PCHOLO Configurations

ONE SYSTEM FOR ALL COHERENT OPTICAL NDT ! The HG7000 Interferometry Computer & The K100 Modular Optical System

Introduction

Recognition Technology, Inc. and Karl Stetson Associates have collaborated to provide the most advanced real-time electronic interferometry available on the market. The combination of an advanced interferometry computing board set (HG7000), a menu driven software package (PCHOLO), plus a unique modular optical system provides the power to do essentially any type of coherent optical testing.

The underlying processing is based on phase step imaging that presents the user with a true WYGIWYS (What You've Got is What You See) display. The optical components are available as separate heads or as a single unit with interchangeable modules that mount on a composite optics breadboard. The optical head can also accommodate up to a 200mw of green laser integrally mounted for simple point-and-shoot operation.

Phase-Step Imaging

Phase-Step Imaging is the process used in the HG7000 interferometry computer to obtain a high quality interferometric image. It is used for processing the vidoe output of Electronic Speckle Pattern Interferometry (ESPI), of Electronic Shearography, and of Electronic Speckle Correlation, and can also be applied to Projected Fringe Moiré. The key to Phase-Step Imaging is a 90° phase step in between the two beams that interfere to form the pattern in the interferometer. The computer generates an image from the current video frame and the three frames before it. Call these four frames A, B, C, and D. Then the image is computed using the following algorithm:

The efficient pipe-line architecture of the processor allows this to be done at the video frame rate of 30 frames/sec. This process gives a true image of the interference seen by every pixel in the interferometer, and generates interference patterns of photographic quality.

Most common image processors for these interferometers work by subtracting a single stored frame from the incoming video frames. This has the detrimental effect of making the object disappear, and, while it does generate fringes from the deformations of an object, they have considerably poorer quality than the those generated by Phase-Step Imaging. This is 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. The remaining intensity related speckles can be eliminated by a simple speckle averaging process that is activated by capturing an image. Furthermore, Phase-Step Imaging does not make the object disappear - What You've Got Is What You See!

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 the application calls for flaw detection by thermal or vacuum stressing and looking for small irregularities in a fringe pattern, then the traveling fringe pattern will prove invaluable. 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.

For vibration analysis applications by holography, the dynamic range of the Phase-Step Imaging display is a very valuable feature. It is possible to see up to the 60th order Bessel function fringe in an electronic holography display. Because phase stepping is the heart of the process, interferograms can be captured and converted to numerical data without any difficulty, and wrapped phase maps can be displayed in real time! Furthermore, an extensive library puts image processing tools for nearly every operation you can imagine at your fingertips. You can measure flaws on the screen and label them for later evaluation. If you are doing flaw detection by white noise excitation, you can even have the computer size the flaws for you.

The HG7000 interferometry computer is available as three plug-in boards for a conventional PC at a price that is easily affordable. Together with our modularly designed optical system, you can apply any of four optical measurement techniques to your problems.

Modular Optical Design

You shouldn't have to be an optical engineer to apply an optical method to your nondestructive testing problems. On the other hand, if you are an optical engineer, you don't want equipment you can't adapt to your needs and ideas. The K100 modular optical system solves both problems. It is a light-weight optical pallet with optics modules that let you make any of four optical testing systems: Electronic Holography, Electronic Shearography, Electronic Speckle Correlation, and Projected Fringe Moiré. The laser is mounted on board so that it can be permanently aligned with the optics to allow you to just "point and shoot" the optical head at the object under test. The optical head can accomodate up to a 400 mw green laser, which provides enough power to deal with really large objects. The optical sensor is a Charge Coupled Device (CCD) camera equipped with a 6X zoom lens to cover any field of view you need, and you can swap the lens for any of hundreds of standard TV objective lenses. The illumination beam is also expandable over the same range. The unit comes with a protective cover that can be equipped with a safety interlock to shut off the laser when it is opened. Cables are provided for interconnection to auxiliary equipment and to the HG7000 interferometery computer.

