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Phase-Step
Imaging is the HoloFringe300 process the interferometry computer uses to
obtain a high quality interferometric image. The key to Phase-Step
Imaging is the 90°phase step introduced between the two beams that
interfere to form the pattern in the interferometer after each TV frame.
The processor forms an image out of the current TV frame and the three
frames before it. If these four frames A, B, C, and D, the image
is I
= [(A – C)2 + (B – D)2]½. The
efficient architecture of the program allows this to be done at the
speed of the TV camera - 30 frames/sec and gives a true image of the
interference seen by every pixel in the interferometer. Most
common image processors for these interferometers work by subtracting a
single stored frame from the incoming TV frames. This has the
annoying effect of making your object disappear, and, while it does
generate fringes from the deformations of an object, they have
considerably poorer quality than those generated by Phase-Step Imaging
because of the random phase of the speckled fields. With
single-frame subtraction, this phase generates additional speckles in
the image beyond those coming from the intensity of the image, but with
Phase-Step Imaging, these speckles are completely eliminated.
Speckle averaging, process activated by capturing an image, further
reduces the intensity related speckles. The
advantages of Phase-Step Imaging do not stop with achieving the highest
quality image display possible. The displayed fringes can be made
to travel across the object at a constant velocity. If you are
doing flaw detection by thermal or vacuum stressing and looking for
small irregularities in a fringe pattern, you will welcome a
traveling fringe pattern. Small irregularities can be hidden in a
fringe pattern if they fall at the wrong place. With traveling
fringes, they will always show up, and, as the fringe sweeps over the
flaw, its size is easy to evaluate. You may also display the
fringes as a wrapped phase map, which eliminates the intensity pattern
of the object. Because phase stepping is the heart of
the process, interferograms can be captured and converted to numerical
data without any difficulty. You can measure flaws on the screen
and label them for later evaluation. If
you are doing vibration analysis by holography, you will welcome the
dynamic range of the Phase-Step Imaging display. It is possible to
see up to the 60th order Bessel function fringe in an electronic
holography display. The computer hosts a pair of frequency
generators to provide excitation to the object and a bias vibration for
converting the Bessel fringes to numerical data. The frequency of
excitation is displayed on the monitor and can be displayed on the
interferogram as well. |
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