Phase-Step Imaging

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.