The K100 modular optical system can be purchased with a full range of modules or tailored to a specific job. The modules can be removed easily from the pallet and used in free standing systems with other lasers. They are easily mounted on standard post-and-bracket fixtures obtainable from standard optics equipment suppliers. So if you want to experiment with different configurations, these modules will make your work easier and give you more time for productive experimentation. If you want, you can even buy the modules separately.

The Holography Configuration

It is composed of 3 modules: an interferometer module, a beamsplitter module, and an illumination module which can be attached to the beamsplitter module to form a single unit. The interferometer and beamsplitter modules are connected by an optical fiber that provides the reference beam for the interferometer. This is greatly superior to a reference beam propagating in free space because it eliminates misalignment. The fiber preserves the polarization of the light, and once it is aligned, it stays that way.

The design of the interferometer module eliminates problems found in most competing designs. First, the beamsplitter is highly reflective and oriented so that the image forming rays never pass through it. That eliminates image aberations and also eliminates patterns in the reference beam from reflections by the second surface. The primary image forming lens is a standard TV zoom lens on a standard C-mount. This is followed by a field lens and a high-quality relay lens to bring the image to the TV camera with no vignetting and with no aberrations. The imaging lens is mounted on a translation stage to facilitate focusing.

The beamsplitter module also uses a high reflection beamsplitter to direct the majority of the laser light to the object. The remainder is reflected by a piezo-electrically actuated mirror to the fiber optic coupler. Two compact rotary shutters are incorporated to switch the illumination and reference beams on and off independently. During the calibration, the interferometry computer measures the ratio of the object beam to reference beam at the TV camera. The beam ratio can be adjusted over a wide range by defocusing the reference beam on the end of the optical fiber.

The illumination module consists of a pair of lenses, one stationary and one movable along its axis, which can expand or compress the illumination beam over a wide range. A anamorphic prism beam expander is also available to elongate the profile of the beam to provide uniform illumination of oblong shaped objects. The illumination beam also passes through a tilted etalon that deviates the beam without changing its direction of propagation. When this assembly rotates, the object speckles seen by the TV camera change, and this is used to eliminate speckles by averaging.

The layout of the K100/HOL optical head is simple and provides easy access to all adjustments. The protective cover is a set of sides that bolts to the pallet capped with a detachable lid so that the user has instant and easy access to any necessary adjustment. A keyed interlock switch can be installed to shut off the laser if an unauthorized person opens the lid.

The Shearography Configuration

Figure 2. illustrates the modular design of the Electronic Shearography optical head mounded on an 18 inch square pallet. It is composed of 2 modules: a beam expander module and a shearography module. The beam expander module provides a steering mirror and a variable beam expander. The heart of the system is an interferometer assembly which is called the shearography module. This is, in effect, a Michelson interferometer through which an image of the object is relayed to the TV camera. The objective lens, which is a 6x TV zoom lens, forms an image of the object on a field lens. The interferometer consists of a cube beamsplitter and two mirrors, and two relay lenses are used for the image transfer, one at the input to the beamsplitter and one at its output. These are specially designed lenses that form aberation free images when the aperture is located at the focal plane. The lens at the input side of the beamsplitter colimates the rays coming from each image point and sends the colimated ray bundles though the interferometer. At the output side of the beamsplitter, the other lens refocuses the rays into a sharp image on the TV camera. Because the rays pass through the cube beamsplitter as colimated ray bundles, they do not acquire aberations. The initial field lens at the primary image plane is chosen so as to eliminate vignetting of the image. This optical system provides images of much higher resolution than the pixels of the TV camera.

One of the two mirrors in the interferometer has a tilt adjustment and controls the image shear, which the user can set to any magnitude, horizontal, vertical, or at any angle. On-screen calibration provided by the HG7000 interferometry computer allows direct measurement of the image shear with a set of calipers. The other mirror is mounted on a piezo-electric translator and provides 90° phase shifts between TV frames. The HG7000 computer uses these frames to create an interferometric shearography image of the overlap between the two sheared images. As with the Electronic Holography image, the image of the object does not disappear when the shearography processor is active, and the image has noticably less speckle.

