Near Field Imaging
Near Field Imaging is not usually thought of as a nondestructive test method but is being used in that way. For those not familiar with the method the principle is very simple. It is based on the concept of forcing a wave through an aperture smaller than a wavelength of the radiation used. It is called near field because it resolves sub-wavelength structures in the near field, generally within a wavelength or two of the aperture.
The principles were first demonstrated by Eric Ashe but are best known in the form of Scanning Tunneling Microscopy invented by Benning & Rohr who received the Nobel Prize for that invention. Scanning tunneling microscopy uses as the aperture a very sharp point. Tunneling is restricted to the tip because the tunneling current is exponentially related to the gap between the tip and the surface to be imaged. A controller holds the current constant, in one variation, by applying a voltage to a piezoelectric actuator. The control system can be made stable to sub-angstrom levels.
The ability of STM to image individual atoms on the surface of materials and to move individual atoms about, by manipulating the voltage on the tip, have captured the imagination. One area in which STM and the related imaging methods have applications is in nondestructive testing. Particularly in semi-conductor manufacturing, integrated circuits, high power optics, and characterization of surfaces.
A typical STM image is shown above:
This image is of graphite. The x and y axii are 30 angstroms (1x10e-10 meters). The total height is 5 angstroms from darkest blue to brightest.
A little more useful image:
This image is of machined gold and shows the ability of near field imaging to characterize polished surfaces.
A commercial near field imageing vendors home page has better and more images and example applications.
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