RTI HG7000 Interferometry Computing System

OVERVIEW

The Recognition Technology HG7000 is a proven electronic interferometry computing system. It consists of a set of boards and the unique software control program PCHOLO that works with a PC/AT compatible host microcomputer. The HG7000 system combines technology based on optical interferometry and advanced image processing techniques. The system generates quantitative data that can be used to observe either vibration modes or static deformation of an object - in real time. It is capable of opto-electronically producing a true hologram, shearographic image, or speckle or moiré‚ interferogram of an object every 1/30 of a second. 

Practical uses of the HG7000 encompass a wide variety of research and industrial applications including:    

  1. measurement of material deformations, distortions, and in-plane displacements 
  2. vibration analysis 
  3. full field strain measurement 
  4. nondestructive testing and evaluation 
  5. surface measurement and topographical mapping of objects 
  6. correlation of performance data with finite element models in order to validate those models for improved predictability. 

During the test cycle, the part under test is stressed. The part may be stressed by dynamically inducing vibrations of a specific frequency, or by application of pressure, vacuum, heat, cold, bending, or torsion. Images of the part under test are captured with a video camera and processed. Perturbations (the induced displacement of the object) appear as interference fringes superimposed on the object's image, as displayed on a video monitor. Qualitative evaluation of object deformations are visually observable. During operation, the video images are electronically processed to create accurate quantitative data. These data can be used to perform holographic displacement analysis of both the object's static states and its vibratory motions. The system is very well suited for visualization of an object's mode shape or static deflections. It can also be used to inspect composites, laminates, and bonded structures for subsurface flaws and defects such as disbonds, voids, or delaminations.

System Configuration

The electronic video holography system consists of four primary elements, a real-time arithmetic pipelined image processor, host computer, a laser light source, and an electro-optical head which includes camera, lenses, optical elements, and beam delivery subsystem.

During operation, a speckle interferometer combines an image of an object lit by laser light with a mutually coherent reference beam. The images are captured by a video camera operating at the RS170 standard 30 frames per secon video frame rate. The holographic processor detects the interference patterns, and an image is generated from each of four sequential video frames. For each of the video frames the phase of the reference beam is advanced by 90o, - i.e., there is a 90o phase difference between each sequential frame. Pixel values in alternate frames are subtracted from each other, their differences squared, and the results summed. This yields new values which are used to create an image of the object and its characteristic fringe function. During this process black speckles are created as a result of laser light that is reflected from the object's surface irregularities. The undesirable laser speckle reduces the resolution and clarity of the holographic fringes.

However, the HG7000 employs computer controlled movement of the object beam so that speckles are aligned differently between each frame. Combining a series of frames at different illuminations reduces laser speckle, and hence improves image resolution. By employing its advanced speckle averaging technique, The HG7000 virtually eliminates all degradation and fringe resolution is substantially improved.

Time Average Holographic Interferometry

This method is used for real-time analysis of moving objects for the measurement of vibration modes. When using this method, a time-averaged, image plane, hologram reconstruction is generated at a rate of 30 frames per second. The time averaged image is derived from information from the current image and 3 previously captured images. This is accomplished by phase stepping the laser light source by 90o every 1/30th of a second, capturing the irradiance fields which are reflected from the object from 4 successive images (at 90o, 180o, 270 o phase change from the original), analyzing the irradiance fields , and mathematically deriving an image that displays the characteristic vibration function of the part at every point in the displayed image. A hologram is created, the interference fringes are revealed, and deformations in the fringe patterns indicate modal information, and/or subsurface flaws which may exist.

Static Mode (Double Exposure or Concomitant) Holographic Interferometry

Static Mode Holographic Interferometry is used to test an object by imposing a static deformation between two states - the reference or unstressed state, and the stressed state. Initially, 4 images of the object in its unstressed state are captured using the 90o phase step methodology described above. Those 4 images are stored and added to 4 corresponding images which have been captured with the object under a static load. The results are then processed and new pixel values are calculated. A holographic image that displays the interference fringes is derived. The fringe patterns can now be examined and analyzed, as previously described. This method is particularly useful for small fragile objects, or large objects which cannot be vibrated.

Imaging Hardware 

The HG7000 Interferometry Computing System (see Fig.1 ) accomplishes all image processing and display functions. It is interfaced to Pentium compatible microcomputer that serves as the host. All functionality necessary for global system operation, I/O control, data storage, and communication is managed by the host.

The HG7000 is a high speed pipeline image processor dedicated to holographic applications. It consists of an RTI HP7000 Holography Processor card that works with an RTI IP7000 Image Processing System. The IP7000 is configured with an additional daughter card: the IP7005 Digital Storage Module/16x16 Look Up Table Module.

The input to the HG7000 is the video output of the interferometer's CCD camera which is integral to the optical head. The video signal is digitized and converted into a 512x480 pixel image. A full frame digital interferogram image is produced every 1/30th of a second. The digital interferogram is then passed to the HP7000 which produces the electronic hologram. For a moving surface, the magnitude of the displacement vector for a pixel relative to its neighbor is computed. By operating on the input image and its three predecessor images, a true hologram is produced. This hologram is now passed on to the IP7000 which accomplishes averaging in order to reduce noise. After noise reduction, the processed electronic hologram is displayed on the video monitor.

The RTI HG 7000 provides a cost effective means to optimally deal with the extreme computational complexity of electronic holographic image creation, processing, and analysis.

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Fig. 1 - Typical NDE System Schematic

Optical Head: Holography

The holography system's optical head (see Fig. 2) consists of the following components in order of their appearance in the beam paths: 

    The integrated laser is a frequency doubled diode pumped YAG laser. It produces a green beam at a wavelength of 532nm with optional power ratings of 50~200mW. 

    This laser beam passes through a beam splitter that creates individual object and reference beam paths. 

    Object Beam: The object beam is directed into the expansion optics by a mirror. The expansion optics unit can be adjusted to obtain the required illumination pattern. The beam can be adjusted for wide, narrow, or oblong illumination. The object beam then passes through a rotary etalon. The etalon consists of beam displacing prisms which are rotated by a closed-loop servo driven motorized rotational stage. It provides beam displacement during the speckle averaging operation. Next, the light strikes the object under test. The light reflected by the object is captured by the wide angle macro-zoom lens. It is then directed into the interferometer. It passes through a focus lens that aims the beam onto the mixing wedge plate, and then onto the focal plane of the CCD video camera. 

    Reference Beam: The reference beam first strikes the phase shifting piezo mirror, then it is launched into a polarization preserving fiber via a fiber coupler. The fiber length is calibrated to equalize path lengths between the two beams, as measured from the splitter to the camera focal plane. The output from the fiber is directed through a fiber positioner onto a wedge plate, and finally, to the camera focal plane. 

The optical head enclosure is electrically interlocked to the laser power supply. There are internal baffles and shutters in each beam path for calibration purposes, and as an additional safety precaution.

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Fig. 2 - Holography Optical Head Layout

Optical Head: Shearography

The shearography head (see Fig. 3) uses a single beam optical arrangement. Like the holography head, it incorporates speckle averaging for improved fringe resolution. The beam path is identical to that of the object beam in the holography head up to the point where it passes through the wide-angle macro-zoom lens. At that point, the beam passes through a copy lens and into a Michelson interferometer. This initially splits the object beam in two and creates a phase shift in one of the two beams. It then recombines the beams to produce a phase-modulated beam of light. This modulated beam then passes through another copy lens and into the CCD camera. The result is a sheared image. The amount of image shear is controlled by the mirror adjustments in the interferometer.

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Figure 3 - Shearography Optical Head Layout