page_title: Microwave imaging
section_title: Procedures
hierarchical_section_title: Microwave imaging / Microwave testing / Procedures
caption_attribution_description: Deutsch: Messgerät FSC1 7 zur zerstörungsfreien Messung von Lackdicken auf CFK, hier auf einem Aerobatic-Flugzeug
context_page_description: Microwave imaging is a science which has been evolved from older detecting/locating techniques in order to evaluate hidden or embedded objects in a structure using electromagnetic waves in microwave regime. Engineering and application oriented microwave imaging for non-destructive testing is called microwave testing, see below.
  Microwave imaging techniques can be classified as either quantitative or qualitative. Quantitative imaging techniques give the electrical and geometrical parameters of an imaged object by solving a nonlinear inverse problem. The nonlinear inverse problem is converted into a linear inverse problem by using Born or distorted Born approximations. Despite the fact that direct matrix inversion methods can be invoked to solve the inversion problem, this will be so costly when the size of the problem is so big. To overcome this problem, direct inversion is replaced with iterative solvers. Techniques in this class are called forward iterative methods which are usually time consuming.
  On the other hand, qualitative microwave imaging methods calculate a qualitative profile to represent the hidden object.
context_section_description: Besides the reflection method also the through transmission method is possible, in which separate transmit and receive antennas are used. The backside of the device under test (DUT) must be accessible and the method gives no information about the depth of a defect within the DUT.
  Microwave tests are possible with constant frequency (CW) or with continuously tuned frequency (FMCW). FMCW is advantageous to determine the depth of defects within the DUT.
  A test probe attached to the DUT's surface gives information about the material distribution below the point of contact. When moving over the DUT surface point by point many such information is stored and then evaluated to give an overall image. This takes time. Directly imaging procedures are faster: Microwave versions are either electronic or make use of planar microwave detector consisting of a microwave absorbing foil and an infrared camera (NIDIT procedure).
caption: Gauge FSC fort the non-destructive measurement of paint thickness on CFRP, here on an aerobatic aircraft
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