Holographic methods permit use of very large effective apertures so that weak signals can be collected over wide area and integrated to form image. Technique, modification of side-looking radar principle, can be used at very short ranges needed for nondestructive inspection of test specimens

Cribbs, R. W.

Lamb, B. L.

English

NASA Technical Reports Server

September 1973
	
B7340379 
•	 NASA TECH BRIEF 
Ames Research Center
	 4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Microwave Holography for Nondestructive Testing 
Microwaves are of interest for nondestructive test-
ing of propellant grains because they are cheaper to 
generate than x-rays and easier to manipulate. How-
ever, means must be found to form microwave im-
ages of the flaws from severely attenuated signals. 
Fortunately, holographic methods permit use of very 
large effective apertures so that weak signals can be 
collected over a wide area and integrated to form 
an image. 
The microwave holographic technique is a modifi-
cation of the side-looking radar principle, using fre-
quency sweep instead of pulses to obtain depth 
resolution; moreover, the image-forming techniques 
common to side-looking radar can be used at the very 
short ranges needed for nondestructive inspection of 
test specimens. 
In a waveguide system, the detector receives the 
reflected signal from the object and a sample of the 
outgoing signal in order to record a hologram. For 
depth resolution, the CW signal has been replaced 
by a linear full-band frequency sweep (12.4 to 18 
GHz in the experimental work). The detector then 
receives two swept-frequency signals, one of which 
has a time delay proportional to the distance to the 
object. In practice, the sweep rate is adjusted so that 
the two signals overlap over almost all the sweep 
period. The detector output thus has a frequency 
output for each reflector in the beam. However, the 
swept-frequency range resolution does not produce
a series of stacked, one-dimensional holograms as 
does a pulsed side-looking radar, which requires only 
an astigmatic lens system with correction for image-
plane tilt; the hologram is actually two-dimensional 
(position along the scan line and distance to the 
specimen) and is difficult to process. 
The method for image formation is described in 
reference to a sandbox simulator, in which rods or 
other representations of flaws can be inserted. To 
form the image in the optical processor, the trans-
form plane filter is made by photographing the trans-
form of a hologram made with no flaws in the sim-
ulator. This eliminates most of the background and 
greatly attenuates the back surface of the box; the 
front surface is completely eliminated. A cylinder 
lens performs focusing in one direction to produce 
the image, but first the lens is removed and the lateral 
focus of the image is observed. If, for example, three 
rods have been placed in the sandbox, they cannot all 
be brought into focus at once, and so they are re-
corded at different positions along the optical bench, 
appearing as bright vertical lines. Then a compromise 
focus is selected and the cylinder lens is replaced. 
The lens can focus all the images in the depth dimen-
sion and focus them in the same plane; it is moved 
to focus vertically in the plane selected as a compro-
mise focus. In the resulting image, the lateral focus is 
better for some objects than others, but all are well 
resolved.
(continued overleaf) 
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Note: 
Requests for further information may be directed 
to
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 78-10379
Patent status: 
NASA has decided not to apply for a patent. 
Source: Robert W. Cribbs and Billy L. Lamb of 
Electra-Physics Laboratories, Inc.
under contract to
Ames Research Center
(ARC-10774)
. 
B73-10379	 Category 03


Microwave holography for nondestructive testing

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