The Electronic Speckle Correlation Configuration

Figure 3 illustrates the modular design of the Electronic Speckle Correlation optical head mounted on an 18 inch square pallet. It is composed of 5 small modules which manipulate the laser beam so as to divide it into two illumination beams that illuminate the object from two equal and opposite angles. The illumination angles are adjustable, depending on the size of the object, from ± 5°; to ± 30°;. The TV camera records the sum of the two fields that are reflected by the object, each corresponding to its respective illumination beam. The mirror in one of the illumination beams is piezoelectrically actuated to step the phase of that beam by 90° between TV frames, and this modulates the interference of the two fields at the camera in the proper way to serve as input to the HG7000 interferometry processor.

This interferometer configuration is sensitive only to horizontal translations of the object at right angles to the bisector of the two illumination beams. It is the ideal interferometer for measurement of strain on a flat surface, or for meaurement of transverse vibration.

The Projected Fringe Moiré Configuration

Figure 4. illustrates the modular design of the Projected Fringe Moir optical head mounted on an 18 inch square pallet. Laser light is directed into a shearing interferometer which generates two interfering beams that form a fringes that fan out to illuminate the object at an angle to the observation direction of the TV camera. The spatial frequency of the fringes is adjustable to any horizontal or vertical spacing, and one of the mirrors in the interferometer is mounted on a piezoelectric actuator. The fringes are given a quarter cycle displacement between TV frames so that they appear to sweep across the object.

When these TV frames are fed into the HG7000 interferometry computer, they are processed in the same way as for holography, shearography, or speckle correslation. A motion of the object out of its surface plane causes the fringes to shift sideways in the camera's image, and this causes a phase shift analogous to holographic interferometry, except that the displacements are now on the order of the projected fringe spacing. The HG7000 interferometry computer can, therefore, display vibration fringes or static displacement fringes for really large motions. Unlike holography, however, one object can be replaced with another because the surface microstructure is not part of the image being used. (In fact it is necessaray to suppress object speckle to make this process work properly.) It is possible, therefore, to compare the surface contours of manufactured parts against a master, or monitor a tool or die for wear.

Because the projected fringe pattern has to be resolved by the camera, and the sensitivity to displacement is governed by the fringe spacing, the resolution is related to the field of view. If the displacements are to be detected visually as fringe displacements, the resolution is in the order of 1/1000 of the field of view. If the displacements are to be detected numerically by data processing, the resolution can be 1/5000 to 1/10,000 of the field of view.

ELECTRONIC PULSED LASER HOLOGRAPHY

A system is now available for electronic recording and display of pulsed laser holographic interferograms. It consists of an optics module that can be integrated into a conventional holographic setup and an electronic image processing board that is installed in a PC type computer. The optics module records holographic interferograms on a CCD TV camera with a low angular offset to the reference beam. The electronics board coordinates the double-pulse laser Q-switching with a reset signal to the TV camera. This stores the image from the first laser pulse in the interline transfer buffer of the TV camera so that it is separated from the exposure of the second pulse, and allows the two to be read out separately on sequential frames of the camera. Laser pulse separation can be as short as 20 m sec. The camera operates in progressive scan so that each frame contains a full array of image pixels.

Subtraction of the two frames yields an image display with sinusoidal fringes indicating contours of constant displacement, and these fringes are displayed instantly after each laser firing. (No more waiting for film to be developed to know if the test was successful.) The holographic patterns obtained with the low angle reference beam can be analyzed with fast fourier transform (FFT) processing to extract the interference phase. Subtraction of the two phases gives the phase change due to the object deformation in numerical form.

Instantaneous fringe display and rapid conversion of fringes into data - this brings pulsed laser holography into the electronic age! Contact us for more information and quotation of prices.