in the standards just to see what others have done.Eric Ashe's Scanned Acoustic Microscopy would be a good starting point.

Regards,
Rocky

: Dear all,

: Could anyone give me a precise definition or where to find
: information on the spatial and time resolution of an ultrasonic
: sensor (EMAT, OBD, PZT)and eventually which parameters
: affect this performance characteristic.

: Thanks in advance!




    
 
 

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